[Clang Interpreter] Initial patch for the constexpr interpreter

Summary:
This patch introduces the skeleton of the constexpr interpreter,
capable of evaluating a simple constexpr functions consisting of
if statements. The interpreter is described in more detail in the
RFC. Further patches will add more features.

Reviewers: Bigcheese, jfb, rsmith

Subscribers: bruno, uenoku, ldionne, Tyker, thegameg, tschuett, dexonsmith, mgorny, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64146

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@371834 91177308-0d34-0410-b5e6-96231b3b80d8

docs/ConstantInterpreter.rst

@@ -0,0 +1,194 @@
+====================
+Constant Interpreter
+====================
+
+.. contents::
+   :local:
+
+Introduction
+============
+
+The constexpr interpreter aims to replace the existing tree evaluator in clang, improving performance on constructs which are executed inefficiently by the evaluator. The interpreter is activated using the following flags:
+
+* ``-fexperimental-new-constant-interpreter`` enables the interpreter, falling back to the evaluator for unsupported features
+* ``-fforce-experimental-new-constant-interpreter`` forces the use of the interpreter, bailing out if an unsupported feature is encountered
+
+Bytecode Compilation
+====================
+
+Bytecode compilation is handled in ``ByteCodeStmtGen.h`` for statements and ``ByteCodeExprGen.h`` for expressions. The compiler has two different backends: one to generate bytecode for functions (``ByteCodeEmitter``) and one to directly evaluate expressions as they are compiled, without generating bytecode (``EvalEmitter``). All functions are compiled to bytecode, while toplevel expressions used in constant contexts are directly evaluated since the bytecode would never be reused. This mechanism aims to pave the way towards replacing the evaluator, improving its performance on functions and loops, while being just as fast on single-use toplevel expressions.
+
+The interpreter relies on stack-based, strongly-typed opcodes. The glue logic between the code generator, along with the enumeration and description of opcodes, can be found in ``Opcodes.td``. The opcodes are implemented as generic template methods in ``Interp.h`` and instantiated with the relevant primitive types by the interpreter loop or by the evaluating emitter.
+
+Primitive Types
+---------------
+
+* ``PT_{U|S}int{8|16|32|64}``
+
+  Signed or unsigned integers of a specific bit width, implemented using the ```Integral``` type.
+
+* ``PT_{U|S}intFP``
+
+  Signed or unsigned integers of an arbitrary, but fixed width used to implement
+  integral types which are required by the target, but are not supported by the host.
+  Under the hood, they rely on APValue. The ``Integral`` specialisation for these
+  types is required by opcodes to share an implementation with fixed integrals.
+
+* ``PT_Bool``
+
+  Representation for boolean types, essentially a 1-bit unsigned ``Integral``.
+
+* ``PT_RealFP``
+
+  Arbitrary, but fixed precision floating point numbers. Could be specialised in
+  the future similarly to integers in order to improve floating point performance.
+
+* ``PT_Ptr``
+
+  Pointer type, defined in ``"Pointer.h"``.
+
+* ``PT_FnPtr``
+
+  Function pointer type, can also be a null function pointer. Defined in ``"Pointer.h"``.
+
+* ``PT_MemPtr``
+
+  Member pointer type, can also be a null member pointer. Defined in ``"Pointer.h"``
+
+Composite types
+---------------
+
+The interpreter distinguishes two kinds of composite types: arrays and records. Unions are represented as records, except a single field can be marked as active. The contents of inactive fields are kept until they
+are reactivated and overwritten.
+
+
+Bytecode Execution
+==================
+
+Bytecode is executed using a stack-based interpreter. The execution context consists of an ``InterpStack``, along with a chain of ``InterpFrame`` objects storing the call frames. Frames are built by call instructions and destroyed by return instructions. They perform one allocation to reserve space for all locals in a single block. These objects store all the required information to emit stack traces whenever evaluation fails.
+
+Memory Organisation
+===================
+
+Memory management in the interpreter relies on 3 data structures: ``Block``
+object which store the data and associated inline metadata, ``Pointer`` objects
+which refer to or into blocks, and ``Descriptor`` structures which describe
+blocks and subobjects nested inside blocks.
+
+Blocks
+------
+
+Blocks contain data interleaved with metadata. They are allocated either statically
+in the code generator (globals, static members, dummy parameter values etc.) or
+dynamically in the interpreter, when creating the frame containing the local variables
+of a function. Blocks are associated with a descriptor that characterises the entire
+allocation, along with a few additional attributes:
+
+* ``IsStatic`` indicates whether the block has static duration in the interpreter, i.e. it is not a local in a frame.
+
+* ``IsExtern`` indicates that the block was created for an extern and the storage cannot be read or written.
+
+* ``DeclID`` identifies each global declaration (it is set to an invalid and irrelevant value for locals) in order to prevent illegal writes and reads involving globals and temporaries with static storage duration.
+
+Static blocks are never deallocated, but local ones might be deallocated even when there are live pointers to them. Pointers are only valid as long as the blocks they point to are valid, so a block with pointers to it whose lifetime ends is kept alive until all pointers to it go out of scope. Since the frame is destroyed on function exit, such blocks are turned into a ``DeadBlock`` and copied to storage managed by the interpreter itself, not the frame. Reads and writes to these blocks are illegal and cause an appropriate diagnostic to be emitted. When the last pointer goes out of scope, dead blocks are also deallocated.
+
+The lifetime of blocks is managed through 3 methods stored in the descriptor of the block:
+
+* **CtorFn**: initializes the metadata which is store in the block, alongside actual data. Invokes the default constructors of objects which are not trivial (``Pointer``, ``RealFP``, etc.)
+* **DtorFn**: invokes the destructors of non-trivial objects.
+* **MoveFn**: moves a block to dead storage.
+
+Non-static blocks track all the pointers into them through an intrusive doubly-linked list, this is required in order to adjust all pointers when transforming a block into a dead block.
+
+Descriptors
+-----------
+
+Descriptor are generated at bytecode compilation time and contain information required to determine if a particular memory access is allowed in constexpr. Even though there is a single descriptor object, it encodes information for several kinds of objects:
+
+* **Primitives**
+
+  A block containing a primitive reserved storage only for the primitive.
+
+* **Arrays of primitives**
+
+  An array of primitives contains a pointer to an ``InitMap`` storage as its first field: the initialisation map is a bit map indicating all elements of the array which were initialised. If the pointer is null, no elements were initialised, while a value of ``(InitMap)-1`` indicates that the object was fully initialised. when all fields are initialised, the map is deallocated and replaced with that token.
+
+  Array elements are stored sequentially, without padding, after the pointer to the map.
+
+* **Arrays of composites and records**
+
+  Each element in an array of composites is preceded by an ``InlineDescriptor``. Descriptors and elements are stored sequentially in the block. Records are laid out identically to arrays of composites: each field and base class is preceded by an inline descriptor. The ``InlineDescriptor`` has the following field:
+
+ * **Offset**: byte offset into the array or record, used to step back to the parent array or record.
+ * **IsConst**: flag indicating if the field is const-qualified.
+ * **IsInitialized**: flag indicating whether the field or element was initialized. For non-primitive fields, this is only relevant for base classes.
+ * **IsBase**: flag indicating whether the record is a base class. In that case, the offset can be used to identify the derived class.
+ * **IsActive**: indicates if the field is the active field of a union.
+ * **IsMutable**: indicates if the field is marked as mutable.
+
+Inline descriptors are filled in by the `CtorFn` of blocks, which leaves storage in an uninitialised, but valid state.
+
+Pointers
+--------
+
+Pointers track a ``Pointee``, the block to which they point or ``nullptr`` for null pointers, along with a ``Base`` and an ``Offset``. The base identifies the innermost field, while the offset points to an array element relative to the base (including one-past-end pointers). Most subobject the pointer points to in block, while the offset identifies the array element the pointer points to. These two fields allow all pointers to be uniquely identified and disambiguated.
+
+As an example, consider the following structure:
+
+.. code-block:: c
+
+    struct A {
+        struct B {
+            int x;
+            int y;
+        } b;
+        struct C {
+            int a;
+            int b;
+        } c[2];
+        int z;
+    };
+    constexpr A a;
+
+On the target, ``&a`` and ``&a.b.x`` are equal. So are ``&a.c[0]`` and ``&a.c[0].a``. In the interpreter, all these pointers must be distinguished since the are all allowed to address distinct range of memory.
+
+In the interpreter, the object would require 240 bytes of storage and would have its field interleaved with metadata. The pointers which can be derived to the object are illustrated in the following diagram:
+
+::
+
+      0   16  32  40  56  64  80  96  112 120 136 144 160 176 184 200 208 224 240
+  +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
+  + B | D | D | x | D | y | D | D | D | a | D | b | D | D | a | D | b | D | z |
+  +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
+      ^   ^   ^       ^       ^   ^   ^       ^       ^   ^       ^       ^
+      |   |   |       |       |   |   |   &a.c[0].b   |   |   &a.c[1].b   |
+      a   |&a.b.x   &a.y    &a.c  |&a.c[0].a          |&a.c[1].a          |
+        &a.b                   &a.c[0]            &a.c[1]               &a.z
+
+The ``Base`` offset of all pointers points to the start of a field or an array and is preceded by an inline descriptor (unless ``Base == 0``, pointing to the root). All the relevant attributes can be read from either the inline descriptor or the descriptor of the block.
+
+Array elements are identified by the ``Offset`` field of pointers, pointing to past the inline descriptors for composites and before the actual data in the case of primitive arrays. The ``Offset`` points to the offset where primitives can be read from. As an example, ``a.c + 1`` would have the same base as ``a.c`` since it is an element of ``a.c``, but its offset would point to ``&a.c[1]``. The ``*`` operation narrows the scope of the pointer, adjusting the base to ``&a.c[1]``. The reverse operator, ``&``, expands the scope of ``&a.c[1]``, turning it into ``a.c + 1``. When a one-past-end pointer is narrowed, its offset is set to ``-1`` to indicate that it is an invalid value (expanding returns the past-the-end pointer). As a special case, narrowing ``&a.c`` results in ``&a.c[0]``. The `narrow` and `expand` methods can be used to follow the chain of equivalent pointers.
+
+TODO
+====
+
+Missing Language Features
+-------------------------
+
+* Definition of externs must override previous declaration
+* Changing the active field of unions
+* Union copy constructors
+* ``typeid``
+* ``volatile``
+* ``__builtin_constant_p``
+* ``std::initializer_list``
+* lambdas
+* range-based for loops
+* ``vector_size``
+* ``dynamic_cast``
+
+Known Bugs
+----------
+
+* Pointer comparison for equality needs to narrow/expand pointers
+* If execution fails, memory storing APInts and APFloats is leaked when the stack is cleared

docs/index.rst

@@ -88,6 +88,7 @@ Design Documents
    PCHInternals
    ItaniumMangleAbiTags
    HardwareAssistedAddressSanitizerDesign.rst
+   ConstantInterpreter
 
 
 Indices and tables

include/clang/AST/ASTContext.h

@@ -139,6 +139,12 @@ class FullComment;
 
 } // namespace comments
 
+namespace interp {
+
+class Context;
+
+} // namespace interp
+
 struct TypeInfo {
   uint64_t Width = 0;
   unsigned Align = 0;
@@ -564,6 +570,7 @@ private:
   const TargetInfo *Target = nullptr;
   const TargetInfo *AuxTarget = nullptr;
   clang::PrintingPolicy PrintingPolicy;
+  std::unique_ptr<interp::Context> InterpContext;
 
 public:
   IdentifierTable &Idents;
@@ -573,6 +580,9 @@ public:
   IntrusiveRefCntPtr<ExternalASTSource> ExternalSource;
   ASTMutationListener *Listener = nullptr;
 
+  /// Returns the clang bytecode interpreter context.
+  interp::Context &getInterpContext();
+
   /// Container for either a single DynTypedNode or for an ArrayRef to
   /// DynTypedNode. For use with ParentMap.
   class DynTypedNodeList {

include/clang/AST/OptionalDiagnostic.h

@@ -0,0 +1,78 @@
+//===- OptionalDiagnostic.h - An optional diagnostic ------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+/// \file
+/// Implements a partial diagnostic which may not be emitted.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_AST_OPTIONALDIAGNOSTIC_H
+#define LLVM_CLANG_AST_OPTIONALDIAGNOSTIC_H
+
+#include "clang/AST/APValue.h"
+#include "clang/Basic/PartialDiagnostic.h"
+#include "llvm/ADT/APFloat.h"
+#include "llvm/ADT/APSInt.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/StringRef.h"
+
+namespace clang {
+
+/// A partial diagnostic which we might know in advance that we are not going
+/// to emit.
+class OptionalDiagnostic {
+  PartialDiagnostic *Diag;
+
+public:
+  explicit OptionalDiagnostic(PartialDiagnostic *Diag = nullptr) : Diag(Diag) {}
+
+  template <typename T> OptionalDiagnostic &operator<<(const T &v) {
+    if (Diag)
+      *Diag << v;
+    return *this;
+  }
+
+  OptionalDiagnostic &operator<<(const llvm::APSInt &I) {
+    if (Diag) {
+      SmallVector<char, 32> Buffer;
+      I.toString(Buffer);
+      *Diag << StringRef(Buffer.data(), Buffer.size());
+    }
+    return *this;
+  }
+
+  OptionalDiagnostic &operator<<(const llvm::APFloat &F) {
+    if (Diag) {
+      // FIXME: Force the precision of the source value down so we don't
+      // print digits which are usually useless (we don't really care here if
+      // we truncate a digit by accident in edge cases).  Ideally,
+      // APFloat::toString would automatically print the shortest
+      // representation which rounds to the correct value, but it's a bit
+      // tricky to implement. Could use std::to_chars.
+      unsigned precision = llvm::APFloat::semanticsPrecision(F.getSemantics());
+      precision = (precision * 59 + 195) / 196;
+      SmallVector<char, 32> Buffer;
+      F.toString(Buffer, precision);
+      *Diag << StringRef(Buffer.data(), Buffer.size());
+    }
+    return *this;
+  }
+
+  OptionalDiagnostic &operator<<(const APFixedPoint &FX) {
+    if (Diag) {
+      SmallVector<char, 32> Buffer;
+      FX.toString(Buffer);
+      *Diag << StringRef(Buffer.data(), Buffer.size());
+    }
+    return *this;
+  }
+};
+
+} // namespace clang
+
+#endif

include/clang/Basic/DiagnosticASTKinds.td

@@ -228,6 +228,8 @@ def note_constexpr_bit_cast_invalid_subtype : Note<
 def note_constexpr_bit_cast_indet_dest : Note<
   "indeterminate value can only initialize an object of type 'unsigned char'"
   "%select{, 'char',|}1 or 'std::byte'; %0 is invalid">;
+def err_experimental_clang_interp_failed : Error<
+  "the experimental clang interpreter failed to evaluate an expression">;
 
 def warn_integer_constant_overflow : Warning<
   "overflow in expression; result is %0 with type %1">,

include/clang/Basic/LangOptions.def

@@ -290,6 +290,10 @@ BENIGN_LANGOPT(ConstexprCallDepth, 32, 512,
                "maximum constexpr call depth")
 BENIGN_LANGOPT(ConstexprStepLimit, 32, 1048576,
                "maximum constexpr evaluation steps")
+BENIGN_LANGOPT(EnableNewConstInterp, 1, 0,
+               "enable the experimental new constant interpreter")
+BENIGN_LANGOPT(ForceNewConstInterp, 1, 0,
+               "force the use of the experimental new constant interpreter")
 BENIGN_LANGOPT(BracketDepth, 32, 256,
                "maximum bracket nesting depth")
 BENIGN_LANGOPT(NumLargeByValueCopy, 32, 0,

include/clang/Driver/Options.td

@@ -838,6 +838,10 @@ def fconstant_cfstrings : Flag<["-"], "fconstant-cfstrings">, Group<f_Group>;
 def fconstant_string_class_EQ : Joined<["-"], "fconstant-string-class=">, Group<f_Group>;
 def fconstexpr_depth_EQ : Joined<["-"], "fconstexpr-depth=">, Group<f_Group>;
 def fconstexpr_steps_EQ : Joined<["-"], "fconstexpr-steps=">, Group<f_Group>;
+def fexperimental_new_constant_interpreter : Flag<["-"], "fexperimental-new-constant-interpreter">, Group<f_Group>,
+  HelpText<"Enable the experimental new constant interpreter">, Flags<[CC1Option]>;
+def fforce_experimental_new_constant_interpreter : Flag<["-"], "fforce-experimental-new-constant-interpreter">, Group<f_Group>,
+  HelpText<"Force the use of the experimental new constant interpreter, failing on missing features">, Flags<[CC1Option]>;
 def fconstexpr_backtrace_limit_EQ : Joined<["-"], "fconstexpr-backtrace-limit=">,
                                     Group<f_Group>;
 def fno_crash_diagnostics : Flag<["-"], "fno-crash-diagnostics">, Group<f_clang_Group>, Flags<[NoArgumentUnused, CoreOption]>,

lib/AST/ASTContext.cpp

@@ -12,6 +12,7 @@
 
 #include "clang/AST/ASTContext.h"
 #include "CXXABI.h"
+#include "Interp/Context.h"
 #include "clang/AST/APValue.h"
 #include "clang/AST/ASTMutationListener.h"
 #include "clang/AST/ASTTypeTraits.h"
@@ -783,6 +784,13 @@ CXXABI *ASTContext::createCXXABI(const TargetInfo &T) {
   llvm_unreachable("Invalid CXXABI type!");
 }
 
+interp::Context &ASTContext::getInterpContext() {
+  if (!InterpContext) {
+    InterpContext.reset(new interp::Context(*this));
+  }
+  return *InterpContext.get();
+}
+
 static const LangASMap *getAddressSpaceMap(const TargetInfo &T,
                                            const LangOptions &LOpts) {
   if (LOpts.FakeAddressSpaceMap) {

lib/AST/CMakeLists.txt

@@ -4,6 +4,14 @@ set(LLVM_LINK_COMPONENTS
   Support
   )
 
+# FIXME: the entry points to the interpreter should be moved out of clangAST
+# into the parser or Sema in order to allow the interpreter to be moved to
+# another library which depends on clangAST.
+clang_tablegen(Opcodes.inc
+  -gen-clang-opcodes
+  SOURCE Interp/Opcodes.td
+  TARGET Opcodes)
+
 add_clang_library(clangAST
   APValue.cpp
   ASTConsumer.cpp
@@ -15,7 +23,6 @@ add_clang_library(clangAST
   ASTStructuralEquivalence.cpp
   ASTTypeTraits.cpp
   AttrImpl.cpp
-  CXXInheritance.cpp
   Comment.cpp
   CommentBriefParser.cpp
   CommentCommandTraits.cpp
@@ -23,6 +30,7 @@ add_clang_library(clangAST
   CommentParser.cpp
   CommentSema.cpp
   ComparisonCategories.cpp
+  CXXInheritance.cpp
   DataCollection.cpp
   Decl.cpp
   DeclarationName.cpp
@@ -43,6 +51,27 @@ add_clang_library(clangAST
   ExternalASTSource.cpp
   FormatString.cpp
   InheritViz.cpp
+  Interp/Block.cpp
+  Interp/ByteCodeEmitter.cpp
+  Interp/ByteCodeExprGen.cpp
+  Interp/ByteCodeGenError.cpp
+  Interp/ByteCodeStmtGen.cpp
+  Interp/Context.cpp
+  Interp/Descriptor.cpp
+  Interp/Disasm.cpp
+  Interp/EvalEmitter.cpp
+  Interp/Frame.cpp
+  Interp/Function.cpp
+  Interp/Interp.cpp
+  Interp/InterpFrame.cpp
+  Interp/InterpStack.cpp
+  Interp/InterpState.cpp
+  Interp/Pointer.cpp
+  Interp/PrimType.cpp
+  Interp/Program.cpp
+  Interp/Record.cpp
+  Interp/Source.cpp
+  Interp/State.cpp
   ItaniumCXXABI.cpp
   ItaniumMangle.cpp
   JSONNodeDumper.cpp
@@ -52,8 +81,8 @@ add_clang_library(clangAST
   NestedNameSpecifier.cpp
   NSAPI.cpp
   ODRHash.cpp
-  OSLog.cpp
   OpenMPClause.cpp
+  OSLog.cpp
   ParentMap.cpp
   PrintfFormatString.cpp
   QualTypeNames.cpp
@@ -82,4 +111,7 @@ add_clang_library(clangAST
   LINK_LIBS
   clangBasic
   clangLex
+
+  DEPENDS
+  Opcodes
   )

lib/AST/ExprConstant.cpp

@@ -32,6 +32,11 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include <cstring>
+#include <functional>
+#include "Interp/Context.h"
+#include "Interp/Frame.h"
+#include "Interp/State.h"
 #include "clang/AST/APValue.h"
 #include "clang/AST/ASTContext.h"
 #include "clang/AST/ASTDiagnostic.h"
@@ -41,6 +46,7 @@
 #include "clang/AST/CurrentSourceLocExprScope.h"
 #include "clang/AST/Expr.h"
 #include "clang/AST/OSLog.h"
+#include "clang/AST/OptionalDiagnostic.h"
 #include "clang/AST/RecordLayout.h"
 #include "clang/AST/StmtVisitor.h"
 #include "clang/AST/TypeLoc.h"
@@ -51,8 +57,6 @@
 #include "llvm/ADT/SmallBitVector.h"
 #include "llvm/Support/SaveAndRestore.h"
 #include "llvm/Support/raw_ostream.h"
-#include <cstring>
-#include <functional>
 
 #define DEBUG_TYPE "exprconstant"
 
@@ -66,8 +70,8 @@ static bool IsGlobalLValue(APValue::LValueBase B);
 
 namespace {
   struct LValue;
-  struct CallStackFrame;
-  struct EvalInfo;
+  class CallStackFrame;
+  class EvalInfo;
 
   using SourceLocExprScopeGuard =
       CurrentSourceLocExprScope::SourceLocExprScopeGuard;
@@ -222,12 +226,6 @@ namespace {
     return MostDerivedLength;
   }
 
-  // The order of this enum is important for diagnostics.
-  enum CheckSubobjectKind {
-    CSK_Base, CSK_Derived, CSK_Field, CSK_ArrayToPointer, CSK_ArrayIndex,
-    CSK_Real, CSK_Imag
-  };
-
   /// A path from a glvalue to a subobject of that glvalue.
   struct SubobjectDesignator {
     /// True if the subobject was named in a manner not supported by C++11. Such
@@ -480,7 +478,8 @@ namespace {
   };
 
   /// A stack frame in the constexpr call stack.
-  struct CallStackFrame {
+  class CallStackFrame : public interp::Frame {
+  public:
     EvalInfo &Info;
 
     /// Parent - The caller of this stack frame.
@@ -574,6 +573,12 @@ namespace {
     }
 
     APValue &createTemporary(const void *Key, bool IsLifetimeExtended);
+
+    void describe(llvm::raw_ostream &OS) override;
+
+    Frame *getCaller() const override { return Caller; }
+    SourceLocation getCallLocation() const override { return CallLoc; }
+    const FunctionDecl *getCallee() const override { return Callee; }
   };
 
   /// Temporarily override 'this'.
@@ -592,59 +597,6 @@ namespace {
     const LValue *OldThis;
   };
 
-  /// A partial diagnostic which we might know in advance that we are not going
-  /// to emit.
-  class OptionalDiagnostic {
-    PartialDiagnostic *Diag;
-
-  public:
-    explicit OptionalDiagnostic(PartialDiagnostic *Diag = nullptr)
-      : Diag(Diag) {}
-
-    template<typename T>
-    OptionalDiagnostic &operator<<(const T &v) {
-      if (Diag)
-        *Diag << v;
-      return *this;
-    }
-
-    OptionalDiagnostic &operator<<(const APSInt &I) {
-      if (Diag) {
-        SmallVector<char, 32> Buffer;
-        I.toString(Buffer);
-        *Diag << StringRef(Buffer.data(), Buffer.size());
-      }
-      return *this;
-    }
-
-    OptionalDiagnostic &operator<<(const APFloat &F) {
-      if (Diag) {
-        // FIXME: Force the precision of the source value down so we don't
-        // print digits which are usually useless (we don't really care here if
-        // we truncate a digit by accident in edge cases).  Ideally,
-        // APFloat::toString would automatically print the shortest
-        // representation which rounds to the correct value, but it's a bit
-        // tricky to implement.
-        unsigned precision =
-            llvm::APFloat::semanticsPrecision(F.getSemantics());
-        precision = (precision * 59 + 195) / 196;
-        SmallVector<char, 32> Buffer;
-        F.toString(Buffer, precision);
-        *Diag << StringRef(Buffer.data(), Buffer.size());
-      }
-      return *this;
-    }
-
-    OptionalDiagnostic &operator<<(const APFixedPoint &FX) {
-      if (Diag) {
-        SmallVector<char, 32> Buffer;
-        FX.toString(Buffer);
-        *Diag << StringRef(Buffer.data(), Buffer.size());
-      }
-      return *this;
-    }
-  };
-
   /// A cleanup, and a flag indicating whether it is lifetime-extended.
   class Cleanup {
     llvm::PointerIntPair<APValue*, 1, bool> Value;
@@ -707,7 +659,8 @@ namespace {
   /// rules.  For example, the RHS of (0 && foo()) is not evaluated.  We can
   /// evaluate the expression regardless of what the RHS is, but C only allows
   /// certain things in certain situations.
-  struct EvalInfo {
+  class EvalInfo : public interp::State {
+  public:
     ASTContext &Ctx;
 
     /// EvalStatus - Contains information about the evaluation.
@@ -727,6 +680,13 @@ namespace {
     /// we will evaluate.
     unsigned StepsLeft;
 
+    /// Force the use of the experimental new constant interpreter, bailing out
+    /// with an error if a feature is not supported.
+    bool ForceNewConstInterp;
+
+    /// Enable the experimental new constant interpreter.
+    bool EnableNewConstInterp;
+
     /// BottomFrame - The frame in which evaluation started. This must be
     /// initialized after CurrentCall and CallStackDepth.
     CallStackFrame BottomFrame;
@@ -827,32 +787,35 @@ namespace {
 
     /// Are we checking whether the expression is a potential constant
     /// expression?
-    bool checkingPotentialConstantExpression() const {
+    bool checkingPotentialConstantExpression() const override  {
       return CheckingPotentialConstantExpression;
     }
 
     /// Are we checking an expression for overflow?
     // FIXME: We should check for any kind of undefined or suspicious behavior
     // in such constructs, not just overflow.
-    bool checkingForUndefinedBehavior() { return CheckingForUndefinedBehavior; }
+    bool checkingForUndefinedBehavior() const override {
+      return CheckingForUndefinedBehavior;
+    }
 
     EvalInfo(const ASTContext &C, Expr::EvalStatus &S, EvaluationMode Mode)
-      : Ctx(const_cast<ASTContext &>(C)), EvalStatus(S), CurrentCall(nullptr),
-        CallStackDepth(0), NextCallIndex(1),
-        StepsLeft(getLangOpts().ConstexprStepLimit),
-        BottomFrame(*this, SourceLocation(), nullptr, nullptr, nullptr),
-        EvaluatingDecl((const ValueDecl *)nullptr),
-        EvaluatingDeclValue(nullptr), HasActiveDiagnostic(false),
-        HasFoldFailureDiagnostic(false),
-        InConstantContext(false), EvalMode(Mode) {}
+        : Ctx(const_cast<ASTContext &>(C)), EvalStatus(S), CurrentCall(nullptr),
+          CallStackDepth(0), NextCallIndex(1),
+          StepsLeft(getLangOpts().ConstexprStepLimit),
+          ForceNewConstInterp(getLangOpts().ForceNewConstInterp),
+          EnableNewConstInterp(ForceNewConstInterp ||
+                               getLangOpts().EnableNewConstInterp),
+          BottomFrame(*this, SourceLocation(), nullptr, nullptr, nullptr),
+          EvaluatingDecl((const ValueDecl *)nullptr),
+          EvaluatingDeclValue(nullptr), HasActiveDiagnostic(false),
+          HasFoldFailureDiagnostic(false), InConstantContext(false),
+          EvalMode(Mode) {}
 
     void setEvaluatingDecl(APValue::LValueBase Base, APValue &Value) {
       EvaluatingDecl = Base;
       EvaluatingDeclValue = &Value;
     }
 
-    const LangOptions &getLangOpts() const { return Ctx.getLangOpts(); }
-
     bool CheckCallLimit(SourceLocation Loc) {
       // Don't perform any constexpr calls (other than the call we're checking)
       // when checking a potential constant expression.
@@ -896,115 +859,47 @@ namespace {
     }
 
   private:
-    /// Add a diagnostic to the diagnostics list.
-    PartialDiagnostic &addDiag(SourceLocation Loc, diag::kind DiagId) {
-      PartialDiagnostic PD(DiagId, Ctx.getDiagAllocator());
-      EvalStatus.Diag->push_back(std::make_pair(Loc, PD));
-      return EvalStatus.Diag->back().second;
-    }
-
-    /// Add notes containing a call stack to the current point of evaluation.
-    void addCallStack(unsigned Limit);
-
-  private:
-    OptionalDiagnostic Diag(SourceLocation Loc, diag::kind DiagId,
-                            unsigned ExtraNotes, bool IsCCEDiag) {
-
-      if (EvalStatus.Diag) {
-        // If we have a prior diagnostic, it will be noting that the expression
-        // isn't a constant expression. This diagnostic is more important,
-        // unless we require this evaluation to produce a constant expression.
-        //
-        // FIXME: We might want to show both diagnostics to the user in
-        // EM_ConstantFold mode.
-        if (!EvalStatus.Diag->empty()) {
-          switch (EvalMode) {
-          case EM_ConstantFold:
-          case EM_IgnoreSideEffects:
-            if (!HasFoldFailureDiagnostic)
-              break;
-            // We've already failed to fold something. Keep that diagnostic.
-            LLVM_FALLTHROUGH;
-          case EM_ConstantExpression:
-          case EM_ConstantExpressionUnevaluated:
-            HasActiveDiagnostic = false;
-            return OptionalDiagnostic();
-          }
+    interp::Frame *getCurrentFrame() override { return CurrentCall; }
+    const interp::Frame *getBottomFrame() const override { return &BottomFrame; }
+
+    bool hasActiveDiagnostic() override { return HasActiveDiagnostic; }
+    void setActiveDiagnostic(bool Flag) override { HasActiveDiagnostic = Flag; }
+
+    void setFoldFailureDiagnostic(bool Flag) override {
+      HasFoldFailureDiagnostic = Flag;
+    }
+
+    Expr::EvalStatus &getEvalStatus() const override { return EvalStatus; }
+
+    ASTContext &getCtx() const override { return Ctx; }
+
+    // If we have a prior diagnostic, it will be noting that the expression
+    // isn't a constant expression. This diagnostic is more important,
+    // unless we require this evaluation to produce a constant expression.
+    //
+    // FIXME: We might want to show both diagnostics to the user in
+    // EM_ConstantFold mode.
+    bool hasPriorDiagnostic() override {
+      if (!EvalStatus.Diag->empty()) {
+        switch (EvalMode) {
+        case EM_ConstantFold:
+        case EM_IgnoreSideEffects:
+          if (!HasFoldFailureDiagnostic)
+            break;
+          // We've already failed to fold something. Keep that diagnostic.
+          LLVM_FALLTHROUGH;
+        case EM_ConstantExpression:
+        case EM_ConstantExpressionUnevaluated:
+          setActiveDiagnostic(false);
+          return true;
         }
-
-        unsigned CallStackNotes = CallStackDepth - 1;
-        unsigned Limit = Ctx.getDiagnostics().getConstexprBacktraceLimit();
-        if (Limit)
-          CallStackNotes = std::min(CallStackNotes, Limit + 1);
-        if (checkingPotentialConstantExpression())
-          CallStackNotes = 0;
-
-        HasActiveDiagnostic = true;
-        HasFoldFailureDiagnostic = !IsCCEDiag;
-        EvalStatus.Diag->clear();
-        EvalStatus.Diag->reserve(1 + ExtraNotes + CallStackNotes);
-        addDiag(Loc, DiagId);
-        if (!checkingPotentialConstantExpression())
-          addCallStack(Limit);
-        return OptionalDiagnostic(&(*EvalStatus.Diag)[0].second);
-      }
-      HasActiveDiagnostic = false;
-      return OptionalDiagnostic();
-    }
-  public:
-    // Diagnose that the evaluation could not be folded (FF => FoldFailure)
-    OptionalDiagnostic
-    FFDiag(SourceLocation Loc,
-          diag::kind DiagId = diag::note_invalid_subexpr_in_const_expr,
-          unsigned ExtraNotes = 0) {
-      return Diag(Loc, DiagId, ExtraNotes, false);
-    }
-
-    OptionalDiagnostic FFDiag(const Expr *E, diag::kind DiagId
-                              = diag::note_invalid_subexpr_in_const_expr,
-                            unsigned ExtraNotes = 0) {
-      if (EvalStatus.Diag)
-        return Diag(E->getExprLoc(), DiagId, ExtraNotes, /*IsCCEDiag*/false);
-      HasActiveDiagnostic = false;
-      return OptionalDiagnostic();
-    }
-
-    /// Diagnose that the evaluation does not produce a C++11 core constant
-    /// expression.
-    ///
-    /// FIXME: Stop evaluating if we're in EM_ConstantExpression mode
-    /// and we produce one of these.
-    OptionalDiagnostic CCEDiag(SourceLocation Loc, diag::kind DiagId
-                                 = diag::note_invalid_subexpr_in_const_expr,
-                               unsigned ExtraNotes = 0) {
-      // Don't override a previous diagnostic. Don't bother collecting
-      // diagnostics if we're evaluating for overflow.
-      if (!EvalStatus.Diag || !EvalStatus.Diag->empty()) {
-        HasActiveDiagnostic = false;
-        return OptionalDiagnostic();
       }
-      return Diag(Loc, DiagId, ExtraNotes, true);
-    }
-    OptionalDiagnostic CCEDiag(const Expr *E, diag::kind DiagId
-                                 = diag::note_invalid_subexpr_in_const_expr,
-                               unsigned ExtraNotes = 0) {
-      return CCEDiag(E->getExprLoc(), DiagId, ExtraNotes);
-    }
-    /// Add a note to a prior diagnostic.
-    OptionalDiagnostic Note(SourceLocation Loc, diag::kind DiagId) {
-      if (!HasActiveDiagnostic)
-        return OptionalDiagnostic();
-      return OptionalDiagnostic(&addDiag(Loc, DiagId));
+      return false;
     }
 
-    /// Add a stack of notes to a prior diagnostic.
-    void addNotes(ArrayRef<PartialDiagnosticAt> Diags) {
-      if (HasActiveDiagnostic) {
-        EvalStatus.Diag->insert(EvalStatus.Diag->end(),
-                                Diags.begin(), Diags.end());
-      }
-    }
+    unsigned getCallStackDepth() override { return CallStackDepth; }
 
+  public:
     /// Should we continue evaluation after encountering a side-effect that we
     /// couldn't model?
     bool keepEvaluatingAfterSideEffect() {
@@ -1047,14 +942,14 @@ namespace {
     /// Note that we hit something that was technically undefined behavior, but
     /// that we can evaluate past it (such as signed overflow or floating-point
     /// division by zero.)
-    bool noteUndefinedBehavior() {
+    bool noteUndefinedBehavior() override {
       EvalStatus.HasUndefinedBehavior = true;
       return keepEvaluatingAfterUndefinedBehavior();
     }
 
     /// Should we continue evaluation as much as possible after encountering a
     /// construct which can't be reduced to a value?
-    bool keepEvaluatingAfterFailure() {
+    bool keepEvaluatingAfterFailure() const override {
       if (!StepsLeft)
         return false;
 
@@ -1297,62 +1192,6 @@ APValue &CallStackFrame::createTemporary(const void *Key,
   return Result;
 }
 
-static void describeCall(CallStackFrame *Frame, raw_ostream &Out);
-
-void EvalInfo::addCallStack(unsigned Limit) {
-  // Determine which calls to skip, if any.
-  unsigned ActiveCalls = CallStackDepth - 1;
-  unsigned SkipStart = ActiveCalls, SkipEnd = SkipStart;
-  if (Limit && Limit < ActiveCalls) {
-    SkipStart = Limit / 2 + Limit % 2;
-    SkipEnd = ActiveCalls - Limit / 2;
-  }
-
-  // Walk the call stack and add the diagnostics.
-  unsigned CallIdx = 0;
-  for (CallStackFrame *Frame = CurrentCall; Frame != &BottomFrame;
-       Frame = Frame->Caller, ++CallIdx) {
-    // Skip this call?
-    if (CallIdx >= SkipStart && CallIdx < SkipEnd) {
-      if (CallIdx == SkipStart) {
-        // Note that we're skipping calls.
-        addDiag(Frame->CallLoc, diag::note_constexpr_calls_suppressed)
-          << unsigned(ActiveCalls - Limit);
-      }
-      continue;
-    }
-
-    // Use a different note for an inheriting constructor, because from the
-    // user's perspective it's not really a function at all.
-    if (auto *CD = dyn_cast_or_null<CXXConstructorDecl>(Frame->Callee)) {
-      if (CD->isInheritingConstructor()) {
-        addDiag(Frame->CallLoc, diag::note_constexpr_inherited_ctor_call_here)
-          << CD->getParent();
-        continue;
-      }
-    }
-
-    SmallVector<char, 128> Buffer;
-    llvm::raw_svector_ostream Out(Buffer);
-    describeCall(Frame, Out);
-    addDiag(Frame->CallLoc, diag::note_constexpr_call_here) << Out.str();
-  }
-}
-
-/// Kinds of access we can perform on an object, for diagnostics. Note that
-/// we consider a member function call to be a kind of access, even though
-/// it is not formally an access of the object, because it has (largely) the
-/// same set of semantic restrictions.
-enum AccessKinds {
-  AK_Read,
-  AK_Assign,
-  AK_Increment,
-  AK_Decrement,
-  AK_MemberCall,
-  AK_DynamicCast,
-  AK_TypeId,
-};
-
 static bool isModification(AccessKinds AK) {
   switch (AK) {
   case AK_Read:
@@ -1720,36 +1559,36 @@ static void negateAsSigned(APSInt &Int) {
 }
 
 /// Produce a string describing the given constexpr call.
-static void describeCall(CallStackFrame *Frame, raw_ostream &Out) {
+void CallStackFrame::describe(raw_ostream &Out) {
   unsigned ArgIndex = 0;
-  bool IsMemberCall = isa<CXXMethodDecl>(Frame->Callee) &&
-                      !isa<CXXConstructorDecl>(Frame->Callee) &&
-                      cast<CXXMethodDecl>(Frame->Callee)->isInstance();
+  bool IsMemberCall = isa<CXXMethodDecl>(Callee) &&
+                      !isa<CXXConstructorDecl>(Callee) &&
+                      cast<CXXMethodDecl>(Callee)->isInstance();
 
   if (!IsMemberCall)
-    Out << *Frame->Callee << '(';
+    Out << *Callee << '(';
 
-  if (Frame->This && IsMemberCall) {
+  if (This && IsMemberCall) {
     APValue Val;
-    Frame->This->moveInto(Val);
-    Val.printPretty(Out, Frame->Info.Ctx,
-                    Frame->This->Designator.MostDerivedType);
+    This->moveInto(Val);
+    Val.printPretty(Out, Info.Ctx,
+                    This->Designator.MostDerivedType);
     // FIXME: Add parens around Val if needed.
-    Out << "->" << *Frame->Callee << '(';
+    Out << "->" << *Callee << '(';
     IsMemberCall = false;
   }
 
-  for (FunctionDecl::param_const_iterator I = Frame->Callee->param_begin(),
-       E = Frame->Callee->param_end(); I != E; ++I, ++ArgIndex) {
+  for (FunctionDecl::param_const_iterator I = Callee->param_begin(),
+       E = Callee->param_end(); I != E; ++I, ++ArgIndex) {
     if (ArgIndex > (unsigned)IsMemberCall)
       Out << ", ";
 
     const ParmVarDecl *Param = *I;
-    const APValue &Arg = Frame->Arguments[ArgIndex];
-    Arg.printPretty(Out, Frame->Info.Ctx, Param->getType());
+    const APValue &Arg = Arguments[ArgIndex];
+    Arg.printPretty(Out, Info.Ctx, Param->getType());
 
     if (ArgIndex == 0 && IsMemberCall)
-      Out << "->" << *Frame->Callee << '(';
+      Out << "->" << *Callee << '(';
   }
 
   Out << ')';
@@ -12251,6 +12090,18 @@ static bool EvaluateInPlace(APValue &Result, EvalInfo &Info, const LValue &This,
 /// EvaluateAsRValue - Try to evaluate this expression, performing an implicit
 /// lvalue-to-rvalue cast if it is an lvalue.
 static bool EvaluateAsRValue(EvalInfo &Info, const Expr *E, APValue &Result) {
+   if (Info.EnableNewConstInterp) {
+    auto &InterpCtx = Info.Ctx.getInterpContext();
+    switch (InterpCtx.evaluateAsRValue(Info, E, Result)) {
+    case interp::InterpResult::Success:
+      return true;
+    case interp::InterpResult::Fail:
+      return false;
+    case interp::InterpResult::Bail:
+      break;
+    }
+  }
+
   if (E->getType().isNull())
     return false;
 
@@ -12458,11 +12309,29 @@ bool Expr::EvaluateAsInitializer(APValue &Value, const ASTContext &Ctx,
   Expr::EvalStatus EStatus;
   EStatus.Diag = &Notes;
 
-  EvalInfo InitInfo(Ctx, EStatus, VD->isConstexpr()
+  EvalInfo Info(Ctx, EStatus, VD->isConstexpr()
                                       ? EvalInfo::EM_ConstantExpression
                                       : EvalInfo::EM_ConstantFold);
-  InitInfo.setEvaluatingDecl(VD, Value);
-  InitInfo.InConstantContext = true;
+  Info.setEvaluatingDecl(VD, Value);
+  Info.InConstantContext = true;
+
+  SourceLocation DeclLoc = VD->getLocation();
+  QualType DeclTy = VD->getType();
+
+  if (Info.EnableNewConstInterp) {
+    auto &InterpCtx = const_cast<ASTContext &>(Ctx).getInterpContext();
+    switch (InterpCtx.evaluateAsInitializer(Info, VD, Value)) {
+    case interp::InterpResult::Fail:
+      // Bail out if an error was encountered.
+      return false;
+    case interp::InterpResult::Success:
+      // Evaluation succeeded and value was set.
+      return CheckConstantExpression(Info, DeclLoc, DeclTy, Value);
+    case interp::InterpResult::Bail:
+      // Evaluate the value again for the tree evaluator to use.
+      break;
+    }
+  }
 
   LValue LVal;
   LVal.set(VD);
@@ -12472,20 +12341,19 @@ bool Expr::EvaluateAsInitializer(APValue &Value, const ASTContext &Ctx,
   //  zero-initialized before any other initialization takes place.
   // This behavior is not present in C.
   if (Ctx.getLangOpts().CPlusPlus && !VD->hasLocalStorage() &&
-      !VD->getType()->isReferenceType()) {
-    ImplicitValueInitExpr VIE(VD->getType());
-    if (!EvaluateInPlace(Value, InitInfo, LVal, &VIE,
+      !DeclTy->isReferenceType()) {
+    ImplicitValueInitExpr VIE(DeclTy);
+    if (!EvaluateInPlace(Value, Info, LVal, &VIE,
                          /*AllowNonLiteralTypes=*/true))
       return false;
   }
 
-  if (!EvaluateInPlace(Value, InitInfo, LVal, this,
+  if (!EvaluateInPlace(Value, Info, LVal, this,
                        /*AllowNonLiteralTypes=*/true) ||
       EStatus.HasSideEffects)
     return false;
 
-  return CheckConstantExpression(InitInfo, VD->getLocation(), VD->getType(),
-                                 Value);
+  return CheckConstantExpression(Info, DeclLoc, DeclTy, Value);
 }
 
 /// isEvaluatable - Call EvaluateAsRValue to see if this expression can be
@@ -13162,6 +13030,18 @@ bool Expr::isPotentialConstantExpr(const FunctionDecl *FD,
   Info.InConstantContext = true;
   Info.CheckingPotentialConstantExpression = true;
 
+  // The constexpr VM attempts to compile all methods to bytecode here.
+  if (Info.EnableNewConstInterp) {
+    auto &InterpCtx = Info.Ctx.getInterpContext();
+    switch (InterpCtx.isPotentialConstantExpr(Info, FD)) {
+    case interp::InterpResult::Success:
+    case interp::InterpResult::Fail:
+      return Diags.empty();
+    case interp::InterpResult::Bail:
+      break;
+    }
+  }
+
   const CXXMethodDecl *MD = dyn_cast<CXXMethodDecl>(FD);
   const CXXRecordDecl *RD = MD ? MD->getParent()->getCanonicalDecl() : nullptr;
 

lib/AST/Interp/Block.cpp

@@ -0,0 +1,87 @@
+//===--- Block.cpp - Allocated blocks for the interpreter -------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Defines the classes describing allocated blocks.
+//
+//===----------------------------------------------------------------------===//
+
+#include "Block.h"
+#include "Pointer.h"
+
+using namespace clang;
+using namespace clang::interp;
+
+
+
+void Block::addPointer(Pointer *P) {
+  if (IsStatic)
+    return;
+  if (Pointers)
+    Pointers->Prev = P;
+  P->Next = Pointers;
+  P->Prev = nullptr;
+  Pointers = P;
+}
+
+void Block::removePointer(Pointer *P) {
+  if (IsStatic)
+    return;
+  if (Pointers == P)
+    Pointers = P->Next;
+  if (P->Prev)
+    P->Prev->Next = P->Next;
+  if (P->Next)
+    P->Next->Prev = P->Prev;
+}
+
+void Block::cleanup() {
+  if (Pointers == nullptr && IsDead)
+    (reinterpret_cast<DeadBlock *>(this + 1) - 1)->free();
+}
+
+void Block::movePointer(Pointer *From, Pointer *To) {
+  if (IsStatic)
+    return;
+  To->Prev = From->Prev;
+  if (To->Prev)
+    To->Prev->Next = To;
+  To->Next = From->Next;
+  if (To->Next)
+    To->Next->Prev = To;
+  if (Pointers == From)
+    Pointers = To;
+
+  From->Prev = nullptr;
+  From->Next = nullptr;
+}
+
+DeadBlock::DeadBlock(DeadBlock *&Root, Block *Blk)
+    : Root(Root), B(Blk->Desc, Blk->IsStatic, Blk->IsExtern, /*isDead=*/true) {
+  // Add the block to the chain of dead blocks.
+  if (Root)
+    Root->Prev = this;
+
+  Next = Root;
+  Prev = nullptr;
+  Root = this;
+
+  // Transfer pointers.
+  B.Pointers = Blk->Pointers;
+  for (Pointer *P = Blk->Pointers; P; P = P->Next)
+    P->Pointee = &B;
+}
+
+void DeadBlock::free() {
+  if (Prev)
+    Prev->Next = Next;
+  if (Next)
+    Next->Prev = Prev;
+  if (Root == this)
+    Root = Next;
+  ::free(this);
+}

lib/AST/Interp/Block.h

@@ -0,0 +1,140 @@
+//===--- Block.h - Allocated blocks for the interpreter ---------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Defines the classes describing allocated blocks.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_AST_INTERP_BLOCK_H
+#define LLVM_CLANG_AST_INTERP_BLOCK_H
+
+#include "Descriptor.h"
+#include "clang/AST/Decl.h"
+#include "clang/AST/DeclCXX.h"
+#include "clang/AST/Expr.h"
+#include "clang/AST/ComparisonCategories.h"
+#include "llvm/ADT/PointerUnion.h"
+#include "llvm/Support/raw_ostream.h"
+
+namespace clang {
+namespace interp {
+class Block;
+class DeadBlock;
+class Context;
+class InterpState;
+class Pointer;
+class Function;
+enum PrimType : unsigned;
+
+/// A memory block, either on the stack or in the heap.
+///
+/// The storage described by the block immediately follows it in memory.
+class Block {
+public:
+  // Creates a new block.
+  Block(const llvm::Optional<unsigned> &DeclID, Descriptor *Desc,
+        bool IsStatic = false, bool IsExtern = false)
+      : DeclID(DeclID), IsStatic(IsStatic), IsExtern(IsExtern), Desc(Desc) {}
+
+  Block(Descriptor *Desc, bool IsStatic = false, bool IsExtern = false)
+      : DeclID((unsigned)-1), IsStatic(IsStatic), IsExtern(IsExtern),
+        Desc(Desc) {}
+
+  /// Returns the block's descriptor.
+  Descriptor *getDescriptor() const { return Desc; }
+  /// Checks if the block has any live pointers.
+  bool hasPointers() const { return Pointers; }
+  /// Checks if the block is extern.
+  bool isExtern() const { return IsExtern; }
+  /// Checks if the block has static storage duration.
+  bool isStatic() const { return IsStatic; }
+  /// Checks if the block is temporary.
+  bool isTemporary() const { return Desc->IsTemporary; }
+  /// Returns the size of the block.
+  InterpSize getSize() const { return Desc->getAllocSize(); }
+  /// Returns the declaration ID.
+  llvm::Optional<unsigned> getDeclID() const { return DeclID; }
+
+  /// Returns a pointer to the stored data.
+  char *data() { return reinterpret_cast<char *>(this + 1); }
+
+  /// Returns a view over the data.
+  template <typename T>
+  T &deref() { return *reinterpret_cast<T *>(data()); }
+
+  /// Invokes the constructor.
+  void invokeCtor() {
+    std::memset(data(), 0, getSize());
+    if (Desc->CtorFn)
+      Desc->CtorFn(this, data(), Desc->IsConst, Desc->IsMutable,
+                   /*isActive=*/true, Desc);
+  }
+
+protected:
+  friend class Pointer;
+  friend class DeadBlock;
+  friend class InterpState;
+
+  Block(Descriptor *Desc, bool IsExtern, bool IsStatic, bool IsDead)
+    : IsStatic(IsStatic), IsExtern(IsExtern), IsDead(true), Desc(Desc) {}
+
+  // Deletes a dead block at the end of its lifetime.
+  void cleanup();
+
+  // Pointer chain management.
+  void addPointer(Pointer *P);
+  void removePointer(Pointer *P);
+  void movePointer(Pointer *From, Pointer *To);
+
+  /// Start of the chain of pointers.
+  Pointer *Pointers = nullptr;
+  /// Unique identifier of the declaration.
+  llvm::Optional<unsigned> DeclID;
+  /// Flag indicating if the block has static storage duration.
+  bool IsStatic = false;
+  /// Flag indicating if the block is an extern.
+  bool IsExtern = false;
+  /// Flag indicating if the pointer is dead.
+  bool IsDead = false;
+  /// Pointer to the stack slot descriptor.
+  Descriptor *Desc;
+};
+
+/// Descriptor for a dead block.
+///
+/// Dead blocks are chained in a double-linked list to deallocate them
+/// whenever pointers become dead.
+class DeadBlock {
+public:
+  /// Copies the block.
+  DeadBlock(DeadBlock *&Root, Block *Blk);
+
+  /// Returns a pointer to the stored data.
+  char *data() { return B.data(); }
+
+private:
+  friend class Block;
+  friend class InterpState;
+
+  void free();
+
+  /// Root pointer of the list.
+  DeadBlock *&Root;
+  /// Previous block in the list.
+  DeadBlock *Prev;
+  /// Next block in the list.
+  DeadBlock *Next;
+
+  /// Actual block storing data and tracking pointers.
+  Block B;
+};
+
+} // namespace interp
+} // namespace clang
+
+#endif

lib/AST/Interp/Boolean.h

@@ -0,0 +1,148 @@
+//===--- Boolean.h - Wrapper for boolean types for the VM -------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_AST_INTERP_BOOLEAN_H
+#define LLVM_CLANG_AST_INTERP_BOOLEAN_H
+
+#include <cstddef>
+#include <cstdint>
+#include "Integral.h"
+#include "clang/AST/APValue.h"
+#include "clang/AST/ComparisonCategories.h"
+#include "llvm/ADT/APSInt.h"
+#include "llvm/Support/MathExtras.h"
+#include "llvm/Support/raw_ostream.h"
+
+namespace clang {
+namespace interp {
+
+/// Wrapper around boolean types.
+class Boolean {
+ private:
+  /// Underlying boolean.
+  bool V;
+
+  /// Construct a wrapper from a boolean.
+  explicit Boolean(bool V) : V(V) {}
+
+ public:
+  /// Zero-initializes a boolean.
+  Boolean() : V(false) {}
+
+  bool operator<(Boolean RHS) const { return V < RHS.V; }
+  bool operator>(Boolean RHS) const { return V > RHS.V; }
+  bool operator<=(Boolean RHS) const { return V <= RHS.V; }
+  bool operator>=(Boolean RHS) const { return V >= RHS.V; }
+  bool operator==(Boolean RHS) const { return V == RHS.V; }
+  bool operator!=(Boolean RHS) const { return V != RHS.V; }
+
+  bool operator>(unsigned RHS) const { return static_cast<unsigned>(V) > RHS; }
+
+  Boolean operator-() const { return Boolean(V); }
+  Boolean operator~() const { return Boolean(true); }
+
+  explicit operator unsigned() const { return V; }
+  explicit operator int64_t() const { return V; }
+  explicit operator uint64_t() const { return V; }
+
+  APSInt toAPSInt() const {
+    return APSInt(APInt(1, static_cast<uint64_t>(V), false), true);
+  }
+  APSInt toAPSInt(unsigned NumBits) const {
+    return APSInt(toAPSInt().zextOrTrunc(NumBits), true);
+  }
+  APValue toAPValue() const { return APValue(toAPSInt()); }
+
+  Boolean toUnsigned() const { return *this; }
+
+  constexpr static unsigned bitWidth() { return true; }
+  bool isZero() const { return !V; }
+  bool isMin() const { return isZero(); }
+
+  constexpr static bool isMinusOne() { return false; }
+
+  constexpr static bool isSigned() { return false; }
+
+  constexpr static bool isNegative() { return false; }
+  constexpr static bool isPositive() { return !isNegative(); }
+
+  ComparisonCategoryResult compare(const Boolean &RHS) const {
+    return Compare(V, RHS.V);
+  }
+
+  unsigned countLeadingZeros() const { return V ? 0 : 1; }
+
+  Boolean truncate(unsigned TruncBits) const { return *this; }
+
+  void print(llvm::raw_ostream &OS) const { OS << (V ? "true" : "false"); }
+
+  static Boolean min(unsigned NumBits) { return Boolean(false); }
+  static Boolean max(unsigned NumBits) { return Boolean(true); }
+
+  template <typename T>
+  static typename std::enable_if<std::is_integral<T>::value, Boolean>::type
+  from(T Value) {
+    return Boolean(Value != 0);
+  }
+
+  template <unsigned SrcBits, bool SrcSign>
+  static typename std::enable_if<SrcBits != 0, Boolean>::type from(
+      Integral<SrcBits, SrcSign> Value) {
+    return Boolean(!Value.isZero());
+  }
+
+  template <bool SrcSign>
+  static Boolean from(Integral<0, SrcSign> Value) {
+    return Boolean(!Value.isZero());
+  }
+
+  static Boolean zero() { return from(false); }
+
+  template <typename T>
+  static Boolean from(T Value, unsigned NumBits) {
+    return Boolean(Value);
+  }
+
+  static bool inRange(int64_t Value, unsigned NumBits) {
+    return Value == 0 || Value == 1;
+  }
+
+  static bool increment(Boolean A, Boolean *R) {
+    *R = Boolean(true);
+    return false;
+  }
+
+  static bool decrement(Boolean A, Boolean *R) {
+    llvm_unreachable("Cannot decrement booleans");
+  }
+
+  static bool add(Boolean A, Boolean B, unsigned OpBits, Boolean *R) {
+    *R = Boolean(A.V || B.V);
+    return false;
+  }
+
+  static bool sub(Boolean A, Boolean B, unsigned OpBits, Boolean *R) {
+    *R = Boolean(A.V ^ B.V);
+    return false;
+  }
+
+  static bool mul(Boolean A, Boolean B, unsigned OpBits, Boolean *R) {
+    *R = Boolean(A.V && B.V);
+    return false;
+  }
+};
+
+inline llvm::raw_ostream &operator<<(llvm::raw_ostream &OS, const Boolean &B) {
+  B.print(OS);
+  return OS;
+}
+
+}  // namespace interp
+}  // namespace clang
+
+#endif

lib/AST/Interp/ByteCodeEmitter.cpp

@@ -0,0 +1,175 @@
+//===--- ByteCodeEmitter.cpp - Instruction emitter for the VM ---*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "ByteCodeEmitter.h"
+#include "Context.h"
+#include "Opcode.h"
+#include "Program.h"
+#include "clang/AST/DeclCXX.h"
+
+using namespace clang;
+using namespace clang::interp;
+
+using APSInt = llvm::APSInt;
+using Error = llvm::Error;
+
+Expected<Function *> ByteCodeEmitter::compileFunc(const FunctionDecl *F) {
+  // Do not try to compile undefined functions.
+  if (!F->isDefined(F) || (!F->hasBody() && F->willHaveBody()))
+    return nullptr;
+
+  // Set up argument indices.
+  unsigned ParamOffset = 0;
+  SmallVector<PrimType, 8> ParamTypes;
+  llvm::DenseMap<unsigned, Function::ParamDescriptor> ParamDescriptors;
+
+  // If the return is not a primitive, a pointer to the storage where the value
+  // is initialized in is passed as the first argument.
+  QualType Ty = F->getReturnType();
+  if (!Ty->isVoidType() && !Ctx.classify(Ty)) {
+    ParamTypes.push_back(PT_Ptr);
+    ParamOffset += align(primSize(PT_Ptr));
+  }
+
+  // Assign descriptors to all parameters.
+  // Composite objects are lowered to pointers.
+  for (const ParmVarDecl *PD : F->parameters()) {
+    PrimType Ty;
+    if (llvm::Optional<PrimType> T = Ctx.classify(PD->getType())) {
+      Ty = *T;
+    } else {
+      Ty = PT_Ptr;
+    }
+
+    Descriptor *Desc = P.createDescriptor(PD, Ty);
+    ParamDescriptors.insert({ParamOffset, {Ty, Desc}});
+    Params.insert({PD, ParamOffset});
+    ParamOffset += align(primSize(Ty));
+    ParamTypes.push_back(Ty);
+  }
+
+  // Create a handle over the emitted code.
+  Function *Func = P.createFunction(F, ParamOffset, std::move(ParamTypes),
+                                    std::move(ParamDescriptors));
+  // Compile the function body.
+  if (!F->isConstexpr() || !visitFunc(F)) {
+    // Return a dummy function if compilation failed.
+    if (BailLocation)
+      return llvm::make_error<ByteCodeGenError>(*BailLocation);
+    else
+      return Func;
+  } else {
+    // Create scopes from descriptors.
+    llvm::SmallVector<Scope, 2> Scopes;
+    for (auto &DS : Descriptors) {
+      Scopes.emplace_back(std::move(DS));
+    }
+
+    // Set the function's code.
+    Func->setCode(NextLocalOffset, std::move(Code), std::move(SrcMap),
+                  std::move(Scopes));
+    return Func;
+  }
+}
+
+Scope::Local ByteCodeEmitter::createLocal(Descriptor *D) {
+  NextLocalOffset += sizeof(Block);
+  unsigned Location = NextLocalOffset;
+  NextLocalOffset += align(D->getAllocSize());
+  return {Location, D};
+}
+
+void ByteCodeEmitter::emitLabel(LabelTy Label) {
+  const size_t Target = Code.size();
+  LabelOffsets.insert({Label, Target});
+  auto It = LabelRelocs.find(Label);
+  if (It != LabelRelocs.end()) {
+    for (unsigned Reloc : It->second) {
+      using namespace llvm::support;
+
+      /// Rewrite the operand of all jumps to this label.
+      void *Location = Code.data() + Reloc - sizeof(int32_t);
+      const int32_t Offset = Target - static_cast<int64_t>(Reloc);
+      endian::write<int32_t, endianness::native, 1>(Location, Offset);
+    }
+    LabelRelocs.erase(It);
+  }
+}
+
+int32_t ByteCodeEmitter::getOffset(LabelTy Label) {
+  // Compute the PC offset which the jump is relative to.
+  const int64_t Position = Code.size() + sizeof(Opcode) + sizeof(int32_t);
+
+  // If target is known, compute jump offset.
+  auto It = LabelOffsets.find(Label);
+  if (It != LabelOffsets.end()) {
+    return It->second - Position;
+  }
+
+  // Otherwise, record relocation and return dummy offset.
+  LabelRelocs[Label].push_back(Position);
+  return 0ull;
+}
+
+bool ByteCodeEmitter::bail(const SourceLocation &Loc) {
+  if (!BailLocation)
+    BailLocation = Loc;
+  return false;
+}
+
+template <typename... Tys>
+bool ByteCodeEmitter::emitOp(Opcode Op, const Tys &... Args, const SourceInfo &SI) {
+  bool Success = true;
+
+  /// Helper to write bytecode and bail out if 32-bit offsets become invalid.
+  auto emit = [this, &Success](const char *Data, size_t Size) {
+    if (Code.size() + Size > std::numeric_limits<unsigned>::max()) {
+      Success = false;
+      return;
+    }
+    Code.insert(Code.end(), Data, Data + Size);
+  };
+
+  /// The opcode is followed by arguments. The source info is
+  /// attached to the address after the opcode.
+  emit(reinterpret_cast<const char *>(&Op), sizeof(Opcode));
+  if (SI)
+    SrcMap.emplace_back(Code.size(), SI);
+
+  /// The initializer list forces the expression to be evaluated
+  /// for each argument in the variadic template, in order.
+  (void)std::initializer_list<int>{
+      (emit(reinterpret_cast<const char *>(&Args), sizeof(Args)), 0)...};
+
+  return Success;
+}
+
+bool ByteCodeEmitter::jumpTrue(const LabelTy &Label) {
+  return emitJt(getOffset(Label), SourceInfo{});
+}
+
+bool ByteCodeEmitter::jumpFalse(const LabelTy &Label) {
+  return emitJf(getOffset(Label), SourceInfo{});
+}
+
+bool ByteCodeEmitter::jump(const LabelTy &Label) {
+  return emitJmp(getOffset(Label), SourceInfo{});
+}
+
+bool ByteCodeEmitter::fallthrough(const LabelTy &Label) {
+  emitLabel(Label);
+  return true;
+}
+
+//===----------------------------------------------------------------------===//
+// Opcode emitters
+//===----------------------------------------------------------------------===//
+
+#define GET_LINK_IMPL
+#include "Opcodes.inc"
+#undef GET_LINK_IMPL

lib/AST/Interp/ByteCodeEmitter.h

@@ -0,0 +1,112 @@
+//===--- ByteCodeEmitter.h - Instruction emitter for the VM ---------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Defines the instruction emitters.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_AST_INTERP_LINKEMITTER_H
+#define LLVM_CLANG_AST_INTERP_LINKEMITTER_H
+
+#include "ByteCodeGenError.h"
+#include "Context.h"
+#include "InterpStack.h"
+#include "InterpState.h"
+#include "PrimType.h"
+#include "Program.h"
+#include "Source.h"
+#include "llvm/Support/Error.h"
+
+namespace clang {
+namespace interp {
+class Context;
+class SourceInfo;
+enum Opcode : uint32_t;
+
+/// An emitter which links the program to bytecode for later use.
+class ByteCodeEmitter {
+protected:
+  using LabelTy = uint32_t;
+  using AddrTy = uintptr_t;
+  using Local = Scope::Local;
+
+public:
+  /// Compiles the function into the module.
+  llvm::Expected<Function *> compileFunc(const FunctionDecl *F);
+
+protected:
+  ByteCodeEmitter(Context &Ctx, Program &P) : Ctx(Ctx), P(P) {}
+
+  virtual ~ByteCodeEmitter() {}
+
+  /// Define a label.
+  void emitLabel(LabelTy Label);
+  /// Create a label.
+  LabelTy getLabel() { return ++NextLabel; }
+
+  /// Methods implemented by the compiler.
+  virtual bool visitFunc(const FunctionDecl *E) = 0;
+  virtual bool visitExpr(const Expr *E) = 0;
+  virtual bool visitDecl(const VarDecl *E) = 0;
+
+  /// Bails out if a given node cannot be compiled.
+  bool bail(const Stmt *S) { return bail(S->getBeginLoc()); }
+  bool bail(const Decl *D) { return bail(D->getBeginLoc()); }
+  bool bail(const SourceLocation &Loc);
+
+  /// Emits jumps.
+  bool jumpTrue(const LabelTy &Label);
+  bool jumpFalse(const LabelTy &Label);
+  bool jump(const LabelTy &Label);
+  bool fallthrough(const LabelTy &Label);
+
+  /// Callback for local registration.
+  Local createLocal(Descriptor *D);
+
+  /// Parameter indices.
+  llvm::DenseMap<const ParmVarDecl *, unsigned> Params;
+  /// Local descriptors.
+  llvm::SmallVector<SmallVector<Local, 8>, 2> Descriptors;
+
+private:
+  /// Current compilation context.
+  Context &Ctx;
+  /// Program to link to.
+  Program &P;
+  /// Index of the next available label.
+  LabelTy NextLabel = 0;
+  /// Offset of the next local variable.
+  unsigned NextLocalOffset = 0;
+  /// Location of a failure.
+  llvm::Optional<SourceLocation> BailLocation;
+  /// Label information for linker.
+  llvm::DenseMap<LabelTy, unsigned> LabelOffsets;
+  /// Location of label relocations.
+  llvm::DenseMap<LabelTy, llvm::SmallVector<unsigned, 5>> LabelRelocs;
+  /// Program code.
+  std::vector<char> Code;
+  /// Opcode to expression mapping.
+  SourceMap SrcMap;
+
+  /// Returns the offset for a jump or records a relocation.
+  int32_t getOffset(LabelTy Label);
+
+  /// Emits an opcode.
+  template <typename... Tys>
+  bool emitOp(Opcode Op, const Tys &... Args, const SourceInfo &L);
+
+protected:
+#define GET_LINK_PROTO
+#include "Opcodes.inc"
+#undef GET_LINK_PROTO
+};
+
+} // namespace interp
+} // namespace clang
+
+#endif

lib/AST/Interp/ByteCodeExprGen.cpp

@@ -0,0 +1,580 @@
+//===--- ByteCodeExprGen.cpp - Code generator for expressions ---*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "ByteCodeExprGen.h"
+#include "ByteCodeEmitter.h"
+#include "ByteCodeGenError.h"
+#include "Context.h"
+#include "Function.h"
+#include "PrimType.h"
+#include "Program.h"
+#include "State.h"
+
+using namespace clang;
+using namespace clang::interp;
+
+using APSInt = llvm::APSInt;
+template <typename T> using Expected = llvm::Expected<T>;
+template <typename T> using Optional = llvm::Optional<T>;
+
+namespace clang {
+namespace interp {
+
+/// Scope used to handle temporaries in toplevel variable declarations.
+template <class Emitter> class DeclScope final : public LocalScope<Emitter> {
+public:
+  DeclScope(ByteCodeExprGen<Emitter> *Ctx, const VarDecl *VD)
+      : LocalScope<Emitter>(Ctx), Scope(Ctx->P, VD) {}
+
+  void addExtended(const Scope::Local &Local) override {
+    return this->addLocal(Local);
+  }
+
+private:
+  Program::DeclScope Scope;
+};
+
+/// Scope used to handle initialization methods.
+template <class Emitter> class OptionScope {
+public:
+  using InitFnRef = typename ByteCodeExprGen<Emitter>::InitFnRef;
+  using ChainedInitFnRef = std::function<bool(InitFnRef)>;
+
+  /// Root constructor, compiling or discarding primitives.
+  OptionScope(ByteCodeExprGen<Emitter> *Ctx, bool NewDiscardResult)
+      : Ctx(Ctx), OldDiscardResult(Ctx->DiscardResult),
+        OldInitFn(std::move(Ctx->InitFn)) {
+    Ctx->DiscardResult = NewDiscardResult;
+    Ctx->InitFn = llvm::Optional<InitFnRef>{};
+  }
+
+  /// Root constructor, setting up compilation state.
+  OptionScope(ByteCodeExprGen<Emitter> *Ctx, InitFnRef NewInitFn)
+      : Ctx(Ctx), OldDiscardResult(Ctx->DiscardResult),
+        OldInitFn(std::move(Ctx->InitFn)) {
+    Ctx->DiscardResult = true;
+    Ctx->InitFn = NewInitFn;
+  }
+
+  /// Extends the chain of initialisation pointers.
+  OptionScope(ByteCodeExprGen<Emitter> *Ctx, ChainedInitFnRef NewInitFn)
+      : Ctx(Ctx), OldDiscardResult(Ctx->DiscardResult),
+        OldInitFn(std::move(Ctx->InitFn)) {
+    assert(OldInitFn && "missing initializer");
+    Ctx->InitFn = [this, NewInitFn] { return NewInitFn(*OldInitFn); };
+  }
+
+  ~OptionScope() {
+    Ctx->DiscardResult = OldDiscardResult;
+    Ctx->InitFn = std::move(OldInitFn);
+  }
+
+private:
+  /// Parent context.
+  ByteCodeExprGen<Emitter> *Ctx;
+  /// Old discard flag to restore.
+  bool OldDiscardResult;
+  /// Old pointer emitter to restore.
+  llvm::Optional<InitFnRef> OldInitFn;
+};
+
+} // namespace interp
+} // namespace clang
+
+template <class Emitter>
+bool ByteCodeExprGen<Emitter>::VisitCastExpr(const CastExpr *CE) {
+  auto *SubExpr = CE->getSubExpr();
+  switch (CE->getCastKind()) {
+
+  case CK_LValueToRValue: {
+    return dereference(
+        CE->getSubExpr(), DerefKind::Read,
+        [](PrimType) {
+          // Value loaded - nothing to do here.
+          return true;
+        },
+        [this, CE](PrimType T) {
+          // Pointer on stack - dereference it.
+          if (!this->emitLoadPop(T, CE))
+            return false;
+          return DiscardResult ? this->emitPop(T, CE) : true;
+        });
+  }
+
+  case CK_ArrayToPointerDecay:
+  case CK_AtomicToNonAtomic:
+  case CK_ConstructorConversion:
+  case CK_FunctionToPointerDecay:
+  case CK_NonAtomicToAtomic:
+  case CK_NoOp:
+  case CK_UserDefinedConversion:
+    return this->Visit(SubExpr);
+
+  case CK_ToVoid:
+    return discard(SubExpr);
+
+  default: {
+    // TODO: implement other casts.
+    return this->bail(CE);
+  }
+  }
+}
+
+template <class Emitter>
+bool ByteCodeExprGen<Emitter>::VisitIntegerLiteral(const IntegerLiteral *LE) {
+  if (DiscardResult)
+    return true;
+
+  auto Val = LE->getValue();
+  QualType LitTy = LE->getType();
+  if (Optional<PrimType> T = classify(LitTy))
+    return emitConst(*T, getIntWidth(LitTy), LE->getValue(), LE);
+  return this->bail(LE);
+}
+
+template <class Emitter>
+bool ByteCodeExprGen<Emitter>::VisitParenExpr(const ParenExpr *PE) {
+  return this->Visit(PE->getSubExpr());
+}
+
+template <class Emitter>
+bool ByteCodeExprGen<Emitter>::VisitBinaryOperator(const BinaryOperator *BO) {
+  const Expr *LHS = BO->getLHS();
+  const Expr *RHS = BO->getRHS();
+
+  // Deal with operations which have composite or void types.
+  switch (BO->getOpcode()) {
+  case BO_Comma:
+    if (!discard(LHS))
+      return false;
+    if (!this->Visit(RHS))
+      return false;
+    return true;
+  default:
+    break;
+  }
+
+  // Typecheck the args.
+  Optional<PrimType> LT = classify(LHS->getType());
+  Optional<PrimType> RT = classify(RHS->getType());
+  if (!LT || !RT) {
+    return this->bail(BO);
+  }
+
+  if (Optional<PrimType> T = classify(BO->getType())) {
+    if (!visit(LHS))
+      return false;
+    if (!visit(RHS))
+      return false;
+
+    auto Discard = [this, T, BO](bool Result) {
+      if (!Result)
+        return false;
+      return DiscardResult ? this->emitPop(*T, BO) : true;
+    };
+
+    switch (BO->getOpcode()) {
+    case BO_EQ:
+      return Discard(this->emitEQ(*LT, BO));
+    case BO_NE:
+      return Discard(this->emitNE(*LT, BO));
+    case BO_LT:
+      return Discard(this->emitLT(*LT, BO));
+    case BO_LE:
+      return Discard(this->emitLE(*LT, BO));
+    case BO_GT:
+      return Discard(this->emitGT(*LT, BO));
+    case BO_GE:
+      return Discard(this->emitGE(*LT, BO));
+    case BO_Sub:
+      return Discard(this->emitSub(*T, BO));
+    case BO_Add:
+      return Discard(this->emitAdd(*T, BO));
+    case BO_Mul:
+      return Discard(this->emitMul(*T, BO));
+    default:
+      return this->bail(BO);
+    }
+  }
+
+  return this->bail(BO);
+}
+
+template <class Emitter>
+bool ByteCodeExprGen<Emitter>::discard(const Expr *E) {
+  OptionScope<Emitter> Scope(this, /*discardResult=*/true);
+  return this->Visit(E);
+}
+
+template <class Emitter>
+bool ByteCodeExprGen<Emitter>::visit(const Expr *E) {
+  OptionScope<Emitter> Scope(this, /*discardResult=*/false);
+  return this->Visit(E);
+}
+
+template <class Emitter>
+bool ByteCodeExprGen<Emitter>::visitBool(const Expr *E) {
+  if (Optional<PrimType> T = classify(E->getType())) {
+    return visit(E);
+  } else {
+    return this->bail(E);
+  }
+}
+
+template <class Emitter>
+bool ByteCodeExprGen<Emitter>::visitZeroInitializer(PrimType T, const Expr *E) {
+  switch (T) {
+  case PT_Bool:
+    return this->emitZeroBool(E);
+  case PT_Sint8:
+    return this->emitZeroSint8(E);
+  case PT_Uint8:
+    return this->emitZeroUint8(E);
+  case PT_Sint16:
+    return this->emitZeroSint16(E);
+  case PT_Uint16:
+    return this->emitZeroUint16(E);
+  case PT_Sint32:
+    return this->emitZeroSint32(E);
+  case PT_Uint32:
+    return this->emitZeroUint32(E);
+  case PT_Sint64:
+    return this->emitZeroSint64(E);
+  case PT_Uint64:
+    return this->emitZeroUint64(E);
+  case PT_Ptr:
+    return this->emitNullPtr(E);
+  }
+  llvm_unreachable("unknown primitive type");
+}
+
+template <class Emitter>
+bool ByteCodeExprGen<Emitter>::dereference(
+    const Expr *LV, DerefKind AK, llvm::function_ref<bool(PrimType)> Direct,
+    llvm::function_ref<bool(PrimType)> Indirect) {
+  if (Optional<PrimType> T = classify(LV->getType())) {
+    if (!LV->refersToBitField()) {
+      // Only primitive, non bit-field types can be dereferenced directly.
+      if (auto *DE = dyn_cast<DeclRefExpr>(LV)) {
+        if (!DE->getDecl()->getType()->isReferenceType()) {
+          if (auto *PD = dyn_cast<ParmVarDecl>(DE->getDecl()))
+            return dereferenceParam(LV, *T, PD, AK, Direct, Indirect);
+          if (auto *VD = dyn_cast<VarDecl>(DE->getDecl()))
+            return dereferenceVar(LV, *T, VD, AK, Direct, Indirect);
+        }
+      }
+    }
+
+    if (!visit(LV))
+      return false;
+    return Indirect(*T);
+  }
+
+  return false;
+}
+
+template <class Emitter>
+bool ByteCodeExprGen<Emitter>::dereferenceParam(
+    const Expr *LV, PrimType T, const ParmVarDecl *PD, DerefKind AK,
+    llvm::function_ref<bool(PrimType)> Direct,
+    llvm::function_ref<bool(PrimType)> Indirect) {
+  auto It = this->Params.find(PD);
+  if (It != this->Params.end()) {
+    unsigned Idx = It->second;
+    switch (AK) {
+    case DerefKind::Read:
+      return DiscardResult ? true : this->emitGetParam(T, Idx, LV);
+
+    case DerefKind::Write:
+      if (!Direct(T))
+        return false;
+      if (!this->emitSetParam(T, Idx, LV))
+        return false;
+      return DiscardResult ? true : this->emitGetPtrParam(Idx, LV);
+
+    case DerefKind::ReadWrite:
+      if (!this->emitGetParam(T, Idx, LV))
+        return false;
+      if (!Direct(T))
+        return false;
+      if (!this->emitSetParam(T, Idx, LV))
+        return false;
+      return DiscardResult ? true : this->emitGetPtrParam(Idx, LV);
+    }
+    return true;
+  }
+
+  // If the param is a pointer, we can dereference a dummy value.
+  if (!DiscardResult && T == PT_Ptr && AK == DerefKind::Read) {
+    if (auto Idx = P.getOrCreateDummy(PD))
+      return this->emitGetPtrGlobal(*Idx, PD);
+    return false;
+  }
+
+  // Value cannot be produced - try to emit pointer and do stuff with it.
+  return visit(LV) && Indirect(T);
+}
+
+template <class Emitter>
+bool ByteCodeExprGen<Emitter>::dereferenceVar(
+    const Expr *LV, PrimType T, const VarDecl *VD, DerefKind AK,
+    llvm::function_ref<bool(PrimType)> Direct,
+    llvm::function_ref<bool(PrimType)> Indirect) {
+  auto It = Locals.find(VD);
+  if (It != Locals.end()) {
+    const auto &L = It->second;
+    switch (AK) {
+    case DerefKind::Read:
+      if (!this->emitGetLocal(T, L.Offset, LV))
+        return false;
+      return DiscardResult ? this->emitPop(T, LV) : true;
+
+    case DerefKind::Write:
+      if (!Direct(T))
+        return false;
+      if (!this->emitSetLocal(T, L.Offset, LV))
+        return false;
+      return DiscardResult ? true : this->emitGetPtrLocal(L.Offset, LV);
+
+    case DerefKind::ReadWrite:
+      if (!this->emitGetLocal(T, L.Offset, LV))
+        return false;
+      if (!Direct(T))
+        return false;
+      if (!this->emitSetLocal(T, L.Offset, LV))
+        return false;
+      return DiscardResult ? true : this->emitGetPtrLocal(L.Offset, LV);
+    }
+  } else if (auto Idx = getGlobalIdx(VD)) {
+    switch (AK) {
+    case DerefKind::Read:
+      if (!this->emitGetGlobal(T, *Idx, LV))
+        return false;
+      return DiscardResult ? this->emitPop(T, LV) : true;
+
+    case DerefKind::Write:
+      if (!Direct(T))
+        return false;
+      if (!this->emitSetGlobal(T, *Idx, LV))
+        return false;
+      return DiscardResult ? true : this->emitGetPtrGlobal(*Idx, LV);
+
+    case DerefKind::ReadWrite:
+      if (!this->emitGetGlobal(T, *Idx, LV))
+        return false;
+      if (!Direct(T))
+        return false;
+      if (!this->emitSetGlobal(T, *Idx, LV))
+        return false;
+      return DiscardResult ? true : this->emitGetPtrGlobal(*Idx, LV);
+    }
+  }
+
+  // If the declaration is a constant value, emit it here even
+  // though the declaration was not evaluated in the current scope.
+  // The access mode can only be read in this case.
+  if (!DiscardResult && AK == DerefKind::Read) {
+    if (VD->hasLocalStorage() && VD->hasInit() && !VD->isConstexpr()) {
+      QualType VT = VD->getType();
+      if (VT.isConstQualified() && VT->isFundamentalType())
+        return this->Visit(VD->getInit());
+    }
+  }
+
+  // Value cannot be produced - try to emit pointer.
+  return visit(LV) && Indirect(T);
+}
+
+template <class Emitter>
+bool ByteCodeExprGen<Emitter>::emitConst(PrimType T, unsigned NumBits,
+                                         const APInt &Value, const Expr *E) {
+  switch (T) {
+  case PT_Sint8:
+    return this->emitConstSint8(Value.getSExtValue(), E);
+  case PT_Uint8:
+    return this->emitConstUint8(Value.getZExtValue(), E);
+  case PT_Sint16:
+    return this->emitConstSint16(Value.getSExtValue(), E);
+  case PT_Uint16:
+    return this->emitConstUint16(Value.getZExtValue(), E);
+  case PT_Sint32:
+    return this->emitConstSint32(Value.getSExtValue(), E);
+  case PT_Uint32:
+    return this->emitConstUint32(Value.getZExtValue(), E);
+  case PT_Sint64:
+    return this->emitConstSint64(Value.getSExtValue(), E);
+  case PT_Uint64:
+    return this->emitConstUint64(Value.getZExtValue(), E);
+  case PT_Bool:
+    return this->emitConstBool(Value.getBoolValue(), E);
+  case PT_Ptr:
+    llvm_unreachable("Invalid integral type");
+    break;
+  }
+  llvm_unreachable("unknown primitive type");
+}
+
+template <class Emitter>
+unsigned ByteCodeExprGen<Emitter>::allocateLocalPrimitive(DeclTy &&Src,
+                                                          PrimType Ty,
+                                                          bool IsConst,
+                                                          bool IsExtended) {
+  Descriptor *D = P.createDescriptor(Src, Ty, IsConst, Src.is<const Expr *>());
+  Scope::Local Local = this->createLocal(D);
+  if (auto *VD = dyn_cast_or_null<ValueDecl>(Src.dyn_cast<const Decl *>()))
+    Locals.insert({VD, Local});
+  VarScope->add(Local, IsExtended);
+  return Local.Offset;
+}
+
+template <class Emitter>
+llvm::Optional<unsigned>
+ByteCodeExprGen<Emitter>::allocateLocal(DeclTy &&Src, bool IsExtended) {
+  QualType Ty;
+
+  const ValueDecl *Key = nullptr;
+  bool IsTemporary = false;
+  if (auto *VD = dyn_cast_or_null<ValueDecl>(Src.dyn_cast<const Decl *>())) {
+    Key = VD;
+    Ty = VD->getType();
+  }
+  if (auto *E = Src.dyn_cast<const Expr *>()) {
+    IsTemporary = true;
+    Ty = E->getType();
+  }
+
+  Descriptor *D = P.createDescriptor(Src, Ty.getTypePtr(),
+                                     Ty.isConstQualified(), IsTemporary);
+  if (!D)
+    return {};
+
+  Scope::Local Local = this->createLocal(D);
+  if (Key)
+    Locals.insert({Key, Local});
+  VarScope->add(Local, IsExtended);
+  return Local.Offset;
+}
+
+template <class Emitter>
+bool ByteCodeExprGen<Emitter>::visitInitializer(
+    const Expr *Init, InitFnRef InitFn) {
+  OptionScope<Emitter> Scope(this, InitFn);
+  return this->Visit(Init);
+}
+
+template <class Emitter>
+bool ByteCodeExprGen<Emitter>::getPtrVarDecl(const VarDecl *VD, const Expr *E) {
+  // Generate a pointer to the local, loading refs.
+  if (Optional<unsigned> Idx = getGlobalIdx(VD)) {
+    if (VD->getType()->isReferenceType())
+      return this->emitGetGlobalPtr(*Idx, E);
+    else
+      return this->emitGetPtrGlobal(*Idx, E);
+  }
+  return this->bail(VD);
+}
+
+template <class Emitter>
+llvm::Optional<unsigned>
+ByteCodeExprGen<Emitter>::getGlobalIdx(const VarDecl *VD) {
+  if (VD->isConstexpr()) {
+    // Constexpr decl - it must have already been defined.
+    return P.getGlobal(VD);
+  }
+  if (!VD->hasLocalStorage()) {
+    // Not constexpr, but a global var - can have pointer taken.
+    Program::DeclScope Scope(P, VD);
+    return P.getOrCreateGlobal(VD);
+  }
+  return {};
+}
+
+template <class Emitter>
+const RecordType *ByteCodeExprGen<Emitter>::getRecordTy(QualType Ty) {
+  if (auto *PT = dyn_cast<PointerType>(Ty))
+    return PT->getPointeeType()->getAs<RecordType>();
+  else
+    return Ty->getAs<RecordType>();
+}
+
+template <class Emitter>
+Record *ByteCodeExprGen<Emitter>::getRecord(QualType Ty) {
+  if (auto *RecordTy = getRecordTy(Ty)) {
+    return getRecord(RecordTy->getDecl());
+  }
+  return nullptr;
+}
+
+template <class Emitter>
+Record *ByteCodeExprGen<Emitter>::getRecord(const RecordDecl *RD) {
+  return P.getOrCreateRecord(RD);
+}
+
+template <class Emitter>
+bool ByteCodeExprGen<Emitter>::visitExpr(const Expr *Exp) {
+  ExprScope<Emitter> RootScope(this);
+  if (!visit(Exp))
+    return false;
+
+  if (Optional<PrimType> T = classify(Exp))
+    return this->emitRet(*T, Exp);
+  else
+    return this->emitRetValue(Exp);
+}
+
+template <class Emitter>
+bool ByteCodeExprGen<Emitter>::visitDecl(const VarDecl *VD) {
+  const Expr *Init = VD->getInit();
+
+  if (Optional<unsigned> I = P.createGlobal(VD)) {
+    if (Optional<PrimType> T = classify(VD->getType())) {
+      {
+        // Primitive declarations - compute the value and set it.
+        DeclScope<Emitter> LocalScope(this, VD);
+        if (!visit(Init))
+          return false;
+      }
+
+      // If the declaration is global, save the value for later use.
+      if (!this->emitDup(*T, VD))
+        return false;
+      if (!this->emitInitGlobal(*T, *I, VD))
+        return false;
+      return this->emitRet(*T, VD);
+    } else {
+      {
+        // Composite declarations - allocate storage and initialize it.
+        DeclScope<Emitter> LocalScope(this, VD);
+        if (!visitGlobalInitializer(Init, *I))
+          return false;
+      }
+
+      // Return a pointer to the global.
+      if (!this->emitGetPtrGlobal(*I, VD))
+        return false;
+      return this->emitRetValue(VD);
+    }
+  }
+
+  return this->bail(VD);
+}
+
+template <class Emitter>
+void ByteCodeExprGen<Emitter>::emitCleanup() {
+  for (VariableScope<Emitter> *C = VarScope; C; C = C->getParent())
+    C->emitDestruction();
+}
+
+namespace clang {
+namespace interp {
+
+template class ByteCodeExprGen<ByteCodeEmitter>;
+template class ByteCodeExprGen<EvalEmitter>;
+
+} // namespace interp
+} // namespace clang

lib/AST/Interp/ByteCodeExprGen.h

@@ -0,0 +1,331 @@
+//===--- ByteCodeExprGen.h - Code generator for expressions -----*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Defines the constexpr bytecode compiler.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_AST_INTERP_BYTECODEEXPRGEN_H
+#define LLVM_CLANG_AST_INTERP_BYTECODEEXPRGEN_H
+
+#include "ByteCodeEmitter.h"
+#include "EvalEmitter.h"
+#include "Pointer.h"
+#include "PrimType.h"
+#include "Record.h"
+#include "clang/AST/Decl.h"
+#include "clang/AST/Expr.h"
+#include "clang/AST/StmtVisitor.h"
+#include "llvm/ADT/Optional.h"
+
+namespace clang {
+class QualType;
+
+namespace interp {
+class Function;
+class State;
+
+template <class Emitter> class LocalScope;
+template <class Emitter> class RecordScope;
+template <class Emitter> class VariableScope;
+template <class Emitter> class DeclScope;
+template <class Emitter> class OptionScope;
+
+/// Compilation context for expressions.
+template <class Emitter>
+class ByteCodeExprGen : public ConstStmtVisitor<ByteCodeExprGen<Emitter>, bool>,
+                        public Emitter {
+protected:
+  // Emitters for opcodes of various arities.
+  using NullaryFn = bool (ByteCodeExprGen::*)(const SourceInfo &);
+  using UnaryFn = bool (ByteCodeExprGen::*)(PrimType, const SourceInfo &);
+  using BinaryFn = bool (ByteCodeExprGen::*)(PrimType, PrimType,
+                                             const SourceInfo &);
+
+  // Aliases for types defined in the emitter.
+  using LabelTy = typename Emitter::LabelTy;
+  using AddrTy = typename Emitter::AddrTy;
+
+  // Reference to a function generating the pointer of an initialized object.s
+  using InitFnRef = std::function<bool()>;
+
+  /// Current compilation context.
+  Context &Ctx;
+  /// Program to link to.
+  Program &P;
+
+public:
+  /// Initializes the compiler and the backend emitter.
+  template <typename... Tys>
+  ByteCodeExprGen(Context &Ctx, Program &P, Tys &&... Args)
+      : Emitter(Ctx, P, Args...), Ctx(Ctx), P(P) {}
+
+  // Expression visitors - result returned on stack.
+  bool VisitCastExpr(const CastExpr *E);
+  bool VisitIntegerLiteral(const IntegerLiteral *E);
+  bool VisitParenExpr(const ParenExpr *E);
+  bool VisitBinaryOperator(const BinaryOperator *E);
+
+protected:
+  bool visitExpr(const Expr *E) override;
+  bool visitDecl(const VarDecl *VD) override;
+
+protected:
+  /// Emits scope cleanup instructions.
+  void emitCleanup();
+
+  /// Returns a record type from a record or pointer type.
+  const RecordType *getRecordTy(QualType Ty);
+
+  /// Returns a record from a record or pointer type.
+  Record *getRecord(QualType Ty);
+  Record *getRecord(const RecordDecl *RD);
+
+  /// Returns the size int bits of an integer.
+  unsigned getIntWidth(QualType Ty) {
+    auto &ASTContext = Ctx.getASTContext();
+    return ASTContext.getIntWidth(Ty);
+  }
+
+  /// Returns the value of CHAR_BIT.
+  unsigned getCharBit() const {
+    auto &ASTContext = Ctx.getASTContext();
+    return ASTContext.getTargetInfo().getCharWidth();
+  }
+
+  /// Classifies a type.
+  llvm::Optional<PrimType> classify(const Expr *E) const {
+    return E->isGLValue() ? PT_Ptr : classify(E->getType());
+  }
+  llvm::Optional<PrimType> classify(QualType Ty) const {
+    return Ctx.classify(Ty);
+  }
+
+  /// Checks if a pointer needs adjustment.
+  bool needsAdjust(QualType Ty) const {
+    return true;
+  }
+
+  /// Classifies a known primitive type
+  PrimType classifyPrim(QualType Ty) const {
+    if (auto T = classify(Ty)) {
+      return *T;
+    }
+    llvm_unreachable("not a primitive type");
+  }
+
+  /// Evaluates an expression for side effects and discards the result.
+  bool discard(const Expr *E);
+  /// Evaluates an expression and places result on stack.
+  bool visit(const Expr *E);
+  /// Compiles an initializer for a local.
+  bool visitInitializer(const Expr *E, InitFnRef GenPtr);
+
+  /// Visits an expression and converts it to a boolean.
+  bool visitBool(const Expr *E);
+
+  /// Visits an initializer for a local.
+  bool visitLocalInitializer(const Expr *Init, unsigned I) {
+    return visitInitializer(Init, [this, I, Init] {
+      return this->emitGetPtrLocal(I, Init);
+    });
+  }
+
+  /// Visits an initializer for a global.
+  bool visitGlobalInitializer(const Expr *Init, unsigned I) {
+    return visitInitializer(Init, [this, I, Init] {
+      return this->emitGetPtrGlobal(I, Init);
+    });
+  }
+
+  /// Visits a delegated initializer.
+  bool visitThisInitializer(const Expr *I) {
+    return visitInitializer(I, [this, I] { return this->emitThis(I); });
+  }
+
+  /// Creates a local primitive value.
+  unsigned allocateLocalPrimitive(DeclTy &&Decl, PrimType Ty, bool IsMutable,
+                                  bool IsExtended = false);
+
+  /// Allocates a space storing a local given its type.
+  llvm::Optional<unsigned> allocateLocal(DeclTy &&Decl,
+                                         bool IsExtended = false);
+
+private:
+  friend class VariableScope<Emitter>;
+  friend class LocalScope<Emitter>;
+  friend class RecordScope<Emitter>;
+  friend class DeclScope<Emitter>;
+  friend class OptionScope<Emitter>;
+
+  /// Emits a zero initializer.
+  bool visitZeroInitializer(PrimType T, const Expr *E);
+
+  enum class DerefKind {
+    /// Value is read and pushed to stack.
+    Read,
+    /// Direct method generates a value which is written. Returns pointer.
+    Write,
+    /// Direct method receives the value, pushes mutated value. Returns pointer.
+    ReadWrite,
+  };
+
+  /// Method to directly load a value. If the value can be fetched directly,
+  /// the direct handler is called. Otherwise, a pointer is left on the stack
+  /// and the indirect handler is expected to operate on that.
+  bool dereference(const Expr *LV, DerefKind AK,
+                   llvm::function_ref<bool(PrimType)> Direct,
+                   llvm::function_ref<bool(PrimType)> Indirect);
+  bool dereferenceParam(const Expr *LV, PrimType T, const ParmVarDecl *PD,
+                        DerefKind AK,
+                        llvm::function_ref<bool(PrimType)> Direct,
+                        llvm::function_ref<bool(PrimType)> Indirect);
+  bool dereferenceVar(const Expr *LV, PrimType T, const VarDecl *PD,
+                      DerefKind AK, llvm::function_ref<bool(PrimType)> Direct,
+                      llvm::function_ref<bool(PrimType)> Indirect);
+
+  /// Emits an APInt constant.
+  bool emitConst(PrimType T, unsigned NumBits, const llvm::APInt &Value,
+                 const Expr *E);
+
+  /// Emits an integer constant.
+  template <typename T> bool emitConst(const Expr *E, T Value) {
+    QualType Ty = E->getType();
+    unsigned NumBits = getIntWidth(Ty);
+    APInt WrappedValue(NumBits, Value, std::is_signed<T>::value);
+    return emitConst(*Ctx.classify(Ty), NumBits, WrappedValue, E);
+  }
+
+  /// Returns a pointer to a variable declaration.
+  bool getPtrVarDecl(const VarDecl *VD, const Expr *E);
+
+  /// Returns the index of a global.
+  llvm::Optional<unsigned> getGlobalIdx(const VarDecl *VD);
+
+  /// Emits the initialized pointer.
+  bool emitInitFn() {
+    assert(InitFn && "missing initializer");
+    return (*InitFn)();
+  }
+
+protected:
+  /// Variable to storage mapping.
+  llvm::DenseMap<const ValueDecl *, Scope::Local> Locals;
+
+  /// OpaqueValueExpr to location mapping.
+  llvm::DenseMap<const OpaqueValueExpr *, unsigned> OpaqueExprs;
+
+  /// Current scope.
+  VariableScope<Emitter> *VarScope = nullptr;
+
+  /// Current argument index.
+  llvm::Optional<uint64_t> ArrayIndex;
+
+  /// Flag indicating if return value is to be discarded.
+  bool DiscardResult = false;
+
+  /// Expression being initialized.
+  llvm::Optional<InitFnRef> InitFn = {};
+};
+
+extern template class ByteCodeExprGen<ByteCodeEmitter>;
+extern template class ByteCodeExprGen<EvalEmitter>;
+
+/// Scope chain managing the variable lifetimes.
+template <class Emitter> class VariableScope {
+public:
+  virtual ~VariableScope() { Ctx->VarScope = this->Parent; }
+
+  void add(const Scope::Local &Local, bool IsExtended) {
+    if (IsExtended)
+      this->addExtended(Local);
+    else
+      this->addLocal(Local);
+  }
+
+  virtual void addLocal(const Scope::Local &Local) {
+    if (this->Parent)
+      this->Parent->addLocal(Local);
+  }
+
+  virtual void addExtended(const Scope::Local &Local) {
+    if (this->Parent)
+      this->Parent->addExtended(Local);
+  }
+
+  virtual void emitDestruction() {}
+
+  VariableScope *getParent() { return Parent; }
+
+protected:
+  VariableScope(ByteCodeExprGen<Emitter> *Ctx)
+      : Ctx(Ctx), Parent(Ctx->VarScope) {
+    Ctx->VarScope = this;
+  }
+
+  /// ByteCodeExprGen instance.
+  ByteCodeExprGen<Emitter> *Ctx;
+  /// Link to the parent scope.
+  VariableScope *Parent;
+};
+
+/// Scope for local variables.
+///
+/// When the scope is destroyed, instructions are emitted to tear down
+/// all variables declared in this scope.
+template <class Emitter> class LocalScope : public VariableScope<Emitter> {
+public:
+  LocalScope(ByteCodeExprGen<Emitter> *Ctx) : VariableScope<Emitter>(Ctx) {}
+
+  ~LocalScope() override { this->emitDestruction(); }
+
+  void addLocal(const Scope::Local &Local) override {
+    if (!Idx.hasValue()) {
+      Idx = this->Ctx->Descriptors.size();
+      this->Ctx->Descriptors.emplace_back();
+    }
+
+    this->Ctx->Descriptors[*Idx].emplace_back(Local);
+  }
+
+  void emitDestruction() override {
+    if (!Idx.hasValue())
+      return;
+    this->Ctx->emitDestroy(*Idx, SourceInfo{});
+  }
+
+protected:
+  /// Index of the scope in the chain.
+  Optional<unsigned> Idx;
+};
+
+/// Scope for storage declared in a compound statement.
+template <class Emitter> class BlockScope final : public LocalScope<Emitter> {
+public:
+  BlockScope(ByteCodeExprGen<Emitter> *Ctx) : LocalScope<Emitter>(Ctx) {}
+
+  void addExtended(const Scope::Local &Local) override {
+    llvm_unreachable("Cannot create temporaries in full scopes");
+  }
+};
+
+/// Expression scope which tracks potentially lifetime extended
+/// temporaries which are hoisted to the parent scope on exit.
+template <class Emitter> class ExprScope final : public LocalScope<Emitter> {
+public:
+  ExprScope(ByteCodeExprGen<Emitter> *Ctx) : LocalScope<Emitter>(Ctx) {}
+
+  void addExtended(const Scope::Local &Local) override {
+    this->Parent->addLocal(Local);
+  }
+};
+
+} // namespace interp
+} // namespace clang
+
+#endif

lib/AST/Interp/ByteCodeGenError.cpp

@@ -0,0 +1,14 @@
+//===--- ByteCodeGenError.h - Byte code generation error --------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "ByteCodeGenError.h"
+
+using namespace clang;
+using namespace clang::interp;
+
+char ByteCodeGenError::ID;

lib/AST/Interp/ByteCodeGenError.h

@@ -0,0 +1,46 @@
+//===--- ByteCodeGenError.h - Byte code generation error ----------*- C -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_AST_INTERP_BYTECODEGENERROR_H
+#define LLVM_CLANG_AST_INTERP_BYTECODEGENERROR_H
+
+#include "clang/AST/Decl.h"
+#include "clang/AST/Stmt.h"
+#include "clang/Basic/SourceLocation.h"
+#include "llvm/Support/Error.h"
+
+namespace clang {
+namespace interp {
+
+/// Error thrown by the compiler.
+struct ByteCodeGenError : public llvm::ErrorInfo<ByteCodeGenError> {
+public:
+  ByteCodeGenError(SourceLocation Loc) : Loc(Loc) {}
+  ByteCodeGenError(const Stmt *S) : ByteCodeGenError(S->getBeginLoc()) {}
+  ByteCodeGenError(const Decl *D) : ByteCodeGenError(D->getBeginLoc()) {}
+
+  void log(raw_ostream &OS) const override { OS << "unimplemented feature"; }
+
+  const SourceLocation &getLoc() const { return Loc; }
+
+  static char ID;
+
+private:
+  // Start of the item where the error occurred.
+  SourceLocation Loc;
+
+  // Users are not expected to use error_code.
+  std::error_code convertToErrorCode() const override {
+    return llvm::inconvertibleErrorCode();
+  }
+};
+
+} // namespace interp
+} // namespace clang
+
+#endif

lib/AST/Interp/ByteCodeStmtGen.cpp

@@ -0,0 +1,265 @@
+//===--- ByteCodeStmtGen.cpp - Code generator for expressions ---*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "ByteCodeStmtGen.h"
+#include "ByteCodeEmitter.h"
+#include "ByteCodeGenError.h"
+#include "Context.h"
+#include "Function.h"
+#include "PrimType.h"
+#include "Program.h"
+#include "State.h"
+
+using namespace clang;
+using namespace clang::interp;
+
+template <typename T> using Expected = llvm::Expected<T>;
+template <typename T> using Optional = llvm::Optional<T>;
+
+namespace clang {
+namespace interp {
+
+/// Scope managing label targets.
+template <class Emitter> class LabelScope {
+public:
+  virtual ~LabelScope() {  }
+
+protected:
+  LabelScope(ByteCodeStmtGen<Emitter> *Ctx) : Ctx(Ctx) {}
+  /// ByteCodeStmtGen instance.
+  ByteCodeStmtGen<Emitter> *Ctx;
+};
+
+/// Sets the context for break/continue statements.
+template <class Emitter> class LoopScope final : public LabelScope<Emitter> {
+public:
+  using LabelTy = typename ByteCodeStmtGen<Emitter>::LabelTy;
+  using OptLabelTy = typename ByteCodeStmtGen<Emitter>::OptLabelTy;
+
+  LoopScope(ByteCodeStmtGen<Emitter> *Ctx, LabelTy BreakLabel,
+            LabelTy ContinueLabel)
+      : LabelScope<Emitter>(Ctx), OldBreakLabel(Ctx->BreakLabel),
+        OldContinueLabel(Ctx->ContinueLabel) {
+    this->Ctx->BreakLabel = BreakLabel;
+    this->Ctx->ContinueLabel = ContinueLabel;
+  }
+
+  ~LoopScope() {
+    this->Ctx->BreakLabel = OldBreakLabel;
+    this->Ctx->ContinueLabel = OldContinueLabel;
+  }
+
+private:
+  OptLabelTy OldBreakLabel;
+  OptLabelTy OldContinueLabel;
+};
+
+// Sets the context for a switch scope, mapping labels.
+template <class Emitter> class SwitchScope final : public LabelScope<Emitter> {
+public:
+  using LabelTy = typename ByteCodeStmtGen<Emitter>::LabelTy;
+  using OptLabelTy = typename ByteCodeStmtGen<Emitter>::OptLabelTy;
+  using CaseMap = typename ByteCodeStmtGen<Emitter>::CaseMap;
+
+  SwitchScope(ByteCodeStmtGen<Emitter> *Ctx, CaseMap &&CaseLabels,
+              LabelTy BreakLabel, OptLabelTy DefaultLabel)
+      : LabelScope<Emitter>(Ctx), OldBreakLabel(Ctx->BreakLabel),
+        OldDefaultLabel(this->Ctx->DefaultLabel),
+        OldCaseLabels(std::move(this->Ctx->CaseLabels)) {
+    this->Ctx->BreakLabel = BreakLabel;
+    this->Ctx->DefaultLabel = DefaultLabel;
+    this->Ctx->CaseLabels = std::move(CaseLabels);
+  }
+
+  ~SwitchScope() {
+    this->Ctx->BreakLabel = OldBreakLabel;
+    this->Ctx->DefaultLabel = OldDefaultLabel;
+    this->Ctx->CaseLabels = std::move(OldCaseLabels);
+  }
+
+private:
+  OptLabelTy OldBreakLabel;
+  OptLabelTy OldDefaultLabel;
+  CaseMap OldCaseLabels;
+};
+
+} // namespace interp
+} // namespace clang
+
+template <class Emitter>
+bool ByteCodeStmtGen<Emitter>::visitFunc(const FunctionDecl *F) {
+  // Classify the return type.
+  ReturnType = this->classify(F->getReturnType());
+
+  // Set up fields and context if a constructor.
+  if (auto *MD = dyn_cast<CXXMethodDecl>(F))
+    return this->bail(MD);
+
+  if (auto *Body = F->getBody())
+    if (!visitStmt(Body))
+      return false;
+
+  // Emit a guard return to protect against a code path missing one.
+  if (F->getReturnType()->isVoidType())
+    return this->emitRetVoid(SourceInfo{});
+  else
+    return this->emitNoRet(SourceInfo{});
+}
+
+template <class Emitter>
+bool ByteCodeStmtGen<Emitter>::visitStmt(const Stmt *S) {
+  switch (S->getStmtClass()) {
+  case Stmt::CompoundStmtClass:
+    return visitCompoundStmt(cast<CompoundStmt>(S));
+  case Stmt::DeclStmtClass:
+    return visitDeclStmt(cast<DeclStmt>(S));
+  case Stmt::ReturnStmtClass:
+    return visitReturnStmt(cast<ReturnStmt>(S));
+  case Stmt::IfStmtClass:
+    return visitIfStmt(cast<IfStmt>(S));
+  case Stmt::NullStmtClass:
+    return true;
+  default: {
+    if (auto *Exp = dyn_cast<Expr>(S))
+      return this->discard(Exp);
+    return this->bail(S);
+  }
+  }
+}
+
+template <class Emitter>
+bool ByteCodeStmtGen<Emitter>::visitCompoundStmt(
+    const CompoundStmt *CompoundStmt) {
+  BlockScope<Emitter> Scope(this);
+  for (auto *InnerStmt : CompoundStmt->body())
+    if (!visitStmt(InnerStmt))
+      return false;
+  return true;
+}
+
+template <class Emitter>
+bool ByteCodeStmtGen<Emitter>::visitDeclStmt(const DeclStmt *DS) {
+  for (auto *D : DS->decls()) {
+    // Variable declarator.
+    if (auto *VD = dyn_cast<VarDecl>(D)) {
+      if (!visitVarDecl(VD))
+        return false;
+      continue;
+    }
+
+    // Decomposition declarator.
+    if (auto *DD = dyn_cast<DecompositionDecl>(D)) {
+      return this->bail(DD);
+    }
+  }
+
+  return true;
+}
+
+template <class Emitter>
+bool ByteCodeStmtGen<Emitter>::visitReturnStmt(const ReturnStmt *RS) {
+  if (const Expr *RE = RS->getRetValue()) {
+    ExprScope<Emitter> RetScope(this);
+    if (ReturnType) {
+      // Primitive types are simply returned.
+      if (!this->visit(RE))
+        return false;
+      this->emitCleanup();
+      return this->emitRet(*ReturnType, RS);
+    } else {
+      // RVO - construct the value in the return location.
+      auto ReturnLocation = [this, RE] { return this->emitGetParamPtr(0, RE); };
+      if (!this->visitInitializer(RE, ReturnLocation))
+        return false;
+      this->emitCleanup();
+      return this->emitRetVoid(RS);
+    }
+  } else {
+    this->emitCleanup();
+    if (!this->emitRetVoid(RS))
+      return false;
+    return true;
+  }
+}
+
+template <class Emitter>
+bool ByteCodeStmtGen<Emitter>::visitIfStmt(const IfStmt *IS) {
+  BlockScope<Emitter> IfScope(this);
+  if (auto *CondInit = IS->getInit())
+    if (!visitStmt(IS->getInit()))
+      return false;
+
+  if (const DeclStmt *CondDecl = IS->getConditionVariableDeclStmt())
+    if (!visitDeclStmt(CondDecl))
+      return false;
+
+  if (!this->visitBool(IS->getCond()))
+    return false;
+
+  if (const Stmt *Else = IS->getElse()) {
+    LabelTy LabelElse = this->getLabel();
+    LabelTy LabelEnd = this->getLabel();
+    if (!this->jumpFalse(LabelElse))
+      return false;
+    if (!visitStmt(IS->getThen()))
+      return false;
+    if (!this->jump(LabelEnd))
+      return false;
+    this->emitLabel(LabelElse);
+    if (!visitStmt(Else))
+      return false;
+    this->emitLabel(LabelEnd);
+  } else {
+    LabelTy LabelEnd = this->getLabel();
+    if (!this->jumpFalse(LabelEnd))
+      return false;
+    if (!visitStmt(IS->getThen()))
+      return false;
+    this->emitLabel(LabelEnd);
+  }
+
+  return true;
+}
+
+template <class Emitter>
+bool ByteCodeStmtGen<Emitter>::visitVarDecl(const VarDecl *VD) {
+  auto DT = VD->getType();
+
+  if (!VD->hasLocalStorage()) {
+    // No code generation required.
+    return true;
+  }
+
+  // Integers, pointers, primitives.
+  if (Optional<PrimType> T = this->classify(DT)) {
+    auto Off = this->allocateLocalPrimitive(VD, *T, DT.isConstQualified());
+    // Compile the initialiser in its own scope.
+    {
+      ExprScope<Emitter> Scope(this);
+      if (!this->visit(VD->getInit()))
+        return false;
+    }
+    // Set the value.
+    return this->emitSetLocal(*T, Off, VD);
+  } else {
+    // Composite types - allocate storage and initialize it.
+    if (auto Off = this->allocateLocal(VD)) {
+      return this->visitLocalInitializer(VD->getInit(), *Off);
+    } else {
+      return this->bail(VD);
+    }
+  }
+}
+
+namespace clang {
+namespace interp {
+
+template class ByteCodeStmtGen<ByteCodeEmitter>;
+
+} // namespace interp
+} // namespace clang

lib/AST/Interp/ByteCodeStmtGen.h

@@ -0,0 +1,89 @@
+//===--- ByteCodeStmtGen.h - Code generator for expressions -----*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Defines the constexpr bytecode compiler.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_AST_INTERP_BYTECODESTMTGEN_H
+#define LLVM_CLANG_AST_INTERP_BYTECODESTMTGEN_H
+
+#include "ByteCodeEmitter.h"
+#include "ByteCodeExprGen.h"
+#include "EvalEmitter.h"
+#include "Pointer.h"
+#include "PrimType.h"
+#include "Record.h"
+#include "clang/AST/Decl.h"
+#include "clang/AST/Expr.h"
+#include "clang/AST/StmtVisitor.h"
+#include "llvm/ADT/Optional.h"
+
+namespace clang {
+class QualType;
+
+namespace interp {
+class Function;
+class State;
+
+template <class Emitter> class LoopScope;
+template <class Emitter> class SwitchScope;
+template <class Emitter> class LabelScope;
+
+/// Compilation context for statements.
+template <class Emitter>
+class ByteCodeStmtGen : public ByteCodeExprGen<Emitter> {
+  using LabelTy = typename Emitter::LabelTy;
+  using AddrTy = typename Emitter::AddrTy;
+  using OptLabelTy = llvm::Optional<LabelTy>;
+  using CaseMap = llvm::DenseMap<const SwitchCase *, LabelTy>;
+
+public:
+  template<typename... Tys>
+  ByteCodeStmtGen(Tys&&... Args)
+      : ByteCodeExprGen<Emitter>(std::forward<Tys>(Args)...) {}
+
+protected:
+  bool visitFunc(const FunctionDecl *F) override;
+
+private:
+  friend class LabelScope<Emitter>;
+  friend class LoopScope<Emitter>;
+  friend class SwitchScope<Emitter>;
+
+  // Statement visitors.
+  bool visitStmt(const Stmt *S);
+  bool visitCompoundStmt(const CompoundStmt *S);
+  bool visitDeclStmt(const DeclStmt *DS);
+  bool visitReturnStmt(const ReturnStmt *RS);
+  bool visitIfStmt(const IfStmt *IS);
+
+  /// Compiles a variable declaration.
+  bool visitVarDecl(const VarDecl *VD);
+
+private:
+  /// Type of the expression returned by the function.
+  llvm::Optional<PrimType> ReturnType;
+
+  /// Switch case mapping.
+  CaseMap CaseLabels;
+
+  /// Point to break to.
+  OptLabelTy BreakLabel;
+  /// Point to continue to.
+  OptLabelTy ContinueLabel;
+  /// Default case label.
+  OptLabelTy DefaultLabel;
+};
+
+extern template class ByteCodeExprGen<EvalEmitter>;
+
+} // namespace interp
+} // namespace clang
+
+#endif

lib/AST/Interp/Context.cpp

@@ -0,0 +1,148 @@
+//===--- Context.cpp - Context for the constexpr VM -------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "Context.h"
+#include "ByteCodeEmitter.h"
+#include "ByteCodeExprGen.h"
+#include "ByteCodeStmtGen.h"
+#include "EvalEmitter.h"
+#include "Interp.h"
+#include "InterpFrame.h"
+#include "InterpStack.h"
+#include "PrimType.h"
+#include "Program.h"
+#include "clang/AST/Expr.h"
+
+using namespace clang;
+using namespace clang::interp;
+
+Context::Context(ASTContext &Ctx)
+    : Ctx(Ctx), ForceInterp(getLangOpts().ForceNewConstInterp),
+      P(new Program(*this)) {}
+
+Context::~Context() {}
+
+InterpResult Context::isPotentialConstantExpr(State &Parent,
+                                              const FunctionDecl *FD) {
+  Function *Func = P->getFunction(FD);
+  if (!Func) {
+    if (auto R = ByteCodeStmtGen<ByteCodeEmitter>(*this, *P).compileFunc(FD)) {
+      Func = *R;
+    } else if (ForceInterp) {
+      handleAllErrors(R.takeError(), [&Parent](ByteCodeGenError &Err) {
+        Parent.FFDiag(Err.getLoc(), diag::err_experimental_clang_interp_failed);
+      });
+      return InterpResult::Fail;
+    } else {
+      consumeError(R.takeError());
+      return InterpResult::Bail;
+    }
+  }
+
+  if (!Func->isConstexpr())
+    return InterpResult::Fail;
+
+  APValue Dummy;
+  return Run(Parent, Func, Dummy);
+}
+
+InterpResult Context::evaluateAsRValue(State &Parent, const Expr *E,
+                                       APValue &Result) {
+  ByteCodeExprGen<EvalEmitter> C(*this, *P, Parent, Stk, Result);
+  return Check(Parent, C.interpretExpr(E));
+}
+
+InterpResult Context::evaluateAsInitializer(State &Parent, const VarDecl *VD,
+                                            APValue &Result) {
+  ByteCodeExprGen<EvalEmitter> C(*this, *P, Parent, Stk, Result);
+  return Check(Parent, C.interpretDecl(VD));
+}
+
+const LangOptions &Context::getLangOpts() const { return Ctx.getLangOpts(); }
+
+llvm::Optional<PrimType> Context::classify(QualType T) {
+  if (T->isReferenceType() || T->isPointerType()) {
+    return PT_Ptr;
+  }
+
+  if (T->isBooleanType())
+    return PT_Bool;
+
+  if (T->isSignedIntegerOrEnumerationType()) {
+    switch (Ctx.getIntWidth(T)) {
+    case 64:
+      return PT_Sint64;
+    case 32:
+      return PT_Sint32;
+    case 16:
+      return PT_Sint16;
+    case 8:
+      return PT_Sint8;
+    default:
+      return {};
+    }
+  }
+
+  if (T->isUnsignedIntegerOrEnumerationType()) {
+    switch (Ctx.getIntWidth(T)) {
+    case 64:
+      return PT_Uint64;
+    case 32:
+      return PT_Uint32;
+    case 16:
+      return PT_Uint16;
+    case 8:
+      return PT_Uint8;
+    default:
+      return {};
+    }
+  }
+
+  if (T->isNullPtrType())
+    return PT_Ptr;
+
+  if (auto *AT = dyn_cast<AtomicType>(T))
+    return classify(AT->getValueType());
+
+  return {};
+}
+
+unsigned Context::getCharBit() const {
+  return Ctx.getTargetInfo().getCharWidth();
+}
+
+InterpResult Context::Run(State &Parent, Function *Func, APValue &Result) {
+  InterpResult Flag;
+  {
+    InterpState State(Parent, *P, Stk, *this);
+    State.Current = new InterpFrame(State, Func, nullptr, {}, {});
+    if (Interpret(State, Result)) {
+      Flag = InterpResult::Success;
+    } else {
+      Flag = InterpResult::Fail;
+    }
+  }
+
+  if (Flag != InterpResult::Success)
+    Stk.clear();
+  return Flag;
+}
+
+InterpResult Context::Check(State &Parent, llvm::Expected<bool> &&R) {
+  if (R) {
+    return *R ? InterpResult::Success : InterpResult::Fail;
+  } else if (ForceInterp) {
+    handleAllErrors(R.takeError(), [&Parent](ByteCodeGenError &Err) {
+      Parent.FFDiag(Err.getLoc(), diag::err_experimental_clang_interp_failed);
+    });
+    return InterpResult::Fail;
+  } else {
+    consumeError(R.takeError());
+    return InterpResult::Bail;
+  }
+}

lib/AST/Interp/Context.h

@@ -0,0 +1,100 @@
+//===--- Context.h - Context for the constexpr VM ---------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Defines the constexpr execution context.
+//
+// The execution context manages cached bytecode and the global context.
+// It invokes the compiler and interpreter, propagating errors.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_AST_INTERP_CONTEXT_H
+#define LLVM_CLANG_AST_INTERP_CONTEXT_H
+
+#include "Context.h"
+#include "InterpStack.h"
+#include "clang/AST/APValue.h"
+#include "llvm/ADT/PointerIntPair.h"
+
+namespace clang {
+class ASTContext;
+class LangOptions;
+class Stmt;
+class FunctionDecl;
+class VarDecl;
+
+namespace interp {
+class Function;
+class Program;
+class State;
+enum PrimType : unsigned;
+
+/// Wrapper around interpreter termination results.
+enum class InterpResult {
+  /// Interpreter successfully computed a value.
+  Success,
+  /// Interpreter encountered an error and quit.
+  Fail,
+  /// Interpreter encountered an unimplemented feature, AST fallback.
+  Bail,
+};
+
+/// Holds all information required to evaluate constexpr code in a module.
+class Context {
+public:
+  /// Initialises the constexpr VM.
+  Context(ASTContext &Ctx);
+
+  /// Cleans up the constexpr VM.
+  ~Context();
+
+  /// Checks if a function is a potential constant expression.
+  InterpResult isPotentialConstantExpr(State &Parent,
+                                       const FunctionDecl *FnDecl);
+
+  /// Evaluates a toplevel expression as an rvalue.
+  InterpResult evaluateAsRValue(State &Parent, const Expr *E, APValue &Result);
+
+  /// Evaluates a toplevel initializer.
+  InterpResult evaluateAsInitializer(State &Parent, const VarDecl *VD,
+                                     APValue &Result);
+
+  /// Returns the AST context.
+  ASTContext &getASTContext() const { return Ctx; }
+  /// Returns the language options.
+  const LangOptions &getLangOpts() const;
+  /// Returns the interpreter stack.
+  InterpStack &getStack() { return Stk; }
+  /// Returns CHAR_BIT.
+  unsigned getCharBit() const;
+
+  /// Classifies an expression.
+  llvm::Optional<PrimType> classify(QualType T);
+
+private:
+  /// Runs a function.
+  InterpResult Run(State &Parent, Function *Func, APValue &Result);
+
+  /// Checks a result fromt the interpreter.
+  InterpResult Check(State &Parent, llvm::Expected<bool> &&R);
+
+private:
+  /// Current compilation context.
+  ASTContext &Ctx;
+  /// Flag to indicate if the use of the interpreter is mandatory.
+  bool ForceInterp;
+  /// Interpreter stack, shared across invocations.
+  InterpStack Stk;
+  /// Constexpr program.
+  std::unique_ptr<Program> P;
+};
+
+} // namespace interp
+} // namespace clang
+
+#endif

lib/AST/Interp/Descriptor.cpp

@@ -0,0 +1,292 @@
+//===--- Descriptor.cpp - Types for the constexpr VM ------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "Descriptor.h"
+#include "Pointer.h"
+#include "PrimType.h"
+#include "Record.h"
+
+using namespace clang;
+using namespace clang::interp;
+
+template <typename T>
+static void ctorTy(Block *, char *Ptr, bool, bool, bool, Descriptor *) {
+  new (Ptr) T();
+}
+
+template <typename T> static void dtorTy(Block *, char *Ptr, Descriptor *) {
+  reinterpret_cast<T *>(Ptr)->~T();
+}
+
+template <typename T>
+static void moveTy(Block *, char *Src, char *Dst, Descriptor *) {
+  auto *SrcPtr = reinterpret_cast<T *>(Src);
+  auto *DstPtr = reinterpret_cast<T *>(Dst);
+  new (DstPtr) T(std::move(*SrcPtr));
+}
+
+template <typename T>
+static void ctorArrayTy(Block *, char *Ptr, bool, bool, bool, Descriptor *D) {
+  for (unsigned I = 0, NE = D->getNumElems(); I < NE; ++I) {
+    new (&reinterpret_cast<T *>(Ptr)[I]) T();
+  }
+}
+
+template <typename T>
+static void dtorArrayTy(Block *, char *Ptr, Descriptor *D) {
+  for (unsigned I = 0, NE = D->getNumElems(); I < NE; ++I) {
+    reinterpret_cast<T *>(Ptr)[I].~T();
+  }
+}
+
+template <typename T>
+static void moveArrayTy(Block *, char *Src, char *Dst, Descriptor *D) {
+  for (unsigned I = 0, NE = D->getNumElems(); I < NE; ++I) {
+    auto *SrcPtr = &reinterpret_cast<T *>(Src)[I];
+    auto *DstPtr = &reinterpret_cast<T *>(Dst)[I];
+    new (DstPtr) T(std::move(*SrcPtr));
+  }
+}
+
+static void ctorArrayDesc(Block *B, char *Ptr, bool IsConst, bool IsMutable,
+                          bool IsActive, Descriptor *D) {
+  const unsigned NumElems = D->getNumElems();
+  const unsigned ElemSize =
+      D->ElemDesc->getAllocSize() + sizeof(InlineDescriptor);
+
+  unsigned ElemOffset = 0;
+  for (unsigned I = 0; I < NumElems; ++I, ElemOffset += ElemSize) {
+    auto *ElemPtr = Ptr + ElemOffset;
+    auto *Desc = reinterpret_cast<InlineDescriptor *>(ElemPtr);
+    auto *ElemLoc = reinterpret_cast<char *>(Desc + 1);
+    auto *SD = D->ElemDesc;
+
+    Desc->Offset = ElemOffset + sizeof(InlineDescriptor);
+    Desc->Desc = SD;
+    Desc->IsInitialized = true;
+    Desc->IsBase = false;
+    Desc->IsActive = IsActive;
+    Desc->IsConst = IsConst || D->IsConst;
+    Desc->IsMutable = IsMutable || D->IsMutable;
+    if (auto Fn = D->ElemDesc->CtorFn)
+      Fn(B, ElemLoc, Desc->IsConst, Desc->IsMutable, IsActive, D->ElemDesc);
+  }
+}
+
+static void dtorArrayDesc(Block *B, char *Ptr, Descriptor *D) {
+  const unsigned NumElems = D->getNumElems();
+  const unsigned ElemSize =
+      D->ElemDesc->getAllocSize() + sizeof(InlineDescriptor);
+
+  unsigned ElemOffset = 0;
+  for (unsigned I = 0; I < NumElems; ++I, ElemOffset += ElemSize) {
+    auto *ElemPtr = Ptr + ElemOffset;
+    auto *Desc = reinterpret_cast<InlineDescriptor *>(ElemPtr);
+    auto *ElemLoc = reinterpret_cast<char *>(Desc + 1);
+    if (auto Fn = D->ElemDesc->DtorFn)
+      Fn(B, ElemLoc, D->ElemDesc);
+  }
+}
+
+static void moveArrayDesc(Block *B, char *Src, char *Dst, Descriptor *D) {
+  const unsigned NumElems = D->getNumElems();
+  const unsigned ElemSize =
+      D->ElemDesc->getAllocSize() + sizeof(InlineDescriptor);
+
+  unsigned ElemOffset = 0;
+  for (unsigned I = 0; I < NumElems; ++I, ElemOffset += ElemSize) {
+    auto *SrcPtr = Src + ElemOffset;
+    auto *DstPtr = Dst + ElemOffset;
+
+    auto *SrcDesc = reinterpret_cast<InlineDescriptor *>(SrcPtr);
+    auto *SrcElemLoc = reinterpret_cast<char *>(SrcDesc + 1);
+    auto *DstDesc = reinterpret_cast<InlineDescriptor *>(DstPtr);
+    auto *DstElemLoc = reinterpret_cast<char *>(DstDesc + 1);
+
+    *DstDesc = *SrcDesc;
+    if (auto Fn = D->ElemDesc->MoveFn)
+      Fn(B, SrcElemLoc, DstElemLoc, D->ElemDesc);
+  }
+}
+
+static void ctorRecord(Block *B, char *Ptr, bool IsConst, bool IsMutable,
+                       bool IsActive, Descriptor *D) {
+  const bool IsUnion = D->ElemRecord->isUnion();
+  auto CtorSub = [=](unsigned SubOff, Descriptor *F, bool IsBase) {
+    auto *Desc = reinterpret_cast<InlineDescriptor *>(Ptr + SubOff) - 1;
+    Desc->Offset = SubOff;
+    Desc->Desc = F;
+    Desc->IsInitialized = (B->isStatic() || F->IsArray) && !IsBase;
+    Desc->IsBase = IsBase;
+    Desc->IsActive = IsActive && !IsUnion;
+    Desc->IsConst = IsConst || F->IsConst;
+    Desc->IsMutable = IsMutable || F->IsMutable;
+    if (auto Fn = F->CtorFn)
+      Fn(B, Ptr + SubOff, Desc->IsConst, Desc->IsMutable, Desc->IsActive, F);
+  };
+  for (const auto &B : D->ElemRecord->bases())
+    CtorSub(B.Offset, B.Desc, /*isBase=*/true);
+  for (const auto &F : D->ElemRecord->fields())
+    CtorSub(F.Offset, F.Desc, /*isBase=*/false);
+  for (const auto &V : D->ElemRecord->virtual_bases())
+    CtorSub(V.Offset, V.Desc, /*isBase=*/true);
+}
+
+static void dtorRecord(Block *B, char *Ptr, Descriptor *D) {
+  auto DtorSub = [=](unsigned SubOff, Descriptor *F) {
+    if (auto Fn = F->DtorFn)
+      Fn(B, Ptr + SubOff, F);
+  };
+  for (const auto &F : D->ElemRecord->bases())
+    DtorSub(F.Offset, F.Desc);
+  for (const auto &F : D->ElemRecord->fields())
+    DtorSub(F.Offset, F.Desc);
+  for (const auto &F : D->ElemRecord->virtual_bases())
+    DtorSub(F.Offset, F.Desc);
+}
+
+static void moveRecord(Block *B, char *Src, char *Dst, Descriptor *D) {
+  for (const auto &F : D->ElemRecord->fields()) {
+    auto FieldOff = F.Offset;
+    auto FieldDesc = F.Desc;
+
+    *(reinterpret_cast<Descriptor **>(Dst + FieldOff) - 1) = FieldDesc;
+    if (auto Fn = FieldDesc->MoveFn)
+      Fn(B, Src + FieldOff, Dst + FieldOff, FieldDesc);
+  }
+}
+
+static BlockCtorFn getCtorPrim(PrimType Type) {
+  COMPOSITE_TYPE_SWITCH(Type, return ctorTy<T>, return nullptr);
+}
+
+static BlockDtorFn getDtorPrim(PrimType Type) {
+  COMPOSITE_TYPE_SWITCH(Type, return dtorTy<T>, return nullptr);
+}
+
+static BlockMoveFn getMovePrim(PrimType Type) {
+  COMPOSITE_TYPE_SWITCH(Type, return moveTy<T>, return nullptr);
+}
+
+static BlockCtorFn getCtorArrayPrim(PrimType Type) {
+  COMPOSITE_TYPE_SWITCH(Type, return ctorArrayTy<T>, return nullptr);
+}
+
+static BlockDtorFn getDtorArrayPrim(PrimType Type) {
+  COMPOSITE_TYPE_SWITCH(Type, return dtorArrayTy<T>, return nullptr);
+}
+
+static BlockMoveFn getMoveArrayPrim(PrimType Type) {
+  COMPOSITE_TYPE_SWITCH(Type, return moveArrayTy<T>, return nullptr);
+}
+
+Descriptor::Descriptor(const DeclTy &D, PrimType Type, bool IsConst,
+                       bool IsTemporary, bool IsMutable)
+    : Source(D), ElemSize(primSize(Type)), Size(ElemSize), AllocSize(Size),
+      IsConst(IsConst), IsMutable(IsMutable), IsTemporary(IsTemporary),
+      CtorFn(getCtorPrim(Type)), DtorFn(getDtorPrim(Type)),
+      MoveFn(getMovePrim(Type)) {
+  assert(Source && "Missing source");
+}
+
+Descriptor::Descriptor(const DeclTy &D, PrimType Type, size_t NumElems,
+                       bool IsConst, bool IsTemporary, bool IsMutable)
+    : Source(D), ElemSize(primSize(Type)), Size(ElemSize * NumElems),
+      AllocSize(align(Size) + sizeof(InitMap *)), IsConst(IsConst),
+      IsMutable(IsMutable), IsTemporary(IsTemporary), IsArray(true),
+      CtorFn(getCtorArrayPrim(Type)), DtorFn(getDtorArrayPrim(Type)),
+      MoveFn(getMoveArrayPrim(Type)) {
+  assert(Source && "Missing source");
+}
+
+Descriptor::Descriptor(const DeclTy &D, PrimType Type, bool IsTemporary,
+                       UnknownSize)
+    : Source(D), ElemSize(primSize(Type)), Size(UnknownSizeMark),
+      AllocSize(alignof(void *)), IsConst(true), IsMutable(false),
+      IsTemporary(IsTemporary), IsArray(true), CtorFn(getCtorArrayPrim(Type)),
+      DtorFn(getDtorArrayPrim(Type)), MoveFn(getMoveArrayPrim(Type)) {
+  assert(Source && "Missing source");
+}
+
+Descriptor::Descriptor(const DeclTy &D, Descriptor *Elem, unsigned NumElems,
+                       bool IsConst, bool IsTemporary, bool IsMutable)
+    : Source(D), ElemSize(Elem->getAllocSize() + sizeof(InlineDescriptor)),
+      Size(ElemSize * NumElems),
+      AllocSize(std::max<size_t>(alignof(void *), Size)), ElemDesc(Elem),
+      IsConst(IsConst), IsMutable(IsMutable), IsTemporary(IsTemporary),
+      IsArray(true), CtorFn(ctorArrayDesc), DtorFn(dtorArrayDesc),
+      MoveFn(moveArrayDesc) {
+  assert(Source && "Missing source");
+}
+
+Descriptor::Descriptor(const DeclTy &D, Descriptor *Elem, bool IsTemporary,
+                       UnknownSize)
+    : Source(D), ElemSize(Elem->getAllocSize() + sizeof(InlineDescriptor)),
+      Size(UnknownSizeMark), AllocSize(alignof(void *)), ElemDesc(Elem),
+      IsConst(true), IsMutable(false), IsTemporary(IsTemporary), IsArray(true),
+      CtorFn(ctorArrayDesc), DtorFn(dtorArrayDesc), MoveFn(moveArrayDesc) {
+  assert(Source && "Missing source");
+}
+
+Descriptor::Descriptor(const DeclTy &D, Record *R, bool IsConst,
+                       bool IsTemporary, bool IsMutable)
+    : Source(D), ElemSize(std::max<size_t>(alignof(void *), R->getFullSize())),
+      Size(ElemSize), AllocSize(Size), ElemRecord(R), IsConst(IsConst),
+      IsMutable(IsMutable), IsTemporary(IsTemporary), CtorFn(ctorRecord),
+      DtorFn(dtorRecord), MoveFn(moveRecord) {
+  assert(Source && "Missing source");
+}
+
+QualType Descriptor::getType() const {
+  if (auto *E = asExpr())
+    return E->getType();
+  if (auto *D = asValueDecl())
+    return D->getType();
+  llvm_unreachable("Invalid descriptor type");
+}
+
+SourceLocation Descriptor::getLocation() const {
+  if (auto *D = Source.dyn_cast<const Decl *>())
+    return D->getLocation();
+  if (auto *E = Source.dyn_cast<const Expr *>())
+    return E->getExprLoc();
+  llvm_unreachable("Invalid descriptor type");
+}
+
+InitMap::InitMap(unsigned N) : UninitFields(N) {
+  for (unsigned I = 0; I < N / PER_FIELD; ++I) {
+    data()[I] = 0;
+  }
+}
+
+InitMap::T *InitMap::data() {
+  auto *Start = reinterpret_cast<char *>(this) + align(sizeof(InitMap));
+  return reinterpret_cast<T *>(Start);
+}
+
+bool InitMap::initialize(unsigned I) {
+  unsigned Bucket = I / PER_FIELD;
+  unsigned Mask = 1ull << static_cast<uint64_t>(I % PER_FIELD);
+  if (!(data()[Bucket] & Mask)) {
+    data()[Bucket] |= Mask;
+    UninitFields -= 1;
+  }
+  return UninitFields == 0;
+}
+
+bool InitMap::isInitialized(unsigned I) {
+  unsigned Bucket = I / PER_FIELD;
+  unsigned Mask = 1ull << static_cast<uint64_t>(I % PER_FIELD);
+  return data()[Bucket] & Mask;
+}
+
+InitMap *InitMap::allocate(unsigned N) {
+  const size_t NumFields = ((N + PER_FIELD - 1) / PER_FIELD);
+  const size_t Size = align(sizeof(InitMap)) + NumFields * PER_FIELD;
+  return new (malloc(Size)) InitMap(N);
+}

lib/AST/Interp/Descriptor.h

@@ -0,0 +1,220 @@
+//===--- Descriptor.h - Types for the constexpr VM --------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Defines descriptors which characterise allocations.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_AST_INTERP_DESCRIPTOR_H
+#define LLVM_CLANG_AST_INTERP_DESCRIPTOR_H
+
+#include "clang/AST/Decl.h"
+#include "clang/AST/Expr.h"
+
+namespace clang {
+namespace interp {
+class Block;
+class Record;
+struct Descriptor;
+enum PrimType : unsigned;
+
+using DeclTy = llvm::PointerUnion<const Decl *, const Expr *>;
+
+/// Invoked whenever a block is created. The constructor method fills in the
+/// inline descriptors of all fields and array elements. It also initializes
+/// all the fields which contain non-trivial types.
+using BlockCtorFn = void (*)(Block *Storage, char *FieldPtr, bool IsConst,
+                             bool IsMutable, bool IsActive,
+                             Descriptor *FieldDesc);
+
+/// Invoked when a block is destroyed. Invokes the destructors of all
+/// non-trivial nested fields of arrays and records.
+using BlockDtorFn = void (*)(Block *Storage, char *FieldPtr,
+                             Descriptor *FieldDesc);
+
+/// Invoked when a block with pointers referencing it goes out of scope. Such
+/// blocks are persisted: the move function copies all inline descriptors and
+/// non-trivial fields, as existing pointers might need to reference those
+/// descriptors. Data is not copied since it cannot be legally read.
+using BlockMoveFn = void (*)(Block *Storage, char *SrcFieldPtr,
+                             char *DstFieldPtr, Descriptor *FieldDesc);
+
+/// Object size as used by the interpreter.
+using InterpSize = unsigned;
+
+/// Describes a memory block created by an allocation site.
+struct Descriptor {
+private:
+  /// Original declaration, used to emit the error message.
+  const DeclTy Source;
+  /// Size of an element, in host bytes.
+  const InterpSize ElemSize;
+  /// Size of the storage, in host bytes.
+  const InterpSize Size;
+  /// Size of the allocation (storage + metadata), in host bytes.
+  const InterpSize AllocSize;
+
+  /// Value to denote arrays of unknown size.
+  static constexpr unsigned UnknownSizeMark = (unsigned)-1;
+
+public:
+  /// Token to denote structures of unknown size.
+  struct UnknownSize {};
+
+  /// Pointer to the record, if block contains records.
+  Record *const ElemRecord = nullptr;
+  /// Descriptor of the array element.
+  Descriptor *const ElemDesc = nullptr;
+  /// Flag indicating if the block is mutable.
+  const bool IsConst = false;
+  /// Flag indicating if a field is mutable.
+  const bool IsMutable = false;
+  /// Flag indicating if the block is a temporary.
+  const bool IsTemporary = false;
+  /// Flag indicating if the block is an array.
+  const bool IsArray = false;
+
+  /// Storage management methods.
+  const BlockCtorFn CtorFn = nullptr;
+  const BlockDtorFn DtorFn = nullptr;
+  const BlockMoveFn MoveFn = nullptr;
+
+  /// Allocates a descriptor for a primitive.
+  Descriptor(const DeclTy &D, PrimType Type, bool IsConst, bool IsTemporary,
+             bool IsMutable);
+
+  /// Allocates a descriptor for an array of primitives.
+  Descriptor(const DeclTy &D, PrimType Type, size_t NumElems, bool IsConst,
+             bool IsTemporary, bool IsMutable);
+
+  /// Allocates a descriptor for an array of primitives of unknown size.
+  Descriptor(const DeclTy &D, PrimType Type, bool IsTemporary, UnknownSize);
+
+  /// Allocates a descriptor for an array of composites.
+  Descriptor(const DeclTy &D, Descriptor *Elem, unsigned NumElems, bool IsConst,
+             bool IsTemporary, bool IsMutable);
+
+  /// Allocates a descriptor for an array of composites of unknown size.
+  Descriptor(const DeclTy &D, Descriptor *Elem, bool IsTemporary, UnknownSize);
+
+  /// Allocates a descriptor for a record.
+  Descriptor(const DeclTy &D, Record *R, bool IsConst, bool IsTemporary,
+             bool IsMutable);
+
+  QualType getType() const;
+  SourceLocation getLocation() const;
+
+  const Decl *asDecl() const { return Source.dyn_cast<const Decl *>(); }
+  const Expr *asExpr() const { return Source.dyn_cast<const Expr *>(); }
+
+  const ValueDecl *asValueDecl() const {
+    return dyn_cast_or_null<ValueDecl>(asDecl());
+  }
+
+  const FieldDecl *asFieldDecl() const {
+    return dyn_cast_or_null<FieldDecl>(asDecl());
+  }
+
+  const RecordDecl *asRecordDecl() const {
+    return dyn_cast_or_null<RecordDecl>(asDecl());
+  }
+
+  /// Returns the size of the object without metadata.
+  unsigned getSize() const {
+    assert(!isUnknownSizeArray() && "Array of unknown size");
+    return Size;
+  }
+
+  /// Returns the allocated size, including metadata.
+  unsigned getAllocSize() const { return AllocSize; }
+  /// returns the size of an element when the structure is viewed as an array.
+  unsigned getElemSize()  const { return ElemSize; }
+
+  /// Returns the number of elements stored in the block.
+  unsigned getNumElems() const {
+    return Size == UnknownSizeMark ? 0 : (getSize() / getElemSize());
+  }
+
+  /// Checks if the descriptor is of an array of primitives.
+  bool isPrimitiveArray() const { return IsArray && !ElemDesc; }
+  /// Checks if the descriptor is of an array of zero size.
+  bool isZeroSizeArray() const { return Size == 0; }
+  /// Checks if the descriptor is of an array of unknown size.
+  bool isUnknownSizeArray() const { return Size == UnknownSizeMark; }
+
+  /// Checks if the descriptor is of a primitive.
+  bool isPrimitive() const { return !IsArray && !ElemRecord; }
+
+  /// Checks if the descriptor is of an array.
+  bool isArray() const { return IsArray; }
+};
+
+/// Inline descriptor embedded in structures and arrays.
+///
+/// Such descriptors precede all composite array elements and structure fields.
+/// If the base of a pointer is not zero, the base points to the end of this
+/// structure. The offset field is used to traverse the pointer chain up
+/// to the root structure which allocated the object.
+struct InlineDescriptor {
+  /// Offset inside the structure/array.
+  unsigned Offset;
+
+  /// Flag indicating if the storage is constant or not.
+  /// Relevant for primitive fields.
+  unsigned IsConst : 1;
+  /// For primitive fields, it indicates if the field was initialized.
+  /// Primitive fields in static storage are always initialized.
+  /// Arrays are always initialized, even though their elements might not be.
+  /// Base classes are initialized after the constructor is invoked.
+  unsigned IsInitialized : 1;
+  /// Flag indicating if the field is an embedded base class.
+  unsigned IsBase : 1;
+  /// Flag indicating if the field is the active member of a union.
+  unsigned IsActive : 1;
+  /// Flag indicating if the field is mutable (if in a record).
+  unsigned IsMutable : 1;
+
+  Descriptor *Desc;
+};
+
+/// Bitfield tracking the initialisation status of elements of primitive arrays.
+/// A pointer to this is embedded at the end of all primitive arrays.
+/// If the map was not yet created and nothing was initialied, the pointer to
+/// this structure is 0. If the object was fully initialized, the pointer is -1.
+struct InitMap {
+private:
+  /// Type packing bits.
+  using T = uint64_t;
+  /// Bits stored in a single field.
+  static constexpr uint64_t PER_FIELD = sizeof(T) * CHAR_BIT;
+
+  /// Initializes the map with no fields set.
+  InitMap(unsigned N);
+
+  /// Returns a pointer to storage.
+  T *data();
+
+public:
+  /// Initializes an element. Returns true when object if fully initialized.
+  bool initialize(unsigned I);
+
+  /// Checks if an element was initialized.
+  bool isInitialized(unsigned I);
+
+  /// Allocates a map holding N elements.
+  static InitMap *allocate(unsigned N);
+
+private:
+  /// Number of fields initialized.
+  unsigned UninitFields;
+};
+
+} // namespace interp
+} // namespace clang
+
+#endif

lib/AST/Interp/Disasm.cpp

@@ -0,0 +1,69 @@
+//===--- Disasm.cpp - Disassembler for bytecode functions -------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Dump method for Function which disassembles the bytecode.
+//
+//===----------------------------------------------------------------------===//
+
+#include "Function.h"
+#include "Opcode.h"
+#include "PrimType.h"
+#include "Program.h"
+#include "clang/AST/DeclCXX.h"
+#include "llvm/Support/Compiler.h"
+
+using namespace clang;
+using namespace clang::interp;
+
+LLVM_DUMP_METHOD void Function::dump() const { dump(llvm::errs()); }
+
+LLVM_DUMP_METHOD void Function::dump(llvm::raw_ostream &OS) const {
+  if (F) {
+    if (auto *Cons = dyn_cast<CXXConstructorDecl>(F)) {
+      const std::string &Name = Cons->getParent()->getNameAsString();
+      OS << Name << "::" << Name << ":\n";
+    } else {
+      OS << F->getNameAsString() << ":\n";
+    }
+  } else {
+    OS << "<<expr>>\n";
+  }
+
+  OS << "frame size: " << getFrameSize() << "\n";
+  OS << "arg size:   " << getArgSize() << "\n";
+  OS << "rvo:        " << hasRVO() << "\n";
+
+  auto PrintName = [&OS](const char *Name) {
+    OS << Name;
+    for (long I = 0, N = strlen(Name); I < 30 - N; ++I) {
+      OS << ' ';
+    }
+  };
+
+  for (CodePtr Start = getCodeBegin(), PC = Start; PC != getCodeEnd();) {
+    size_t Addr = PC - Start;
+    auto Op = PC.read<Opcode>();
+    OS << llvm::format("%8d", Addr) << " ";
+    switch (Op) {
+#define GET_DISASM
+#include "Opcodes.inc"
+#undef GET_DISASM
+    }
+  }
+}
+
+LLVM_DUMP_METHOD void Program::dump() const { dump(llvm::errs()); }
+
+LLVM_DUMP_METHOD void Program::dump(llvm::raw_ostream &OS) const {
+  for (auto &Func : Funcs) {
+    Func.second->dump();
+  }
+  for (auto &Anon : AnonFuncs) {
+    Anon->dump();
+  }
+}

lib/AST/Interp/EvalEmitter.cpp

@@ -0,0 +1,253 @@
+//===--- EvalEmitter.cpp - Instruction emitter for the VM -------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "EvalEmitter.h"
+#include "Context.h"
+#include "Interp.h"
+#include "Opcode.h"
+#include "Program.h"
+#include "clang/AST/DeclCXX.h"
+
+using namespace clang;
+using namespace clang::interp;
+
+using APSInt = llvm::APSInt;
+template <typename T> using Expected = llvm::Expected<T>;
+
+EvalEmitter::EvalEmitter(Context &Ctx, Program &P, State &Parent,
+                         InterpStack &Stk, APValue &Result)
+    : Ctx(Ctx), P(P), S(Parent, P, Stk, Ctx, this), Result(Result) {
+  // Create a dummy frame for the interpreter which does not have locals.
+  S.Current = new InterpFrame(S, nullptr, nullptr, CodePtr(), Pointer());
+}
+
+llvm::Expected<bool> EvalEmitter::interpretExpr(const Expr *E) {
+  if (this->visitExpr(E))
+    return true;
+  if (BailLocation)
+    return llvm::make_error<ByteCodeGenError>(*BailLocation);
+  return false;
+}
+
+llvm::Expected<bool> EvalEmitter::interpretDecl(const VarDecl *VD) {
+  if (this->visitDecl(VD))
+    return true;
+  if (BailLocation)
+    return llvm::make_error<ByteCodeGenError>(*BailLocation);
+  return false;
+}
+
+void EvalEmitter::emitLabel(LabelTy Label) {
+  CurrentLabel = Label;
+}
+
+EvalEmitter::LabelTy EvalEmitter::getLabel() { return NextLabel++; }
+
+Scope::Local EvalEmitter::createLocal(Descriptor *D) {
+  // Allocate memory for a local.
+  auto Memory = std::make_unique<char[]>(sizeof(Block) + D->getAllocSize());
+  auto *B = new (Memory.get()) Block(D, /*isStatic=*/false);
+  B->invokeCtor();
+
+  // Register the local.
+  unsigned Off = Locals.size();
+  Locals.insert({Off, std::move(Memory)});
+  return {Off, D};
+}
+
+bool EvalEmitter::bail(const SourceLocation &Loc) {
+  if (!BailLocation)
+    BailLocation = Loc;
+  return false;
+}
+
+bool EvalEmitter::jumpTrue(const LabelTy &Label) {
+  if (isActive()) {
+    if (S.Stk.pop<bool>())
+      ActiveLabel = Label;
+  }
+  return true;
+}
+
+bool EvalEmitter::jumpFalse(const LabelTy &Label) {
+  if (isActive()) {
+    if (!S.Stk.pop<bool>())
+      ActiveLabel = Label;
+  }
+  return true;
+}
+
+bool EvalEmitter::jump(const LabelTy &Label) {
+  if (isActive())
+    CurrentLabel = ActiveLabel = Label;
+  return true;
+}
+
+bool EvalEmitter::fallthrough(const LabelTy &Label) {
+  if (isActive())
+    ActiveLabel = Label;
+  CurrentLabel = Label;
+  return true;
+}
+
+template <PrimType OpType> bool EvalEmitter::emitRet(const SourceInfo &Info) {
+  if (!isActive())
+    return true;
+  using T = typename PrimConv<OpType>::T;
+  return ReturnValue<T>(S.Stk.pop<T>(), Result);
+}
+
+bool EvalEmitter::emitRetVoid(const SourceInfo &Info) { return true; }
+
+bool EvalEmitter::emitRetValue(const SourceInfo &Info) {
+  // Method to recursively traverse composites.
+  std::function<bool(QualType, const Pointer &, APValue &)> Composite;
+  Composite = [this, &Composite](QualType Ty, const Pointer &Ptr, APValue &R) {
+    if (auto *AT = Ty->getAs<AtomicType>())
+      Ty = AT->getValueType();
+
+    if (auto *RT = Ty->getAs<RecordType>()) {
+      auto *Record = Ptr.getRecord();
+      assert(Record && "Missing record descriptor");
+
+      bool Ok = true;
+      if (RT->getDecl()->isUnion()) {
+        const FieldDecl *ActiveField = nullptr;
+        APValue Value;
+        for (auto &F : Record->fields()) {
+          const Pointer &FP = Ptr.atField(F.Offset);
+          QualType FieldTy = F.Decl->getType();
+          if (FP.isActive()) {
+            if (llvm::Optional<PrimType> T = Ctx.classify(FieldTy)) {
+              TYPE_SWITCH(*T, Ok &= ReturnValue<T>(FP.deref<T>(), Value));
+            } else {
+              Ok &= Composite(FieldTy, FP, Value);
+            }
+            break;
+          }
+        }
+        R = APValue(ActiveField, Value);
+      } else {
+        unsigned NF = Record->getNumFields();
+        unsigned NB = Record->getNumBases();
+        unsigned NV = Ptr.isBaseClass() ? 0 : Record->getNumVirtualBases();
+
+        R = APValue(APValue::UninitStruct(), NB, NF);
+
+        for (unsigned I = 0; I < NF; ++I) {
+          const Record::Field *FD = Record->getField(I);
+          QualType FieldTy = FD->Decl->getType();
+          const Pointer &FP = Ptr.atField(FD->Offset);
+          APValue &Value = R.getStructField(I);
+
+          if (llvm::Optional<PrimType> T = Ctx.classify(FieldTy)) {
+            TYPE_SWITCH(*T, Ok &= ReturnValue<T>(FP.deref<T>(), Value));
+          } else {
+            Ok &= Composite(FieldTy, FP, Value);
+          }
+        }
+
+        for (unsigned I = 0; I < NB; ++I) {
+          const Record::Base *BD = Record->getBase(I);
+          QualType BaseTy = Ctx.getASTContext().getRecordType(BD->Decl);
+          const Pointer &BP = Ptr.atField(BD->Offset);
+          Ok &= Composite(BaseTy, BP, R.getStructBase(I));
+        }
+
+        for (unsigned I = 0; I < NV; ++I) {
+          const Record::Base *VD = Record->getVirtualBase(I);
+          QualType VirtBaseTy = Ctx.getASTContext().getRecordType(VD->Decl);
+          const Pointer &VP = Ptr.atField(VD->Offset);
+          Ok &= Composite(VirtBaseTy, VP, R.getStructBase(NB + I));
+        }
+      }
+      return Ok;
+    }
+    if (auto *AT = Ty->getAsArrayTypeUnsafe()) {
+      const size_t NumElems = Ptr.getNumElems();
+      QualType ElemTy = AT->getElementType();
+      R = APValue(APValue::UninitArray{}, NumElems, NumElems);
+
+      bool Ok = true;
+      for (unsigned I = 0; I < NumElems; ++I) {
+        APValue &Slot = R.getArrayInitializedElt(I);
+        const Pointer &EP = Ptr.atIndex(I);
+        if (llvm::Optional<PrimType> T = Ctx.classify(ElemTy)) {
+          TYPE_SWITCH(*T, Ok &= ReturnValue<T>(EP.deref<T>(), Slot));
+        } else {
+          Ok &= Composite(ElemTy, EP.narrow(), Slot);
+        }
+      }
+      return Ok;
+    }
+    llvm_unreachable("invalid value to return");
+  };
+
+  // Return the composite type.
+  const auto &Ptr = S.Stk.pop<Pointer>();
+  return Composite(Ptr.getType(), Ptr, Result);
+}
+
+bool EvalEmitter::emitGetPtrLocal(uint32_t I, const SourceInfo &Info) {
+  if (!isActive())
+    return true;
+
+  auto It = Locals.find(I);
+  assert(It != Locals.end() && "Missing local variable");
+  S.Stk.push<Pointer>(reinterpret_cast<Block *>(It->second.get()));
+  return true;
+}
+
+template <PrimType OpType>
+bool EvalEmitter::emitGetLocal(uint32_t I, const SourceInfo &Info) {
+  if (!isActive())
+    return true;
+
+  using T = typename PrimConv<OpType>::T;
+
+  auto It = Locals.find(I);
+  assert(It != Locals.end() && "Missing local variable");
+  auto *B = reinterpret_cast<Block *>(It->second.get());
+  S.Stk.push<T>(*reinterpret_cast<T *>(B + 1));
+  return true;
+}
+
+template <PrimType OpType>
+bool EvalEmitter::emitSetLocal(uint32_t I, const SourceInfo &Info) {
+  if (!isActive())
+    return true;
+
+  using T = typename PrimConv<OpType>::T;
+
+  auto It = Locals.find(I);
+  assert(It != Locals.end() && "Missing local variable");
+  auto *B = reinterpret_cast<Block *>(It->second.get());
+  *reinterpret_cast<T *>(B + 1) = S.Stk.pop<T>();
+  return true;
+}
+
+bool EvalEmitter::emitDestroy(uint32_t I, const SourceInfo &Info) {
+  if (!isActive())
+    return true;
+
+  for (auto &Local : Descriptors[I]) {
+    auto It = Locals.find(Local.Offset);
+    assert(It != Locals.end() && "Missing local variable");
+    S.deallocate(reinterpret_cast<Block *>(It->second.get()));
+  }
+
+  return true;
+}
+
+//===----------------------------------------------------------------------===//
+// Opcode evaluators
+//===----------------------------------------------------------------------===//
+
+#define GET_EVAL_IMPL
+#include "Opcodes.inc"
+#undef GET_EVAL_IMPL

lib/AST/Interp/EvalEmitter.h

@@ -0,0 +1,129 @@
+//===--- EvalEmitter.h - Instruction emitter for the VM ---------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Defines the instruction emitters.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_AST_INTERP_EVALEMITTER_H
+#define LLVM_CLANG_AST_INTERP_EVALEMITTER_H
+
+#include "ByteCodeGenError.h"
+#include "Context.h"
+#include "InterpStack.h"
+#include "InterpState.h"
+#include "PrimType.h"
+#include "Program.h"
+#include "Source.h"
+#include "llvm/Support/Error.h"
+
+namespace clang {
+class FunctionDecl;
+namespace interp {
+class Context;
+class Function;
+class InterpState;
+class Program;
+class SourceInfo;
+enum Opcode : uint32_t;
+
+/// An emitter which evaluates opcodes as they are emitted.
+class EvalEmitter : public SourceMapper {
+public:
+  using LabelTy = uint32_t;
+  using AddrTy = uintptr_t;
+  using Local = Scope::Local;
+
+  llvm::Expected<bool> interpretExpr(const Expr *E);
+  llvm::Expected<bool> interpretDecl(const VarDecl *VD);
+
+protected:
+  EvalEmitter(Context &Ctx, Program &P, State &Parent, InterpStack &Stk,
+              APValue &Result);
+
+  virtual ~EvalEmitter() {}
+
+  /// Define a label.
+  void emitLabel(LabelTy Label);
+  /// Create a label.
+  LabelTy getLabel();
+
+  /// Methods implemented by the compiler.
+  virtual bool visitExpr(const Expr *E) = 0;
+  virtual bool visitDecl(const VarDecl *VD) = 0;
+
+  bool bail(const Stmt *S) { return bail(S->getBeginLoc()); }
+  bool bail(const Decl *D) { return bail(D->getBeginLoc()); }
+  bool bail(const SourceLocation &Loc);
+
+  /// Emits jumps.
+  bool jumpTrue(const LabelTy &Label);
+  bool jumpFalse(const LabelTy &Label);
+  bool jump(const LabelTy &Label);
+  bool fallthrough(const LabelTy &Label);
+
+  /// Callback for registering a local.
+  Local createLocal(Descriptor *D);
+
+  /// Returns the source location of the current opcode.
+  SourceInfo getSource(Function *F, CodePtr PC) const override {
+    return F ? F->getSource(PC) : CurrentSource;
+  }
+
+  /// Parameter indices.
+  llvm::DenseMap<const ParmVarDecl *, unsigned> Params;
+  /// Local descriptors.
+  llvm::SmallVector<SmallVector<Local, 8>, 2> Descriptors;
+
+private:
+  /// Current compilation context.
+  Context &Ctx;
+  /// Current program.
+  Program &P;
+  /// Callee evaluation state.
+  InterpState S;
+  /// Location to write the result to.
+  APValue &Result;
+
+  /// Temporaries which require storage.
+  llvm::DenseMap<unsigned, std::unique_ptr<char[]>> Locals;
+
+  // The emitter always tracks the current instruction and sets OpPC to a token
+  // value which is mapped to the location of the opcode being evaluated.
+  CodePtr OpPC;
+  /// Location of a failure.
+  llvm::Optional<SourceLocation> BailLocation;
+  /// Location of the current instruction.
+  SourceInfo CurrentSource;
+
+  /// Next label ID to generate - first label is 1.
+  LabelTy NextLabel = 1;
+  /// Label being executed - 0 is the entry label.
+  LabelTy CurrentLabel = 0;
+  /// Active block which should be executed.
+  LabelTy ActiveLabel = 0;
+
+  /// Since expressions can only jump forward, predicated execution is
+  /// used to deal with if-else statements.
+  bool isActive() { return CurrentLabel == ActiveLabel; }
+
+  /// Helper to invoke a method.
+  bool ExecuteCall(Function *F, Pointer &&This, const SourceInfo &Info);
+  /// Helper to emit a diagnostic on a missing method.
+  bool ExecuteNoCall(const FunctionDecl *F, const SourceInfo &Info);
+
+protected:
+#define GET_EVAL_PROTO
+#include "Opcodes.inc"
+#undef GET_EVAL_PROTO
+};
+
+} // namespace interp
+} // namespace clang
+
+#endif

lib/AST/Interp/Frame.cpp

@@ -0,0 +1,14 @@
+//===--- Frame.cpp - Call frame for the VM and AST Walker -------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "Frame.h"
+
+using namespace clang;
+using namespace clang::interp;
+
+Frame::~Frame() {}

lib/AST/Interp/Frame.h

@@ -0,0 +1,45 @@
+//===--- Frame.h - Call frame for the VM and AST Walker ---------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Defines the base class of interpreter and evaluator stack frames.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_AST_INTERP_FRAME_H
+#define LLVM_CLANG_AST_INTERP_FRAME_H
+
+#include "clang/Basic/SourceLocation.h"
+#include "llvm/Support/raw_ostream.h"
+
+namespace clang {
+class FunctionDecl;
+
+namespace interp {
+
+/// Base class for stack frames, shared between VM and walker.
+class Frame {
+public:
+  virtual ~Frame();
+
+  /// Generates a human-readable description of the call site.
+  virtual void describe(llvm::raw_ostream &OS) = 0;
+
+  /// Returns a pointer to the caller frame.
+  virtual Frame *getCaller() const = 0;
+
+  /// Returns the location of the call site.
+  virtual SourceLocation getCallLocation() const = 0;
+
+  /// Returns the called function's declaration.
+  virtual const FunctionDecl *getCallee() const = 0;
+};
+
+} // namespace interp
+} // namespace clang
+
+#endif

lib/AST/Interp/Function.cpp

@@ -0,0 +1,48 @@
+//===--- Function.h - Bytecode function for the VM --------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "Function.h"
+#include "Program.h"
+#include "Opcode.h"
+#include "clang/AST/Decl.h"
+#include "clang/AST/DeclCXX.h"
+
+using namespace clang;
+using namespace clang::interp;
+
+Function::Function(Program &P, const FunctionDecl *F, unsigned ArgSize,
+                   llvm::SmallVector<PrimType, 8> &&ParamTypes,
+                   llvm::DenseMap<unsigned, ParamDescriptor> &&Params)
+    : P(P), Loc(F->getBeginLoc()), F(F), ArgSize(ArgSize),
+      ParamTypes(std::move(ParamTypes)), Params(std::move(Params)) {}
+
+CodePtr Function::getCodeBegin() const { return Code.data(); }
+
+CodePtr Function::getCodeEnd() const { return Code.data() + Code.size(); }
+
+Function::ParamDescriptor Function::getParamDescriptor(unsigned Offset) const {
+  auto It = Params.find(Offset);
+  assert(It != Params.end() && "Invalid parameter offset");
+  return It->second;
+}
+
+SourceInfo Function::getSource(CodePtr PC) const {
+  unsigned Offset = PC - getCodeBegin();
+  using Elem = std::pair<unsigned, SourceInfo>;
+  auto It = std::lower_bound(SrcMap.begin(), SrcMap.end(), Elem{Offset, {}},
+                             [](Elem A, Elem B) { return A.first < B.first; });
+  if (It == SrcMap.end() || It->first != Offset)
+    llvm::report_fatal_error("missing source location");
+  return It->second;
+}
+
+bool Function::isVirtual() const {
+  if (auto *M = dyn_cast<CXXMethodDecl>(F))
+    return M->isVirtual();
+  return false;
+}

lib/AST/Interp/Function.h

@@ -0,0 +1,163 @@
+//===--- Function.h - Bytecode function for the VM --------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Defines the Function class which holds all bytecode function-specific data.
+//
+// The scope class which describes local variables is also defined here.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_AST_INTERP_FUNCTION_H
+#define LLVM_CLANG_AST_INTERP_FUNCTION_H
+
+#include "Pointer.h"
+#include "Source.h"
+#include "clang/AST/Decl.h"
+#include "llvm/Support/raw_ostream.h"
+
+namespace clang {
+namespace interp {
+class Program;
+class ByteCodeEmitter;
+enum PrimType : uint32_t;
+
+/// Describes a scope block.
+///
+/// The block gathers all the descriptors of the locals defined in this block.
+class Scope {
+public:
+  /// Information about a local's storage.
+  struct Local {
+    /// Offset of the local in frame.
+    unsigned Offset;
+    /// Descriptor of the local.
+    Descriptor *Desc;
+  };
+
+  using LocalVectorTy = llvm::SmallVector<Local, 8>;
+
+  Scope(LocalVectorTy &&Descriptors) : Descriptors(std::move(Descriptors)) {}
+
+  llvm::iterator_range<LocalVectorTy::iterator> locals() {
+    return llvm::make_range(Descriptors.begin(), Descriptors.end());
+  }
+
+private:
+  /// Object descriptors in this block.
+  LocalVectorTy Descriptors;
+};
+
+/// Bytecode function.
+///
+/// Contains links to the bytecode of the function, as well as metadata
+/// describing all arguments and stack-local variables.
+class Function {
+public:
+  using ParamDescriptor = std::pair<PrimType, Descriptor *>;
+
+  /// Returns the size of the function's local stack.
+  unsigned getFrameSize() const { return FrameSize; }
+  /// Returns the size of the argument stackx
+  unsigned getArgSize() const { return ArgSize; }
+
+  /// Returns a pointer to the start of the code.
+  CodePtr getCodeBegin() const;
+  /// Returns a pointer to the end of the code.
+  CodePtr getCodeEnd() const;
+
+  /// Returns the original FunctionDecl.
+  const FunctionDecl *getDecl() const { return F; }
+
+  /// Returns the lcoation.
+  SourceLocation getLoc() const { return Loc; }
+
+  /// Returns a parameter descriptor.
+  ParamDescriptor getParamDescriptor(unsigned Offset) const;
+
+  /// Checks if the first argument is a RVO pointer.
+  bool hasRVO() const { return ParamTypes.size() != Params.size(); }
+
+  /// Range over the scope blocks.
+  llvm::iterator_range<llvm::SmallVector<Scope, 2>::iterator> scopes() {
+    return llvm::make_range(Scopes.begin(), Scopes.end());
+  }
+
+  /// Range over argument types.
+  using arg_reverse_iterator = SmallVectorImpl<PrimType>::reverse_iterator;
+  llvm::iterator_range<arg_reverse_iterator> args_reverse() {
+    return llvm::make_range(ParamTypes.rbegin(), ParamTypes.rend());
+  }
+
+  /// Returns a specific scope.
+  Scope &getScope(unsigned Idx) { return Scopes[Idx]; }
+
+  /// Returns the source information at a given PC.
+  SourceInfo getSource(CodePtr PC) const;
+
+  /// Checks if the function is valid to call in constexpr.
+  bool isConstexpr() const { return IsValid; }
+
+  /// Checks if the function is virtual.
+  bool isVirtual() const;
+
+  /// Checks if the function is a constructor.
+  bool isConstructor() const { return isa<CXXConstructorDecl>(F); }
+
+private:
+  /// Construct a function representing an actual function.
+  Function(Program &P, const FunctionDecl *F, unsigned ArgSize,
+           llvm::SmallVector<PrimType, 8> &&ParamTypes,
+           llvm::DenseMap<unsigned, ParamDescriptor> &&Params);
+
+  /// Sets the code of a function.
+  void setCode(unsigned NewFrameSize, std::vector<char> &&NewCode, SourceMap &&NewSrcMap,
+               llvm::SmallVector<Scope, 2> &&NewScopes) {
+    FrameSize = NewFrameSize;
+    Code = std::move(NewCode);
+    SrcMap = std::move(NewSrcMap);
+    Scopes = std::move(NewScopes);
+    IsValid = true;
+  }
+
+private:
+  friend class Program;
+  friend class ByteCodeEmitter;
+
+  /// Program reference.
+  Program &P;
+  /// Location of the executed code.
+  SourceLocation Loc;
+  /// Declaration this function was compiled from.
+  const FunctionDecl *F;
+  /// Local area size: storage + metadata.
+  unsigned FrameSize;
+  /// Size of the argument stack.
+  unsigned ArgSize;
+  /// Program code.
+  std::vector<char> Code;
+  /// Opcode-to-expression mapping.
+  SourceMap SrcMap;
+  /// List of block descriptors.
+  llvm::SmallVector<Scope, 2> Scopes;
+  /// List of argument types.
+  llvm::SmallVector<PrimType, 8> ParamTypes;
+  /// Map from byte offset to parameter descriptor.
+  llvm::DenseMap<unsigned, ParamDescriptor> Params;
+  /// Flag to indicate if the function is valid.
+  bool IsValid = false;
+
+public:
+  /// Dumps the disassembled bytecode to \c llvm::errs().
+  void dump() const;
+  void dump(llvm::raw_ostream &OS) const;
+};
+
+} // namespace interp
+} // namespace clang
+
+#endif

lib/AST/Interp/Integral.h

@@ -0,0 +1,269 @@
+//===--- Integral.h - Wrapper for numeric types for the VM ------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Defines the VM types and helpers operating on types.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_AST_INTERP_INTEGRAL_H
+#define LLVM_CLANG_AST_INTERP_INTEGRAL_H
+
+#include "clang/AST/ComparisonCategories.h"
+#include "clang/AST/APValue.h"
+#include "llvm/ADT/APSInt.h"
+#include "llvm/Support/MathExtras.h"
+#include "llvm/Support/raw_ostream.h"
+#include <cstddef>
+#include <cstdint>
+
+namespace clang {
+namespace interp {
+
+using APInt = llvm::APInt;
+using APSInt = llvm::APSInt;
+
+/// Helper to compare two comparable types.
+template <typename T>
+ComparisonCategoryResult Compare(const T &X, const T &Y) {
+  if (X < Y)
+    return ComparisonCategoryResult::Less;
+  if (X > Y)
+    return ComparisonCategoryResult::Greater;
+  return ComparisonCategoryResult::Equal;
+}
+
+// Helper structure to select the representation.
+template <unsigned Bits, bool Signed> struct Repr;
+template <> struct Repr<8, false> { using Type = uint8_t; };
+template <> struct Repr<16, false> { using Type = uint16_t; };
+template <> struct Repr<32, false> { using Type = uint32_t; };
+template <> struct Repr<64, false> { using Type = uint64_t; };
+template <> struct Repr<8, true> { using Type = int8_t; };
+template <> struct Repr<16, true> { using Type = int16_t; };
+template <> struct Repr<32, true> { using Type = int32_t; };
+template <> struct Repr<64, true> { using Type = int64_t; };
+
+/// Wrapper around numeric types.
+///
+/// These wrappers are required to shared an interface between APSint and
+/// builtin primitive numeral types, while optimising for storage and
+/// allowing methods operating on primitive type to compile to fast code.
+template <unsigned Bits, bool Signed> class Integral {
+private:
+  template <unsigned OtherBits, bool OtherSigned> friend class Integral;
+
+  // The primitive representing the integral.
+  using T = typename Repr<Bits, Signed>::Type;
+  T V;
+
+  /// Primitive representing limits.
+  static const auto Min = std::numeric_limits<T>::min();
+  static const auto Max = std::numeric_limits<T>::max();
+
+  /// Construct an integral from anything that is convertible to storage.
+  template <typename T> explicit Integral(T V) : V(V) {}
+
+public:
+  /// Zero-initializes an integral.
+  Integral() : V(0) {}
+
+  /// Constructs an integral from another integral.
+  template <unsigned SrcBits, bool SrcSign>
+  explicit Integral(Integral<SrcBits, SrcSign> V) : V(V.V) {}
+
+  /// Construct an integral from a value based on signedness.
+  explicit Integral(const APSInt &V)
+      : V(V.isSigned() ? V.getSExtValue() : V.getZExtValue()) {}
+
+  bool operator<(Integral RHS) const { return V < RHS.V; }
+  bool operator>(Integral RHS) const { return V > RHS.V; }
+  bool operator<=(Integral RHS) const { return V <= RHS.V; }
+  bool operator>=(Integral RHS) const { return V >= RHS.V; }
+  bool operator==(Integral RHS) const { return V == RHS.V; }
+  bool operator!=(Integral RHS) const { return V != RHS.V; }
+
+  bool operator>(unsigned RHS) const {
+    return V >= 0 && static_cast<unsigned>(V) > RHS;
+  }
+
+  Integral operator-() const { return Integral(-V); }
+  Integral operator~() const { return Integral(~V); }
+
+  template <unsigned DstBits, bool DstSign>
+  explicit operator Integral<DstBits, DstSign>() const {
+    return Integral<DstBits, DstSign>(V);
+  }
+
+  explicit operator unsigned() const { return V; }
+  explicit operator int64_t() const { return V; }
+  explicit operator uint64_t() const { return V; }
+
+  APSInt toAPSInt() const {
+    return APSInt(APInt(Bits, static_cast<uint64_t>(V), Signed), !Signed);
+  }
+  APSInt toAPSInt(unsigned NumBits) const {
+    if (Signed)
+      return APSInt(toAPSInt().sextOrTrunc(NumBits), !Signed);
+    else
+      return APSInt(toAPSInt().zextOrTrunc(NumBits), !Signed);
+  }
+  APValue toAPValue() const { return APValue(toAPSInt()); }
+
+  Integral<Bits, false> toUnsigned() const {
+    return Integral<Bits, false>(*this);
+  }
+
+  constexpr static unsigned bitWidth() { return Bits; }
+
+  bool isZero() const { return !V; }
+
+  bool isMin() const { return *this == min(bitWidth()); }
+
+  bool isMinusOne() const { return Signed && V == T(-1); }
+
+  constexpr static bool isSigned() { return Signed; }
+
+  bool isNegative() const { return V < T(0); }
+  bool isPositive() const { return !isNegative(); }
+
+  ComparisonCategoryResult compare(const Integral &RHS) const {
+    return Compare(V, RHS.V);
+  }
+
+  unsigned countLeadingZeros() const { return llvm::countLeadingZeros<T>(V); }
+
+  Integral truncate(unsigned TruncBits) const {
+    if (TruncBits >= Bits)
+      return *this;
+    const T BitMask = (T(1) << T(TruncBits)) - 1;
+    const T SignBit = T(1) << (TruncBits - 1);
+    const T ExtMask = ~BitMask;
+    return Integral((V & BitMask) | (Signed && (V & SignBit) ? ExtMask : 0));
+  }
+
+  void print(llvm::raw_ostream &OS) const { OS << V; }
+
+  static Integral min(unsigned NumBits) {
+    return Integral(Min);
+  }
+  static Integral max(unsigned NumBits) {
+    return Integral(Max);
+  }
+
+  template <typename T>
+  static typename std::enable_if<std::is_integral<T>::value, Integral>::type
+  from(T Value) {
+    return Integral(Value);
+  }
+
+  template <unsigned SrcBits, bool SrcSign>
+  static typename std::enable_if<SrcBits != 0, Integral>::type
+  from(Integral<SrcBits, SrcSign> Value) {
+    return Integral(Value.V);
+  }
+
+  template <bool SrcSign> static Integral from(Integral<0, SrcSign> Value) {
+    if (SrcSign)
+      return Integral(Value.V.getSExtValue());
+    else
+      return Integral(Value.V.getZExtValue());
+  }
+
+  static Integral zero() { return from(0); }
+
+  template <typename T> static Integral from(T Value, unsigned NumBits) {
+    return Integral(Value);
+  }
+
+  static bool inRange(int64_t Value, unsigned NumBits) {
+    return CheckRange<T, Min, Max>(Value);
+  }
+
+  static bool increment(Integral A, Integral *R) {
+    return add(A, Integral(T(1)), A.bitWidth(), R);
+  }
+
+  static bool decrement(Integral A, Integral *R) {
+    return sub(A, Integral(T(1)), A.bitWidth(), R);
+  }
+
+  static bool add(Integral A, Integral B, unsigned OpBits, Integral *R) {
+    return CheckAddUB(A.V, B.V, R->V);
+  }
+
+  static bool sub(Integral A, Integral B, unsigned OpBits, Integral *R) {
+    return CheckSubUB(A.V, B.V, R->V);
+  }
+
+  static bool mul(Integral A, Integral B, unsigned OpBits, Integral *R) {
+    return CheckMulUB(A.V, B.V, R->V);
+  }
+
+private:
+  template <typename T>
+  static typename std::enable_if<std::is_signed<T>::value, bool>::type
+  CheckAddUB(T A, T B, T &R) {
+    return llvm::AddOverflow<T>(A, B, R);
+  }
+
+  template <typename T>
+  static typename std::enable_if<std::is_unsigned<T>::value, bool>::type
+  CheckAddUB(T A, T B, T &R) {
+    R = A + B;
+    return false;
+  }
+
+  template <typename T>
+  static typename std::enable_if<std::is_signed<T>::value, bool>::type
+  CheckSubUB(T A, T B, T &R) {
+    return llvm::SubOverflow<T>(A, B, R);
+  }
+
+  template <typename T>
+  static typename std::enable_if<std::is_unsigned<T>::value, bool>::type
+  CheckSubUB(T A, T B, T &R) {
+    R = A - B;
+    return false;
+  }
+
+  template <typename T>
+  static typename std::enable_if<std::is_signed<T>::value, bool>::type
+  CheckMulUB(T A, T B, T &R) {
+    return llvm::MulOverflow<T>(A, B, R);
+  }
+
+  template <typename T>
+  static typename std::enable_if<std::is_unsigned<T>::value, bool>::type
+  CheckMulUB(T A, T B, T &R) {
+    R = A * B;
+    return false;
+  }
+
+  template <typename T, T Min, T Max>
+  static typename std::enable_if<std::is_signed<T>::value, bool>::type
+  CheckRange(int64_t V) {
+    return Min <= V && V <= Max;
+  }
+
+  template <typename T, T Min, T Max>
+  static typename std::enable_if<std::is_unsigned<T>::value, bool>::type
+  CheckRange(int64_t V) {
+    return V >= 0 && static_cast<uint64_t>(V) <= Max;
+  }
+};
+
+template <unsigned Bits, bool Signed>
+llvm::raw_ostream &operator<<(llvm::raw_ostream &OS, Integral<Bits, Signed> I) {
+  I.print(OS);
+  return OS;
+}
+
+} // namespace interp
+} // namespace clang
+
+#endif

lib/AST/Interp/Interp.cpp

@@ -0,0 +1,417 @@
+//===--- InterpState.cpp - Interpreter for the constexpr VM -----*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "Interp.h"
+#include <limits>
+#include <vector>
+#include "Function.h"
+#include "InterpFrame.h"
+#include "InterpStack.h"
+#include "Opcode.h"
+#include "PrimType.h"
+#include "Program.h"
+#include "State.h"
+#include "clang/AST/ASTContext.h"
+#include "clang/AST/ASTDiagnostic.h"
+#include "clang/AST/CXXInheritance.h"
+#include "clang/AST/Expr.h"
+#include "clang/AST/ExprCXX.h"
+#include "llvm/ADT/APSInt.h"
+
+using namespace clang;
+using namespace clang::interp;
+
+//===----------------------------------------------------------------------===//
+// Ret
+//===----------------------------------------------------------------------===//
+
+template <PrimType Name, class T = typename PrimConv<Name>::T>
+static bool Ret(InterpState &S, CodePtr &PC, APValue &Result) {
+  S.CallStackDepth--;
+  const T &Ret = S.Stk.pop<T>();
+
+  assert(S.Current->getFrameOffset() == S.Stk.size() && "Invalid frame");
+  if (!S.checkingPotentialConstantExpression())
+    S.Current->popArgs();
+
+  if (InterpFrame *Caller = S.Current->Caller) {
+    PC = S.Current->getRetPC();
+    delete S.Current;
+    S.Current = Caller;
+    S.Stk.push<T>(Ret);
+  } else {
+    delete S.Current;
+    S.Current = nullptr;
+    if (!ReturnValue<T>(Ret, Result))
+      return false;
+  }
+  return true;
+}
+
+static bool RetVoid(InterpState &S, CodePtr &PC, APValue &Result) {
+  S.CallStackDepth--;
+
+  assert(S.Current->getFrameOffset() == S.Stk.size() && "Invalid frame");
+  if (!S.checkingPotentialConstantExpression())
+    S.Current->popArgs();
+
+  if (InterpFrame *Caller = S.Current->Caller) {
+    PC = S.Current->getRetPC();
+    delete S.Current;
+    S.Current = Caller;
+  } else {
+    delete S.Current;
+    S.Current = nullptr;
+  }
+  return true;
+}
+
+static bool RetValue(InterpState &S, CodePtr &Pt, APValue &Result) {
+  llvm::report_fatal_error("Interpreter cannot return values");
+}
+
+//===----------------------------------------------------------------------===//
+// Jmp, Jt, Jf
+//===----------------------------------------------------------------------===//
+
+static bool Jmp(InterpState &S, CodePtr &PC, int32_t Offset) {
+  PC += Offset;
+  return true;
+}
+
+static bool Jt(InterpState &S, CodePtr &PC, int32_t Offset) {
+  if (S.Stk.pop<bool>()) {
+    PC += Offset;
+  }
+  return true;
+}
+
+static bool Jf(InterpState &S, CodePtr &PC, int32_t Offset) {
+  if (!S.Stk.pop<bool>()) {
+    PC += Offset;
+  }
+  return true;
+}
+
+static bool CheckInitialized(InterpState &S, CodePtr OpPC, const Pointer &Ptr,
+                             AccessKinds AK) {
+  if (Ptr.isInitialized())
+    return true;
+  if (!S.checkingPotentialConstantExpression()) {
+    const SourceInfo &Loc = S.Current->getSource(OpPC);
+    S.FFDiag(Loc, diag::note_constexpr_access_uninit) << AK << false;
+  }
+  return false;
+}
+
+static bool CheckActive(InterpState &S, CodePtr OpPC, const Pointer &Ptr,
+                        AccessKinds AK) {
+  if (Ptr.isActive())
+    return true;
+
+  // Get the inactive field descriptor.
+  const FieldDecl *InactiveField = Ptr.getField();
+
+  // Walk up the pointer chain to find the union which is not active.
+  Pointer U = Ptr.getBase();
+  while (!U.isActive()) {
+    U = U.getBase();
+  }
+
+  // Find the active field of the union.
+  Record *R = U.getRecord();
+  assert(R && R->isUnion() && "Not a union");
+  const FieldDecl *ActiveField = nullptr;
+  for (unsigned I = 0, N = R->getNumFields(); I < N; ++I) {
+    const Pointer &Field = U.atField(R->getField(I)->Offset);
+    if (Field.isActive()) {
+      ActiveField = Field.getField();
+      break;
+    }
+  }
+
+  const SourceInfo &Loc = S.Current->getSource(OpPC);
+  S.FFDiag(Loc, diag::note_constexpr_access_inactive_union_member)
+      << AK << InactiveField << !ActiveField << ActiveField;
+  return false;
+}
+
+static bool CheckTemporary(InterpState &S, CodePtr OpPC, const Pointer &Ptr,
+                           AccessKinds AK) {
+  if (auto ID = Ptr.getDeclID()) {
+    if (!Ptr.isStaticTemporary())
+      return true;
+
+    if (Ptr.getDeclDesc()->getType().isConstQualified())
+      return true;
+
+    if (S.P.getCurrentDecl() == ID)
+      return true;
+
+    const SourceInfo &E = S.Current->getSource(OpPC);
+    S.FFDiag(E, diag::note_constexpr_access_static_temporary, 1) << AK;
+    S.Note(Ptr.getDeclLoc(), diag::note_constexpr_temporary_here);
+    return false;
+  }
+  return true;
+}
+
+static bool CheckGlobal(InterpState &S, CodePtr OpPC, const Pointer &Ptr) {
+  if (auto ID = Ptr.getDeclID()) {
+    if (!Ptr.isStatic())
+      return true;
+
+    if (S.P.getCurrentDecl() == ID)
+      return true;
+
+    S.FFDiag(S.Current->getLocation(OpPC), diag::note_constexpr_modify_global);
+    return false;
+  }
+  return true;
+}
+
+namespace clang {
+namespace interp {
+
+bool CheckExtern(InterpState &S, CodePtr OpPC, const Pointer &Ptr) {
+  if (!Ptr.isExtern())
+    return true;
+
+  if (!S.checkingPotentialConstantExpression()) {
+    auto *VD = Ptr.getDeclDesc()->asValueDecl();
+    const SourceInfo &Loc = S.Current->getSource(OpPC);
+    S.FFDiag(Loc, diag::note_constexpr_ltor_non_constexpr, 1) << VD;
+    S.Note(VD->getLocation(), diag::note_declared_at);
+  }
+  return false;
+}
+
+bool CheckArray(InterpState &S, CodePtr OpPC, const Pointer &Ptr) {
+  if (!Ptr.isUnknownSizeArray())
+    return true;
+  const SourceInfo &E = S.Current->getSource(OpPC);
+  S.FFDiag(E, diag::note_constexpr_unsized_array_indexed);
+  return false;
+}
+
+bool CheckLive(InterpState &S, CodePtr OpPC, const Pointer &Ptr,
+               AccessKinds AK) {
+  const auto &Src = S.Current->getSource(OpPC);
+  if (Ptr.isZero()) {
+
+    if (Ptr.isField())
+      S.FFDiag(Src, diag::note_constexpr_null_subobject) << CSK_Field;
+    else
+      S.FFDiag(Src, diag::note_constexpr_access_null) << AK;
+
+    return false;
+  }
+
+  if (!Ptr.isLive()) {
+    bool IsTemp = Ptr.isTemporary();
+
+    S.FFDiag(Src, diag::note_constexpr_lifetime_ended, 1) << AK << !IsTemp;
+
+    if (IsTemp)
+      S.Note(Ptr.getDeclLoc(), diag::note_constexpr_temporary_here);
+    else
+      S.Note(Ptr.getDeclLoc(), diag::note_declared_at);
+
+    return false;
+  }
+
+  return true;
+}
+
+bool CheckNull(InterpState &S, CodePtr OpPC, const Pointer &Ptr,
+               CheckSubobjectKind CSK) {
+  if (!Ptr.isZero())
+    return true;
+  const SourceInfo &Loc = S.Current->getSource(OpPC);
+  S.FFDiag(Loc, diag::note_constexpr_null_subobject) << CSK;
+  return false;
+}
+
+bool CheckRange(InterpState &S, CodePtr OpPC, const Pointer &Ptr,
+                AccessKinds AK) {
+  if (!Ptr.isOnePastEnd())
+    return true;
+  const SourceInfo &Loc = S.Current->getSource(OpPC);
+  S.FFDiag(Loc, diag::note_constexpr_access_past_end) << AK;
+  return false;
+}
+
+bool CheckRange(InterpState &S, CodePtr OpPC, const Pointer &Ptr,
+                CheckSubobjectKind CSK) {
+  if (!Ptr.isElementPastEnd())
+    return true;
+  const SourceInfo &Loc = S.Current->getSource(OpPC);
+  S.FFDiag(Loc, diag::note_constexpr_past_end_subobject) << CSK;
+  return false;
+}
+
+bool CheckConst(InterpState &S, CodePtr OpPC, const Pointer &Ptr) {
+  assert(Ptr.isLive() && "Pointer is not live");
+  if (!Ptr.isConst()) {
+    return true;
+  }
+
+  const QualType Ty = Ptr.getType();
+  const SourceInfo &Loc = S.Current->getSource(OpPC);
+  S.FFDiag(Loc, diag::note_constexpr_modify_const_type) << Ty;
+  return false;
+}
+
+bool CheckMutable(InterpState &S, CodePtr OpPC, const Pointer &Ptr) {
+  assert(Ptr.isLive() && "Pointer is not live");
+  if (!Ptr.isMutable()) {
+    return true;
+  }
+
+  const SourceInfo &Loc = S.Current->getSource(OpPC);
+  const FieldDecl *Field = Ptr.getField();
+  S.FFDiag(Loc, diag::note_constexpr_ltor_mutable, 1) << Field;
+  S.Note(Field->getLocation(), diag::note_declared_at);
+  return false;
+}
+
+bool CheckLoad(InterpState &S, CodePtr OpPC, const Pointer &Ptr) {
+  if (!CheckLive(S, OpPC, Ptr, AK_Read))
+    return false;
+  if (!CheckExtern(S, OpPC, Ptr))
+    return false;
+  if (!CheckRange(S, OpPC, Ptr, AK_Read))
+    return false;
+  if (!CheckInitialized(S, OpPC, Ptr, AK_Read))
+    return false;
+  if (!CheckActive(S, OpPC, Ptr, AK_Read))
+    return false;
+  if (!CheckTemporary(S, OpPC, Ptr, AK_Read))
+    return false;
+  if (!CheckMutable(S, OpPC, Ptr))
+    return false;
+  return true;
+}
+
+bool CheckStore(InterpState &S, CodePtr OpPC, const Pointer &Ptr) {
+  if (!CheckLive(S, OpPC, Ptr, AK_Assign))
+    return false;
+  if (!CheckExtern(S, OpPC, Ptr))
+    return false;
+  if (!CheckRange(S, OpPC, Ptr, AK_Assign))
+    return false;
+  if (!CheckGlobal(S, OpPC, Ptr))
+    return false;
+  if (!CheckConst(S, OpPC, Ptr))
+    return false;
+  return true;
+}
+
+bool CheckInvoke(InterpState &S, CodePtr OpPC, const Pointer &Ptr) {
+  if (!CheckLive(S, OpPC, Ptr, AK_MemberCall))
+    return false;
+  if (!CheckExtern(S, OpPC, Ptr))
+    return false;
+  if (!CheckRange(S, OpPC, Ptr, AK_MemberCall))
+    return false;
+  return true;
+}
+
+bool CheckInit(InterpState &S, CodePtr OpPC, const Pointer &Ptr) {
+  if (!CheckLive(S, OpPC, Ptr, AK_Assign))
+    return false;
+  if (!CheckRange(S, OpPC, Ptr, AK_Assign))
+    return false;
+  return true;
+}
+
+bool CheckCallable(InterpState &S, CodePtr OpPC, Function *F) {
+  const SourceLocation &Loc = S.Current->getLocation(OpPC);
+
+  if (F->isVirtual()) {
+    if (!S.getLangOpts().CPlusPlus2a) {
+      S.CCEDiag(Loc, diag::note_constexpr_virtual_call);
+      return false;
+    }
+  }
+
+  if (!F->isConstexpr()) {
+    if (S.getLangOpts().CPlusPlus11) {
+      const FunctionDecl *DiagDecl = F->getDecl();
+
+      // If this function is not constexpr because it is an inherited
+      // non-constexpr constructor, diagnose that directly.
+      auto *CD = dyn_cast<CXXConstructorDecl>(DiagDecl);
+      if (CD && CD->isInheritingConstructor()) {
+        auto *Inherited = CD->getInheritedConstructor().getConstructor();
+        if (!Inherited->isConstexpr())
+          DiagDecl = CD = Inherited;
+      }
+
+      // FIXME: If DiagDecl is an implicitly-declared special member function
+      // or an inheriting constructor, we should be much more explicit about why
+      // it's not constexpr.
+      if (CD && CD->isInheritingConstructor())
+        S.FFDiag(Loc, diag::note_constexpr_invalid_inhctor, 1)
+          << CD->getInheritedConstructor().getConstructor()->getParent();
+      else
+        S.FFDiag(Loc, diag::note_constexpr_invalid_function, 1)
+          << DiagDecl->isConstexpr() << (bool)CD << DiagDecl;
+      S.Note(DiagDecl->getLocation(), diag::note_declared_at);
+    } else {
+      S.FFDiag(Loc, diag::note_invalid_subexpr_in_const_expr);
+    }
+    return false;
+  }
+
+  return true;
+}
+
+bool CheckThis(InterpState &S, CodePtr OpPC, const Pointer &This) {
+  if (!This.isZero())
+    return true;
+
+  const SourceInfo &Loc = S.Current->getSource(OpPC);
+
+  bool IsImplicit = false;
+  if (auto *E = dyn_cast_or_null<CXXThisExpr>(Loc.asExpr()))
+    IsImplicit = E->isImplicit();
+
+  if (S.getLangOpts().CPlusPlus11)
+    S.FFDiag(Loc, diag::note_constexpr_this) << IsImplicit;
+  else
+    S.FFDiag(Loc);
+
+  return false;
+}
+
+bool CheckPure(InterpState &S, CodePtr OpPC, const CXXMethodDecl *MD) {
+  if (!MD->isPure())
+    return true;
+  const SourceInfo &E = S.Current->getSource(OpPC);
+  S.FFDiag(E, diag::note_constexpr_pure_virtual_call, 1) << MD;
+  S.Note(MD->getLocation(), diag::note_declared_at);
+  return false;
+}
+bool Interpret(InterpState &S, APValue &Result) {
+  CodePtr PC = S.Current->getPC();
+
+  for (;;) {
+    auto Op = PC.read<Opcode>();
+    CodePtr OpPC = PC;
+
+    switch (Op) {
+#define GET_INTERP
+#include "Opcodes.inc"
+#undef GET_INTERP
+    }
+  }
+}
+
+} // namespace interp
+} // namespace clang

lib/AST/Interp/Interp.h

@@ -0,0 +1,960 @@
+//===--- Interp.h - Interpreter for the constexpr VM ------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Definition of the interpreter state and entry point.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_AST_INTERP_INTERP_H
+#define LLVM_CLANG_AST_INTERP_INTERP_H
+
+#include <limits>
+#include <vector>
+#include "Function.h"
+#include "InterpFrame.h"
+#include "InterpStack.h"
+#include "InterpState.h"
+#include "Opcode.h"
+#include "PrimType.h"
+#include "Program.h"
+#include "State.h"
+#include "clang/AST/ASTContext.h"
+#include "clang/AST/ASTDiagnostic.h"
+#include "clang/AST/CXXInheritance.h"
+#include "clang/AST/Expr.h"
+#include "llvm/ADT/APFloat.h"
+#include "llvm/ADT/APSInt.h"
+#include "llvm/Support/Endian.h"
+
+namespace clang {
+namespace interp {
+
+using APInt = llvm::APInt;
+using APSInt = llvm::APSInt;
+
+/// Convers a value to an APValue.
+template <typename T> bool ReturnValue(const T &V, APValue &R) {
+  R = V.toAPValue();
+  return true;
+}
+
+/// Checks if the variable has externally defined storage.
+bool CheckExtern(InterpState &S, CodePtr OpPC, const Pointer &Ptr);
+
+/// Checks if the array is offsetable.
+bool CheckArray(InterpState &S, CodePtr OpPC, const Pointer &Ptr);
+
+/// Checks if a pointer is live and accesible.
+bool CheckLive(InterpState &S, CodePtr OpPC, const Pointer &Ptr,
+               AccessKinds AK);
+/// Checks if a pointer is null.
+bool CheckNull(InterpState &S, CodePtr OpPC, const Pointer &Ptr,
+               CheckSubobjectKind CSK);
+
+/// Checks if a pointer is in range.
+bool CheckRange(InterpState &S, CodePtr OpPC, const Pointer &Ptr,
+                AccessKinds AK);
+
+/// Checks if a field from which a pointer is going to be derived is valid.
+bool CheckRange(InterpState &S, CodePtr OpPC, const Pointer &Ptr,
+                CheckSubobjectKind CSK);
+
+/// Checks if a pointer points to const storage.
+bool CheckConst(InterpState &S, CodePtr OpPC, const Pointer &Ptr);
+
+/// Checks if a pointer points to a mutable field.
+bool CheckMutable(InterpState &S, CodePtr OpPC, const Pointer &Ptr);
+
+/// Checks if a value can be loaded from a block.
+bool CheckLoad(InterpState &S, CodePtr OpPC, const Pointer &Ptr);
+
+/// Checks if a value can be stored in a block.
+bool CheckStore(InterpState &S, CodePtr OpPC, const Pointer &Ptr);
+
+/// Checks if a method can be invoked on an object.
+bool CheckInvoke(InterpState &S, CodePtr OpPC, const Pointer &Ptr);
+
+/// Checks if a value can be initialized.
+bool CheckInit(InterpState &S, CodePtr OpPC, const Pointer &Ptr);
+
+/// Checks if a method can be called.
+bool CheckCallable(InterpState &S, CodePtr OpPC, Function *F);
+
+/// Checks the 'this' pointer.
+bool CheckThis(InterpState &S, CodePtr OpPC, const Pointer &This);
+
+/// Checks if a method is pure virtual.
+bool CheckPure(InterpState &S, CodePtr OpPC, const CXXMethodDecl *MD);
+
+template <typename T> inline bool IsTrue(const T &V) { return !V.isZero(); }
+
+//===----------------------------------------------------------------------===//
+// Add, Sub, Mul
+//===----------------------------------------------------------------------===//
+
+template <typename T, bool (*OpFW)(T, T, unsigned, T *),
+          template <typename U> class OpAP>
+bool AddSubMulHelper(InterpState &S, CodePtr OpPC, unsigned Bits, const T &LHS,
+                     const T &RHS) {
+  // Fast path - add the numbers with fixed width.
+  T Result;
+  if (!OpFW(LHS, RHS, Bits, &Result)) {
+    S.Stk.push<T>(Result);
+    return true;
+  }
+
+  // If for some reason evaluation continues, use the truncated results.
+  S.Stk.push<T>(Result);
+
+  // Slow path - compute the result using another bit of precision.
+  APSInt Value = OpAP<APSInt>()(LHS.toAPSInt(Bits), RHS.toAPSInt(Bits));
+
+  // Report undefined behaviour, stopping if required.
+  const Expr *E = S.Current->getExpr(OpPC);
+  QualType Type = E->getType();
+  if (S.checkingForUndefinedBehavior()) {
+    auto Trunc = Value.trunc(Result.bitWidth()).toString(10);
+    auto Loc = E->getExprLoc();
+    S.report(Loc, diag::warn_integer_constant_overflow) << Trunc << Type;
+    return true;
+  } else {
+    S.CCEDiag(E, diag::note_constexpr_overflow) << Value << Type;
+    return S.noteUndefinedBehavior();
+  }
+}
+
+template <PrimType Name, class T = typename PrimConv<Name>::T>
+bool Add(InterpState &S, CodePtr OpPC) {
+  const T &RHS = S.Stk.pop<T>();
+  const T &LHS = S.Stk.pop<T>();
+  const unsigned Bits = RHS.bitWidth() + 1;
+  return AddSubMulHelper<T, T::add, std::plus>(S, OpPC, Bits, LHS, RHS);
+}
+
+template <PrimType Name, class T = typename PrimConv<Name>::T>
+bool Sub(InterpState &S, CodePtr OpPC) {
+  const T &RHS = S.Stk.pop<T>();
+  const T &LHS = S.Stk.pop<T>();
+  const unsigned Bits = RHS.bitWidth() + 1;
+  return AddSubMulHelper<T, T::sub, std::minus>(S, OpPC, Bits, LHS, RHS);
+}
+
+template <PrimType Name, class T = typename PrimConv<Name>::T>
+bool Mul(InterpState &S, CodePtr OpPC) {
+  const T &RHS = S.Stk.pop<T>();
+  const T &LHS = S.Stk.pop<T>();
+  const unsigned Bits = RHS.bitWidth() * 2;
+  return AddSubMulHelper<T, T::mul, std::multiplies>(S, OpPC, Bits, LHS, RHS);
+}
+
+//===----------------------------------------------------------------------===//
+// EQ, NE, GT, GE, LT, LE
+//===----------------------------------------------------------------------===//
+
+using CompareFn = llvm::function_ref<bool(ComparisonCategoryResult)>;
+
+template <typename T>
+bool CmpHelper(InterpState &S, CodePtr OpPC, CompareFn Fn) {
+  using BoolT = PrimConv<PT_Bool>::T;
+  const T &RHS = S.Stk.pop<T>();
+  const T &LHS = S.Stk.pop<T>();
+  S.Stk.push<BoolT>(BoolT::from(Fn(LHS.compare(RHS))));
+  return true;
+}
+
+template <typename T>
+bool CmpHelperEQ(InterpState &S, CodePtr OpPC, CompareFn Fn) {
+  return CmpHelper<T>(S, OpPC, Fn);
+}
+
+template <>
+inline bool CmpHelper<Pointer>(InterpState &S, CodePtr OpPC, CompareFn Fn) {
+  using BoolT = PrimConv<PT_Bool>::T;
+  const Pointer &RHS = S.Stk.pop<Pointer>();
+  const Pointer &LHS = S.Stk.pop<Pointer>();
+
+  if (!Pointer::hasSameBase(LHS, RHS)) {
+    const SourceInfo &Loc = S.Current->getSource(OpPC);
+    S.FFDiag(Loc, diag::note_invalid_subexpr_in_const_expr);
+    return false;
+  } else {
+    unsigned VL = LHS.getByteOffset();
+    unsigned VR = RHS.getByteOffset();
+    S.Stk.push<BoolT>(BoolT::from(Fn(Compare(VL, VR))));
+    return true;
+  }
+}
+
+template <>
+inline bool CmpHelperEQ<Pointer>(InterpState &S, CodePtr OpPC, CompareFn Fn) {
+  using BoolT = PrimConv<PT_Bool>::T;
+  const Pointer &RHS = S.Stk.pop<Pointer>();
+  const Pointer &LHS = S.Stk.pop<Pointer>();
+
+  if (LHS.isZero() || RHS.isZero()) {
+    if (LHS.isZero() && RHS.isZero())
+      S.Stk.push<BoolT>(BoolT::from(Fn(ComparisonCategoryResult::Equal)));
+    else
+      S.Stk.push<BoolT>(BoolT::from(Fn(ComparisonCategoryResult::Nonequal)));
+    return true;
+  }
+
+  if (!Pointer::hasSameBase(LHS, RHS)) {
+    S.Stk.push<BoolT>(BoolT::from(Fn(ComparisonCategoryResult::Unordered)));
+    return true;
+  } else {
+    unsigned VL = LHS.getByteOffset();
+    unsigned VR = RHS.getByteOffset();
+    S.Stk.push<BoolT>(BoolT::from(Fn(Compare(VL, VR))));
+    return true;
+  }
+}
+
+template <PrimType Name, class T = typename PrimConv<Name>::T>
+bool EQ(InterpState &S, CodePtr OpPC) {
+  return CmpHelperEQ<T>(S, OpPC, [](ComparisonCategoryResult R) {
+    return R == ComparisonCategoryResult::Equal;
+  });
+}
+
+template <PrimType Name, class T = typename PrimConv<Name>::T>
+bool NE(InterpState &S, CodePtr OpPC) {
+  return CmpHelperEQ<T>(S, OpPC, [](ComparisonCategoryResult R) {
+    return R != ComparisonCategoryResult::Equal;
+  });
+}
+
+template <PrimType Name, class T = typename PrimConv<Name>::T>
+bool LT(InterpState &S, CodePtr OpPC) {
+  return CmpHelper<T>(S, OpPC, [](ComparisonCategoryResult R) {
+    return R == ComparisonCategoryResult::Less;
+  });
+}
+
+template <PrimType Name, class T = typename PrimConv<Name>::T>
+bool LE(InterpState &S, CodePtr OpPC) {
+  return CmpHelper<T>(S, OpPC, [](ComparisonCategoryResult R) {
+    return R == ComparisonCategoryResult::Less ||
+           R == ComparisonCategoryResult::Equal;
+  });
+}
+
+template <PrimType Name, class T = typename PrimConv<Name>::T>
+bool GT(InterpState &S, CodePtr OpPC) {
+  return CmpHelper<T>(S, OpPC, [](ComparisonCategoryResult R) {
+    return R == ComparisonCategoryResult::Greater;
+  });
+}
+
+template <PrimType Name, class T = typename PrimConv<Name>::T>
+bool GE(InterpState &S, CodePtr OpPC) {
+  return CmpHelper<T>(S, OpPC, [](ComparisonCategoryResult R) {
+    return R == ComparisonCategoryResult::Greater ||
+           R == ComparisonCategoryResult::Equal;
+  });
+}
+
+//===----------------------------------------------------------------------===//
+// InRange
+//===----------------------------------------------------------------------===//
+
+template <PrimType Name, class T = typename PrimConv<Name>::T>
+bool InRange(InterpState &S, CodePtr OpPC) {
+  const T RHS = S.Stk.pop<T>();
+  const T LHS = S.Stk.pop<T>();
+  const T Value = S.Stk.pop<T>();
+
+  S.Stk.push<bool>(LHS <= Value && Value <= RHS);
+  return true;
+}
+
+//===----------------------------------------------------------------------===//
+// Dup, Pop, Test
+//===----------------------------------------------------------------------===//
+
+template <PrimType Name, class T = typename PrimConv<Name>::T>
+bool Dup(InterpState &S, CodePtr OpPC) {
+  S.Stk.push<T>(S.Stk.peek<T>());
+  return true;
+}
+
+template <PrimType Name, class T = typename PrimConv<Name>::T>
+bool Pop(InterpState &S, CodePtr OpPC) {
+  S.Stk.pop<T>();
+  return true;
+}
+
+//===----------------------------------------------------------------------===//
+// Const
+//===----------------------------------------------------------------------===//
+
+template <PrimType Name, class T = typename PrimConv<Name>::T>
+bool Const(InterpState &S, CodePtr OpPC, const T &Arg) {
+  S.Stk.push<T>(Arg);
+  return true;
+}
+
+//===----------------------------------------------------------------------===//
+// Get/Set Local/Param/Global/This
+//===----------------------------------------------------------------------===//
+
+template <PrimType Name, class T = typename PrimConv<Name>::T>
+bool GetLocal(InterpState &S, CodePtr OpPC, uint32_t I) {
+  S.Stk.push<T>(S.Current->getLocal<T>(I));
+  return true;
+}
+
+template <PrimType Name, class T = typename PrimConv<Name>::T>
+bool SetLocal(InterpState &S, CodePtr OpPC, uint32_t I) {
+  S.Current->setLocal<T>(I, S.Stk.pop<T>());
+  return true;
+}
+
+template <PrimType Name, class T = typename PrimConv<Name>::T>
+bool GetParam(InterpState &S, CodePtr OpPC, uint32_t I) {
+  if (S.checkingPotentialConstantExpression()) {
+    return false;
+  }
+  S.Stk.push<T>(S.Current->getParam<T>(I));
+  return true;
+}
+
+template <PrimType Name, class T = typename PrimConv<Name>::T>
+bool SetParam(InterpState &S, CodePtr OpPC, uint32_t I) {
+  S.Current->setParam<T>(I, S.Stk.pop<T>());
+  return true;
+}
+
+template <PrimType Name, class T = typename PrimConv<Name>::T>
+bool GetField(InterpState &S, CodePtr OpPC, uint32_t I) {
+  const Pointer &Obj = S.Stk.peek<Pointer>();
+  if (!CheckNull(S, OpPC, Obj, CSK_Field))
+      return false;
+  if (!CheckRange(S, OpPC, Obj, CSK_Field))
+    return false;
+  const Pointer &Field = Obj.atField(I);
+  if (!CheckLoad(S, OpPC, Field))
+    return false;
+  S.Stk.push<T>(Field.deref<T>());
+  return true;
+}
+
+template <PrimType Name, class T = typename PrimConv<Name>::T>
+bool SetField(InterpState &S, CodePtr OpPC, uint32_t I) {
+  const T &Value = S.Stk.pop<T>();
+  const Pointer &Obj = S.Stk.peek<Pointer>();
+  if (!CheckNull(S, OpPC, Obj, CSK_Field))
+    return false;
+  if (!CheckRange(S, OpPC, Obj, CSK_Field))
+    return false;
+  const Pointer &Field = Obj.atField(I);
+  if (!CheckStore(S, OpPC, Field))
+    return false;
+  Field.deref<T>() = Value;
+  return true;
+}
+
+template <PrimType Name, class T = typename PrimConv<Name>::T>
+bool GetFieldPop(InterpState &S, CodePtr OpPC, uint32_t I) {
+  const Pointer &Obj = S.Stk.pop<Pointer>();
+  if (!CheckNull(S, OpPC, Obj, CSK_Field))
+    return false;
+  if (!CheckRange(S, OpPC, Obj, CSK_Field))
+    return false;
+  const Pointer &Field = Obj.atField(I);
+  if (!CheckLoad(S, OpPC, Field))
+    return false;
+  S.Stk.push<T>(Field.deref<T>());
+  return true;
+}
+
+template <PrimType Name, class T = typename PrimConv<Name>::T>
+bool GetThisField(InterpState &S, CodePtr OpPC, uint32_t I) {
+  if (S.checkingPotentialConstantExpression())
+    return false;
+  const Pointer &This = S.Current->getThis();
+  if (!CheckThis(S, OpPC, This))
+    return false;
+  const Pointer &Field = This.atField(I);
+  if (!CheckLoad(S, OpPC, Field))
+    return false;
+  S.Stk.push<T>(Field.deref<T>());
+  return true;
+}
+
+template <PrimType Name, class T = typename PrimConv<Name>::T>
+bool SetThisField(InterpState &S, CodePtr OpPC, uint32_t I) {
+  if (S.checkingPotentialConstantExpression())
+    return false;
+  const T &Value = S.Stk.pop<T>();
+  const Pointer &This = S.Current->getThis();
+  if (!CheckThis(S, OpPC, This))
+    return false;
+  const Pointer &Field = This.atField(I);
+  if (!CheckStore(S, OpPC, Field))
+    return false;
+  Field.deref<T>() = Value;
+  return true;
+}
+
+template <PrimType Name, class T = typename PrimConv<Name>::T>
+bool GetGlobal(InterpState &S, CodePtr OpPC, uint32_t I) {
+  auto *B = S.P.getGlobal(I);
+  if (B->isExtern())
+    return false;
+  S.Stk.push<T>(B->deref<T>());
+  return true;
+}
+
+template <PrimType Name, class T = typename PrimConv<Name>::T>
+bool SetGlobal(InterpState &S, CodePtr OpPC, uint32_t I) {
+  // TODO: emit warning.
+  return false;
+}
+
+template <PrimType Name, class T = typename PrimConv<Name>::T>
+bool InitGlobal(InterpState &S, CodePtr OpPC, uint32_t I) {
+  S.P.getGlobal(I)->deref<T>() = S.Stk.pop<T>();
+  return true;
+}
+
+template <PrimType Name, class T = typename PrimConv<Name>::T>
+bool InitThisField(InterpState &S, CodePtr OpPC, uint32_t I) {
+  if (S.checkingPotentialConstantExpression())
+    return false;
+  const Pointer &This = S.Current->getThis();
+  if (!CheckThis(S, OpPC, This))
+    return false;
+  const Pointer &Field = This.atField(I);
+  Field.deref<T>() = S.Stk.pop<T>();
+  Field.initialize();
+  return true;
+}
+
+template <PrimType Name, class T = typename PrimConv<Name>::T>
+bool InitThisBitField(InterpState &S, CodePtr OpPC, const Record::Field *F) {
+  if (S.checkingPotentialConstantExpression())
+    return false;
+  const Pointer &This = S.Current->getThis();
+  if (!CheckThis(S, OpPC, This))
+    return false;
+  const Pointer &Field = This.atField(F->Offset);
+  const auto &Value = S.Stk.pop<T>();
+  Field.deref<T>() = Value.truncate(F->Decl->getBitWidthValue(S.getCtx()));
+  Field.initialize();
+  return true;
+}
+
+template <PrimType Name, class T = typename PrimConv<Name>::T>
+bool InitThisFieldActive(InterpState &S, CodePtr OpPC, uint32_t I) {
+  if (S.checkingPotentialConstantExpression())
+    return false;
+  const Pointer &This = S.Current->getThis();
+  if (!CheckThis(S, OpPC, This))
+    return false;
+  const Pointer &Field = This.atField(I);
+  Field.deref<T>() = S.Stk.pop<T>();
+  Field.activate();
+  Field.initialize();
+  return true;
+}
+
+template <PrimType Name, class T = typename PrimConv<Name>::T>
+bool InitField(InterpState &S, CodePtr OpPC, uint32_t I) {
+  const T &Value = S.Stk.pop<T>();
+  const Pointer &Field = S.Stk.pop<Pointer>().atField(I);
+  Field.deref<T>() = Value;
+  Field.activate();
+  Field.initialize();
+  return true;
+}
+
+template <PrimType Name, class T = typename PrimConv<Name>::T>
+bool InitBitField(InterpState &S, CodePtr OpPC, const Record::Field *F) {
+  const T &Value = S.Stk.pop<T>();
+  const Pointer &Field = S.Stk.pop<Pointer>().atField(F->Offset);
+  Field.deref<T>() = Value.truncate(F->Decl->getBitWidthValue(S.getCtx()));
+  Field.activate();
+  Field.initialize();
+  return true;
+}
+
+template <PrimType Name, class T = typename PrimConv<Name>::T>
+bool InitFieldActive(InterpState &S, CodePtr OpPC, uint32_t I) {
+  const T &Value = S.Stk.pop<T>();
+  const Pointer &Ptr = S.Stk.pop<Pointer>();
+  const Pointer &Field = Ptr.atField(I);
+  Field.deref<T>() = Value;
+  Field.activate();
+  Field.initialize();
+  return true;
+}
+
+//===----------------------------------------------------------------------===//
+// GetPtr Local/Param/Global/Field/This
+//===----------------------------------------------------------------------===//
+
+inline bool GetPtrLocal(InterpState &S, CodePtr OpPC, uint32_t I) {
+  S.Stk.push<Pointer>(S.Current->getLocalPointer(I));
+  return true;
+}
+
+inline bool GetPtrParam(InterpState &S, CodePtr OpPC, uint32_t I) {
+  if (S.checkingPotentialConstantExpression()) {
+    return false;
+  }
+  S.Stk.push<Pointer>(S.Current->getParamPointer(I));
+  return true;
+}
+
+inline bool GetPtrGlobal(InterpState &S, CodePtr OpPC, uint32_t I) {
+  S.Stk.push<Pointer>(S.P.getPtrGlobal(I));
+  return true;
+}
+
+inline bool GetPtrField(InterpState &S, CodePtr OpPC, uint32_t Off) {
+  const Pointer &Ptr = S.Stk.pop<Pointer>();
+  if (!CheckNull(S, OpPC, Ptr, CSK_Field))
+    return false;
+  if (!CheckExtern(S, OpPC, Ptr))
+    return false;
+  if (!CheckRange(S, OpPC, Ptr, CSK_Field))
+    return false;
+  S.Stk.push<Pointer>(Ptr.atField(Off));
+  return true;
+}
+
+inline bool GetPtrThisField(InterpState &S, CodePtr OpPC, uint32_t Off) {
+  if (S.checkingPotentialConstantExpression())
+    return false;
+  const Pointer &This = S.Current->getThis();
+  if (!CheckThis(S, OpPC, This))
+    return false;
+  S.Stk.push<Pointer>(This.atField(Off));
+  return true;
+}
+
+inline bool GetPtrActiveField(InterpState &S, CodePtr OpPC, uint32_t Off) {
+  const Pointer &Ptr = S.Stk.pop<Pointer>();
+  if (!CheckNull(S, OpPC, Ptr, CSK_Field))
+    return false;
+  if (!CheckRange(S, OpPC, Ptr, CSK_Field))
+    return false;
+  Pointer Field = Ptr.atField(Off);
+  Ptr.deactivate();
+  Field.activate();
+  S.Stk.push<Pointer>(std::move(Field));
+  return true;
+}
+
+inline bool GetPtrActiveThisField(InterpState &S, CodePtr OpPC, uint32_t Off) {
+ if (S.checkingPotentialConstantExpression())
+    return false;
+  const Pointer &This = S.Current->getThis();
+  if (!CheckThis(S, OpPC, This))
+    return false;
+  Pointer Field = This.atField(Off);
+  This.deactivate();
+  Field.activate();
+  S.Stk.push<Pointer>(std::move(Field));
+  return true;
+}
+
+inline bool GetPtrBase(InterpState &S, CodePtr OpPC, uint32_t Off) {
+  const Pointer &Ptr = S.Stk.pop<Pointer>();
+  if (!CheckNull(S, OpPC, Ptr, CSK_Base))
+    return false;
+  S.Stk.push<Pointer>(Ptr.atField(Off));
+  return true;
+}
+
+inline bool GetPtrThisBase(InterpState &S, CodePtr OpPC, uint32_t Off) {
+  if (S.checkingPotentialConstantExpression())
+    return false;
+  const Pointer &This = S.Current->getThis();
+  if (!CheckThis(S, OpPC, This))
+    return false;
+  S.Stk.push<Pointer>(This.atField(Off));
+  return true;
+}
+
+inline bool VirtBaseHelper(InterpState &S, CodePtr OpPC, const RecordDecl *Decl,
+                           const Pointer &Ptr) {
+  Pointer Base = Ptr;
+  while (Base.isBaseClass())
+    Base = Base.getBase();
+
+  auto *Field = Base.getRecord()->getVirtualBase(Decl);
+  S.Stk.push<Pointer>(Base.atField(Field->Offset));
+  return true;
+}
+
+inline bool GetPtrVirtBase(InterpState &S, CodePtr OpPC, const RecordDecl *D) {
+  const Pointer &Ptr = S.Stk.pop<Pointer>();
+  if (!CheckNull(S, OpPC, Ptr, CSK_Base))
+    return false;
+  return VirtBaseHelper(S, OpPC, D, Ptr);
+}
+
+inline bool GetPtrThisVirtBase(InterpState &S, CodePtr OpPC,
+                               const RecordDecl *D) {
+  if (S.checkingPotentialConstantExpression())
+    return false;
+  const Pointer &This = S.Current->getThis();
+  if (!CheckThis(S, OpPC, This))
+    return false;
+  return VirtBaseHelper(S, OpPC, D, S.Current->getThis());
+}
+
+//===----------------------------------------------------------------------===//
+// Load, Store, Init
+//===----------------------------------------------------------------------===//
+
+template <PrimType Name, class T = typename PrimConv<Name>::T>
+bool Load(InterpState &S, CodePtr OpPC) {
+  const Pointer &Ptr = S.Stk.peek<Pointer>();
+  if (!CheckLoad(S, OpPC, Ptr))
+    return false;
+  S.Stk.push<T>(Ptr.deref<T>());
+  return true;
+}
+
+template <PrimType Name, class T = typename PrimConv<Name>::T>
+bool LoadPop(InterpState &S, CodePtr OpPC) {
+  const Pointer &Ptr = S.Stk.pop<Pointer>();
+  if (!CheckLoad(S, OpPC, Ptr))
+    return false;
+  S.Stk.push<T>(Ptr.deref<T>());
+  return true;
+}
+
+template <PrimType Name, class T = typename PrimConv<Name>::T>
+bool Store(InterpState &S, CodePtr OpPC) {
+  const T &Value = S.Stk.pop<T>();
+  const Pointer &Ptr = S.Stk.peek<Pointer>();
+  if (!CheckStore(S, OpPC, Ptr))
+    return false;
+  Ptr.deref<T>() = Value;
+  return true;
+}
+
+template <PrimType Name, class T = typename PrimConv<Name>::T>
+bool StorePop(InterpState &S, CodePtr OpPC) {
+  const T &Value = S.Stk.pop<T>();
+  const Pointer &Ptr = S.Stk.pop<Pointer>();
+  if (!CheckStore(S, OpPC, Ptr))
+    return false;
+  Ptr.deref<T>() = Value;
+  return true;
+}
+
+template <PrimType Name, class T = typename PrimConv<Name>::T>
+bool StoreBitField(InterpState &S, CodePtr OpPC) {
+  const T &Value = S.Stk.pop<T>();
+  const Pointer &Ptr = S.Stk.peek<Pointer>();
+  if (!CheckStore(S, OpPC, Ptr))
+    return false;
+  if (auto *FD = Ptr.getField()) {
+    Ptr.deref<T>() = Value.truncate(FD->getBitWidthValue(S.getCtx()));
+  } else {
+    Ptr.deref<T>() = Value;
+  }
+  return true;
+}
+
+template <PrimType Name, class T = typename PrimConv<Name>::T>
+bool StoreBitFieldPop(InterpState &S, CodePtr OpPC) {
+  const T &Value = S.Stk.pop<T>();
+  const Pointer &Ptr = S.Stk.pop<Pointer>();
+  if (!CheckStore(S, OpPC, Ptr))
+    return false;
+  if (auto *FD = Ptr.getField()) {
+    Ptr.deref<T>() = Value.truncate(FD->getBitWidthValue(S.getCtx()));
+  } else {
+    Ptr.deref<T>() = Value;
+  }
+  return true;
+}
+
+template <PrimType Name, class T = typename PrimConv<Name>::T>
+bool InitPop(InterpState &S, CodePtr OpPC) {
+  const T &Value = S.Stk.pop<T>();
+  const Pointer &Ptr = S.Stk.pop<Pointer>();
+  if (!CheckInit(S, OpPC, Ptr))
+    return false;
+  Ptr.initialize();
+  new (&Ptr.deref<T>()) T(Value);
+  return true;
+}
+
+template <PrimType Name, class T = typename PrimConv<Name>::T>
+bool InitElem(InterpState &S, CodePtr OpPC, uint32_t Idx) {
+  const T &Value = S.Stk.pop<T>();
+  const Pointer &Ptr = S.Stk.peek<Pointer>().atIndex(Idx);
+  if (!CheckInit(S, OpPC, Ptr))
+    return false;
+  Ptr.initialize();
+  new (&Ptr.deref<T>()) T(Value);
+  return true;
+}
+
+template <PrimType Name, class T = typename PrimConv<Name>::T>
+bool InitElemPop(InterpState &S, CodePtr OpPC, uint32_t Idx) {
+  const T &Value = S.Stk.pop<T>();
+  const Pointer &Ptr = S.Stk.pop<Pointer>().atIndex(Idx);
+  if (!CheckInit(S, OpPC, Ptr))
+    return false;
+  Ptr.initialize();
+  new (&Ptr.deref<T>()) T(Value);
+  return true;
+}
+
+//===----------------------------------------------------------------------===//
+// AddOffset, SubOffset
+//===----------------------------------------------------------------------===//
+
+template <class T, bool Add> bool OffsetHelper(InterpState &S, CodePtr OpPC) {
+  // Fetch the pointer and the offset.
+  const T &Offset = S.Stk.pop<T>();
+  const Pointer &Ptr = S.Stk.pop<Pointer>();
+  if (!CheckNull(S, OpPC, Ptr, CSK_ArrayIndex))
+    return false;
+  if (!CheckRange(S, OpPC, Ptr, CSK_ArrayToPointer))
+    return false;
+
+  // Get a version of the index comparable to the type.
+  T Index = T::from(Ptr.getIndex(), Offset.bitWidth());
+  // A zero offset does not change the pointer, but in the case of an array
+  // it has to be adjusted to point to the first element instead of the array.
+  if (Offset.isZero()) {
+    S.Stk.push<Pointer>(Index.isZero() ? Ptr.atIndex(0) : Ptr);
+    return true;
+  }
+  // Arrays of unknown bounds cannot have pointers into them.
+  if (!CheckArray(S, OpPC, Ptr))
+    return false;
+
+  // Compute the largest index into the array.
+  unsigned MaxIndex = Ptr.getNumElems();
+
+  // Helper to report an invalid offset, computed as APSInt.
+  auto InvalidOffset = [&]() {
+    const unsigned Bits = Offset.bitWidth();
+    APSInt APOffset(Offset.toAPSInt().extend(Bits + 2), false);
+    APSInt APIndex(Index.toAPSInt().extend(Bits + 2), false);
+    APSInt NewIndex = Add ? (APIndex + APOffset) : (APIndex - APOffset);
+    S.CCEDiag(S.Current->getSource(OpPC), diag::note_constexpr_array_index)
+        << NewIndex
+        << /*array*/ static_cast<int>(!Ptr.inArray())
+        << static_cast<unsigned>(MaxIndex);
+    return false;
+  };
+
+  // If the new offset would be negative, bail out.
+  if (Add && Offset.isNegative() && (Offset.isMin() || -Offset > Index))
+    return InvalidOffset();
+  if (!Add && Offset.isPositive() && Index < Offset)
+    return InvalidOffset();
+
+  // If the new offset would be out of bounds, bail out.
+  unsigned MaxOffset = MaxIndex - Ptr.getIndex();
+  if (Add && Offset.isPositive() && Offset > MaxOffset)
+    return InvalidOffset();
+  if (!Add && Offset.isNegative() && (Offset.isMin() || -Offset > MaxOffset))
+    return InvalidOffset();
+
+  // Offset is valid - compute it on unsigned.
+  int64_t WideIndex = static_cast<int64_t>(Index);
+  int64_t WideOffset = static_cast<int64_t>(Offset);
+  int64_t Result = Add ? (WideIndex + WideOffset) : (WideIndex - WideOffset);
+  S.Stk.push<Pointer>(Ptr.atIndex(static_cast<unsigned>(Result)));
+  return true;
+}
+
+template <PrimType Name, class T = typename PrimConv<Name>::T>
+bool AddOffset(InterpState &S, CodePtr OpPC) {
+  return OffsetHelper<T, true>(S, OpPC);
+}
+
+template <PrimType Name, class T = typename PrimConv<Name>::T>
+bool SubOffset(InterpState &S, CodePtr OpPC) {
+  return OffsetHelper<T, false>(S, OpPC);
+}
+
+
+//===----------------------------------------------------------------------===//
+// Destroy
+//===----------------------------------------------------------------------===//
+
+inline bool Destroy(InterpState &S, CodePtr OpPC, uint32_t I) {
+  S.Current->destroy(I);
+  return true;
+}
+
+//===----------------------------------------------------------------------===//
+// Cast, CastFP
+//===----------------------------------------------------------------------===//
+
+template <PrimType TIn, PrimType TOut> bool Cast(InterpState &S, CodePtr OpPC) {
+  using T = typename PrimConv<TIn>::T;
+  using U = typename PrimConv<TOut>::T;
+  S.Stk.push<U>(U::from(S.Stk.pop<T>()));
+  return true;
+}
+
+//===----------------------------------------------------------------------===//
+// Zero, Nullptr
+//===----------------------------------------------------------------------===//
+
+template <PrimType Name, class T = typename PrimConv<Name>::T>
+bool Zero(InterpState &S, CodePtr OpPC) {
+  S.Stk.push<T>(T::zero());
+  return true;
+}
+
+template <PrimType Name, class T = typename PrimConv<Name>::T>
+inline bool Null(InterpState &S, CodePtr OpPC) {
+  S.Stk.push<T>();
+  return true;
+}
+
+//===----------------------------------------------------------------------===//
+// This, ImplicitThis
+//===----------------------------------------------------------------------===//
+
+inline bool This(InterpState &S, CodePtr OpPC) {
+  // Cannot read 'this' in this mode.
+  if (S.checkingPotentialConstantExpression()) {
+    return false;
+  }
+
+  const Pointer &This = S.Current->getThis();
+  if (!CheckThis(S, OpPC, This))
+    return false;
+
+  S.Stk.push<Pointer>(This);
+  return true;
+}
+
+//===----------------------------------------------------------------------===//
+// Shr, Shl
+//===----------------------------------------------------------------------===//
+
+template <PrimType TR, PrimType TL, class T = typename PrimConv<TR>::T>
+unsigned Trunc(InterpState &S, CodePtr OpPC, unsigned Bits, const T &V) {
+  // C++11 [expr.shift]p1: Shift width must be less than the bit width of
+  // the shifted type.
+  if (Bits > 1 && V >= T::from(Bits, V.bitWidth())) {
+    const Expr *E = S.Current->getExpr(OpPC);
+    const APSInt Val = V.toAPSInt();
+    QualType Ty = E->getType();
+    S.CCEDiag(E, diag::note_constexpr_large_shift) << Val << Ty << Bits;
+    return Bits;
+  } else {
+    return static_cast<unsigned>(V);
+  }
+}
+
+template <PrimType TL, PrimType TR, typename T = typename PrimConv<TL>::T>
+inline bool ShiftRight(InterpState &S, CodePtr OpPC, const T &V, unsigned RHS) {
+  if (RHS >= V.bitWidth()) {
+    S.Stk.push<T>(T::from(0, V.bitWidth()));
+  } else {
+    S.Stk.push<T>(T::from(V >> RHS, V.bitWidth()));
+  }
+  return true;
+}
+
+template <PrimType TL, PrimType TR, typename T = typename PrimConv<TL>::T>
+inline bool ShiftLeft(InterpState &S, CodePtr OpPC, const T &V, unsigned RHS) {
+  if (V.isSigned() && !S.getLangOpts().CPlusPlus2a) {
+    // C++11 [expr.shift]p2: A signed left shift must have a non-negative
+    // operand, and must not overflow the corresponding unsigned type.
+    // C++2a [expr.shift]p2: E1 << E2 is the unique value congruent to
+    // E1 x 2^E2 module 2^N.
+    if (V.isNegative()) {
+      const Expr *E = S.Current->getExpr(OpPC);
+      S.CCEDiag(E, diag::note_constexpr_lshift_of_negative) << V.toAPSInt();
+    } else if (V.countLeadingZeros() < RHS) {
+      S.CCEDiag(S.Current->getExpr(OpPC), diag::note_constexpr_lshift_discards);
+    }
+  }
+
+  if (V.bitWidth() == 1) {
+    S.Stk.push<T>(V);
+  } else if (RHS >= V.bitWidth()) {
+    S.Stk.push<T>(T::from(0, V.bitWidth()));
+  } else {
+    S.Stk.push<T>(T::from(V.toUnsigned() << RHS, V.bitWidth()));
+  }
+  return true;
+}
+
+template <PrimType TL, PrimType TR>
+inline bool Shr(InterpState &S, CodePtr OpPC) {
+  const auto &RHS = S.Stk.pop<typename PrimConv<TR>::T>();
+  const auto &LHS = S.Stk.pop<typename PrimConv<TL>::T>();
+  const unsigned Bits = LHS.bitWidth();
+
+  if (RHS.isSigned() && RHS.isNegative()) {
+    const SourceInfo &Loc = S.Current->getSource(OpPC);
+    S.CCEDiag(Loc, diag::note_constexpr_negative_shift) << RHS.toAPSInt();
+    return ShiftLeft<TL, TR>(S, OpPC, LHS, Trunc<TR, TL>(S, OpPC, Bits, -RHS));
+  } else {
+    return ShiftRight<TL, TR>(S, OpPC, LHS, Trunc<TR, TL>(S, OpPC, Bits, RHS));
+  }
+}
+
+template <PrimType TL, PrimType TR>
+inline bool Shl(InterpState &S, CodePtr OpPC) {
+  const auto &RHS = S.Stk.pop<typename PrimConv<TR>::T>();
+  const auto &LHS = S.Stk.pop<typename PrimConv<TL>::T>();
+  const unsigned Bits = LHS.bitWidth();
+
+  if (RHS.isSigned() && RHS.isNegative()) {
+    const SourceInfo &Loc = S.Current->getSource(OpPC);
+    S.CCEDiag(Loc, diag::note_constexpr_negative_shift) << RHS.toAPSInt();
+    return ShiftRight<TL, TR>(S, OpPC, LHS, Trunc<TR, TL>(S, OpPC, Bits, -RHS));
+  } else {
+    return ShiftLeft<TL, TR>(S, OpPC, LHS, Trunc<TR, TL>(S, OpPC, Bits, RHS));
+  }
+}
+
+//===----------------------------------------------------------------------===//
+// NoRet
+//===----------------------------------------------------------------------===//
+
+inline bool NoRet(InterpState &S, CodePtr OpPC) {
+  SourceLocation EndLoc = S.Current->getCallee()->getEndLoc();
+  S.FFDiag(EndLoc, diag::note_constexpr_no_return);
+  return false;
+}
+
+//===----------------------------------------------------------------------===//
+// NarrowPtr, ExpandPtr
+//===----------------------------------------------------------------------===//
+
+inline bool NarrowPtr(InterpState &S, CodePtr OpPC) {
+  const Pointer &Ptr = S.Stk.pop<Pointer>();
+  S.Stk.push<Pointer>(Ptr.narrow());
+  return true;
+}
+
+inline bool ExpandPtr(InterpState &S, CodePtr OpPC) {
+  const Pointer &Ptr = S.Stk.pop<Pointer>();
+  S.Stk.push<Pointer>(Ptr.expand());
+  return true;
+}
+
+/// Interpreter entry point.
+bool Interpret(InterpState &S, APValue &Result);
+
+} // namespace interp
+} // namespace clang
+
+#endif

lib/AST/Interp/InterpFrame.cpp

@@ -0,0 +1,193 @@
+//===--- InterpFrame.cpp - Call Frame implementation for the VM -*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "InterpFrame.h"
+#include "Function.h"
+#include "Interp.h"
+#include "InterpStack.h"
+#include "PrimType.h"
+#include "Program.h"
+#include "clang/AST/DeclCXX.h"
+
+using namespace clang;
+using namespace clang::interp;
+
+InterpFrame::InterpFrame(InterpState &S, Function *Func, InterpFrame *Caller,
+                         CodePtr RetPC, Pointer &&This)
+    : Caller(Caller), S(S), Func(Func), This(std::move(This)), RetPC(RetPC),
+      ArgSize(Func ? Func->getArgSize() : 0),
+      Args(static_cast<char *>(S.Stk.top())), FrameOffset(S.Stk.size()) {
+  if (Func) {
+    if (unsigned FrameSize = Func->getFrameSize()) {
+      Locals = std::make_unique<char[]>(FrameSize);
+      for (auto &Scope : Func->scopes()) {
+        for (auto &Local : Scope.locals()) {
+          Block *B = new (localBlock(Local.Offset)) Block(Local.Desc);
+          B->invokeCtor();
+        }
+      }
+    }
+  }
+}
+
+InterpFrame::~InterpFrame() {
+  if (Func && Func->isConstructor() && This.isBaseClass())
+    This.initialize();
+  for (auto &Param : Params)
+    S.deallocate(reinterpret_cast<Block *>(Param.second.get()));
+}
+
+void InterpFrame::destroy(unsigned Idx) {
+  for (auto &Local : Func->getScope(Idx).locals()) {
+    S.deallocate(reinterpret_cast<Block *>(localBlock(Local.Offset)));
+  }
+}
+
+void InterpFrame::popArgs() {
+  for (PrimType Ty : Func->args_reverse())
+    TYPE_SWITCH(Ty, S.Stk.discard<T>());
+}
+
+template <typename T>
+static void print(llvm::raw_ostream &OS, const T &V, ASTContext &, QualType) {
+  OS << V;
+}
+
+template <>
+void print(llvm::raw_ostream &OS, const Pointer &P, ASTContext &Ctx,
+           QualType Ty) {
+  if (P.isZero()) {
+    OS << "nullptr";
+    return;
+  }
+
+  auto printDesc = [&OS, &Ctx](Descriptor *Desc) {
+    if (auto *D = Desc->asDecl()) {
+      // Subfields or named values.
+      if (auto *VD = dyn_cast<ValueDecl>(D)) {
+        OS << *VD;
+        return;
+      }
+      // Base classes.
+      if (isa<RecordDecl>(D)) {
+        return;
+      }
+    }
+    // Temporary expression.
+    if (auto *E = Desc->asExpr()) {
+      E->printPretty(OS, nullptr, Ctx.getPrintingPolicy());
+      return;
+    }
+    llvm_unreachable("Invalid descriptor type");
+  };
+
+  if (!Ty->isReferenceType())
+    OS << "&";
+  llvm::SmallVector<Pointer, 2> Levels;
+  for (Pointer F = P; !F.isRoot(); ) {
+    Levels.push_back(F);
+    F = F.isArrayElement() ? F.getArray().expand() : F.getBase();
+  }
+
+  printDesc(P.getDeclDesc());
+  for (auto It = Levels.rbegin(); It != Levels.rend(); ++It) {
+    if (It->inArray()) {
+      OS << "[" << It->expand().getIndex() << "]";
+      continue;
+    }
+    if (auto Index = It->getIndex()) {
+      OS << " + " << Index;
+      continue;
+    }
+    OS << ".";
+    printDesc(It->getFieldDesc());
+  }
+}
+
+void InterpFrame::describe(llvm::raw_ostream &OS) {
+  const FunctionDecl *F = getCallee();
+  auto *M = dyn_cast<CXXMethodDecl>(F);
+  if (M && M->isInstance() && !isa<CXXConstructorDecl>(F)) {
+    print(OS, This, S.getCtx(), S.getCtx().getRecordType(M->getParent()));
+    OS << "->";
+  }
+  OS << *F << "(";
+  unsigned Off = Func->hasRVO() ? primSize(PT_Ptr) : 0;
+  for (unsigned I = 0, N = F->getNumParams(); I < N; ++I) {
+    QualType Ty = F->getParamDecl(I)->getType();
+
+    PrimType PrimTy;
+    if (llvm::Optional<PrimType> T = S.Ctx.classify(Ty)) {
+      PrimTy = *T;
+    } else {
+      PrimTy = PT_Ptr;
+    }
+
+    TYPE_SWITCH(PrimTy, print(OS, stackRef<T>(Off), S.getCtx(), Ty));
+    Off += align(primSize(PrimTy));
+    if (I + 1 != N)
+      OS << ", ";
+  }
+  OS << ")";
+}
+
+Frame *InterpFrame::getCaller() const {
+  if (Caller->Caller)
+    return Caller;
+  return S.getSplitFrame();
+}
+
+SourceLocation InterpFrame::getCallLocation() const {
+  if (!Caller->Func)
+    return S.getLocation(nullptr, {});
+  return S.getLocation(Caller->Func, RetPC - sizeof(uintptr_t));
+}
+
+const FunctionDecl *InterpFrame::getCallee() const {
+  return Func->getDecl();
+}
+
+Pointer InterpFrame::getLocalPointer(unsigned Offset) {
+  assert(Offset < Func->getFrameSize() && "Invalid local offset.");
+  return Pointer(
+      reinterpret_cast<Block *>(Locals.get() + Offset - sizeof(Block)));
+}
+
+Pointer InterpFrame::getParamPointer(unsigned Off) {
+  // Return the block if it was created previously.
+  auto Pt = Params.find(Off);
+  if (Pt != Params.end()) {
+    return Pointer(reinterpret_cast<Block *>(Pt->second.get()));
+  }
+
+  // Allocate memory to store the parameter and the block metadata.
+  const auto &Desc = Func->getParamDescriptor(Off);
+  size_t BlockSize = sizeof(Block) + Desc.second->getAllocSize();
+  auto Memory = std::make_unique<char[]>(BlockSize);
+  auto *B = new (Memory.get()) Block(Desc.second);
+
+  // Copy the initial value.
+  TYPE_SWITCH(Desc.first, new (B->data()) T(stackRef<T>(Off)));
+
+  // Record the param.
+  Params.insert({Off, std::move(Memory)});
+  return Pointer(B);
+}
+
+SourceInfo InterpFrame::getSource(CodePtr PC) const {
+  return S.getSource(Func, PC);
+}
+
+const Expr *InterpFrame::getExpr(CodePtr PC) const {
+  return S.getExpr(Func, PC);
+}
+
+SourceLocation InterpFrame::getLocation(CodePtr PC) const {
+  return S.getLocation(Func, PC);
+}
+

lib/AST/Interp/InterpFrame.h

@@ -0,0 +1,153 @@
+//===--- InterpFrame.h - Call Frame implementation for the VM ---*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Defines the class storing information about stack frames in the interpreter.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_AST_INTERP_INTERPFRAME_H
+#define LLVM_CLANG_AST_INTERP_INTERPFRAME_H
+
+#include "Frame.h"
+#include "Pointer.h"
+#include "Program.h"
+#include "State.h"
+#include <cstdint>
+#include <vector>
+
+namespace clang {
+namespace interp {
+class Function;
+class InterpState;
+
+/// Frame storing local variables.
+class InterpFrame final : public Frame {
+public:
+  /// The frame of the previous function.
+  InterpFrame *Caller;
+
+  /// Creates a new frame for a method call.
+  InterpFrame(InterpState &S, Function *Func, InterpFrame *Caller,
+              CodePtr RetPC, Pointer &&This);
+
+  /// Destroys the frame, killing all live pointers to stack slots.
+  ~InterpFrame();
+
+  /// Invokes the destructors for a scope.
+  void destroy(unsigned Idx);
+
+  /// Pops the arguments off the stack.
+  void popArgs();
+
+  /// Describes the frame with arguments for diagnostic purposes.
+  void describe(llvm::raw_ostream &OS);
+
+  /// Returns the parent frame object.
+  Frame *getCaller() const;
+
+  /// Returns the location of the call to the frame.
+  SourceLocation getCallLocation() const;
+
+  /// Returns the caller.
+  const FunctionDecl *getCallee() const;
+
+  /// Returns the current function.
+  Function *getFunction() const { return Func; }
+
+  /// Returns the offset on the stack at which the frame starts.
+  size_t getFrameOffset() const { return FrameOffset; }
+
+  /// Returns the value of a local variable.
+  template <typename T> const T &getLocal(unsigned Offset) {
+    return localRef<T>(Offset);
+  }
+
+  /// Mutates a local variable.
+  template <typename T> void setLocal(unsigned Offset, const T &Value) {
+    localRef<T>(Offset) = Value;
+  }
+
+  /// Returns a pointer to a local variables.
+  Pointer getLocalPointer(unsigned Offset);
+
+  /// Returns the value of an argument.
+  template <typename T> const T &getParam(unsigned Offset) {
+    auto Pt = Params.find(Offset);
+    if (Pt == Params.end()) {
+      return stackRef<T>(Offset);
+    } else {
+      return Pointer(reinterpret_cast<Block *>(Pt->second.get())).deref<T>();
+    }
+  }
+
+  /// Mutates a local copy of a parameter.
+  template <typename T> void setParam(unsigned Offset, const T &Value) {
+     getParamPointer(Offset).deref<T>() = Value;
+  }
+
+  /// Returns a pointer to an argument - lazily creates a block.
+  Pointer getParamPointer(unsigned Offset);
+
+  /// Returns the 'this' pointer.
+  const Pointer &getThis() const { return This; }
+
+  /// Checks if the frame is a root frame - return should quit the interpreter.
+  bool isRoot() const { return !Func; }
+
+  /// Returns the PC of the frame's code start.
+  CodePtr getPC() const { return Func->getCodeBegin(); }
+
+  /// Returns the return address of the frame.
+  CodePtr getRetPC() const { return RetPC; }
+
+  /// Map a location to a source.
+  virtual SourceInfo getSource(CodePtr PC) const;
+  const Expr *getExpr(CodePtr PC) const;
+  SourceLocation getLocation(CodePtr PC) const;
+
+private:
+  /// Returns an original argument from the stack.
+  template <typename T> const T &stackRef(unsigned Offset) {
+    return *reinterpret_cast<const T *>(Args - ArgSize + Offset);
+  }
+
+  /// Returns an offset to a local.
+  template <typename T> T &localRef(unsigned Offset) {
+    return *reinterpret_cast<T *>(Locals.get() + Offset);
+  }
+
+  /// Returns a pointer to a local's block.
+  void *localBlock(unsigned Offset) {
+    return Locals.get() + Offset - sizeof(Block);
+  }
+
+private:
+  /// Reference to the interpreter state.
+  InterpState &S;
+  /// Reference to the function being executed.
+  Function *Func;
+  /// Current object pointer for methods.
+  Pointer This;
+  /// Return address.
+  CodePtr RetPC;
+  /// The size of all the arguments.
+  const unsigned ArgSize;
+  /// Pointer to the arguments in the callee's frame.
+  char *Args = nullptr;
+  /// Fixed, initial storage for known local variables.
+  std::unique_ptr<char[]> Locals;
+  /// Offset on the stack at entry.
+  const size_t FrameOffset;
+  /// Mapping from arg offsets to their argument blocks.
+  llvm::DenseMap<unsigned, std::unique_ptr<char[]>> Params;
+};
+
+} // namespace interp
+} // namespace clang
+
+#endif

lib/AST/Interp/InterpStack.cpp

@@ -0,0 +1,77 @@
+//===--- InterpStack.cpp - Stack implementation for the VM ------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include <cassert>
+#include "InterpStack.h"
+
+using namespace clang;
+using namespace clang::interp;
+
+InterpStack::~InterpStack() {
+  clear();
+}
+
+void InterpStack::clear() {
+  if (Chunk && Chunk->Next)
+    free(Chunk->Next);
+  if (Chunk)
+    free(Chunk);
+  Chunk = nullptr;
+  StackSize = 0;
+}
+
+void *InterpStack::grow(size_t Size) {
+  assert(Size < ChunkSize - sizeof(StackChunk) && "Object too large");
+
+  if (!Chunk || sizeof(StackChunk) + Chunk->size() + Size > ChunkSize) {
+    if (Chunk && Chunk->Next) {
+      Chunk = Chunk->Next;
+    } else {
+      StackChunk *Next = new (malloc(ChunkSize)) StackChunk(Chunk);
+      if (Chunk)
+        Chunk->Next = Next;
+      Chunk = Next;
+    }
+  }
+
+  auto *Object = reinterpret_cast<void *>(Chunk->End);
+  Chunk->End += Size;
+  StackSize += Size;
+  return Object;
+}
+
+void *InterpStack::peek(size_t Size) {
+  assert(Chunk && "Stack is empty!");
+
+  StackChunk *Ptr = Chunk;
+  while (Size > Ptr->size()) {
+    Size -= Ptr->size();
+    Ptr = Ptr->Prev;
+    assert(Ptr && "Offset too large");
+  }
+
+  return reinterpret_cast<void *>(Ptr->End - Size);
+}
+
+void InterpStack::shrink(size_t Size) {
+  assert(Chunk && "Chunk is empty!");
+
+  while (Size > Chunk->size()) {
+    Size -= Chunk->size();
+    if (Chunk->Next) {
+      free(Chunk->Next);
+      Chunk->Next = nullptr;
+    }
+    Chunk->End = Chunk->start();
+    Chunk = Chunk->Prev;
+    assert(Chunk && "Offset too large");
+  }
+
+  Chunk->End -= Size;
+  StackSize -= Size;
+}

lib/AST/Interp/InterpStack.h

@@ -0,0 +1,113 @@
+//===--- InterpStack.h - Stack implementation for the VM --------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Defines the upwards-growing stack used by the interpreter.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_AST_INTERP_INTERPSTACK_H
+#define LLVM_CLANG_AST_INTERP_INTERPSTACK_H
+
+#include <memory>
+
+namespace clang {
+namespace interp {
+
+/// Stack frame storing temporaries and parameters.
+class InterpStack final {
+public:
+  InterpStack() {}
+
+  /// Destroys the stack, freeing up storage.
+  ~InterpStack();
+
+  /// Constructs a value in place on the top of the stack.
+  template <typename T, typename... Tys> void push(Tys &&... Args) {
+    new (grow(aligned_size<T>())) T(std::forward<Tys>(Args)...);
+  }
+
+  /// Returns the value from the top of the stack and removes it.
+  template <typename T> T pop() {
+    auto *Ptr = &peek<T>();
+    auto Value = std::move(*Ptr);
+    Ptr->~T();
+    shrink(aligned_size<T>());
+    return Value;
+  }
+
+  /// Discards the top value from the stack.
+  template <typename T> void discard() {
+    auto *Ptr = &peek<T>();
+    Ptr->~T();
+    shrink(aligned_size<T>());
+  }
+
+  /// Returns a reference to the value on the top of the stack.
+  template <typename T> T &peek() {
+    return *reinterpret_cast<T *>(peek(aligned_size<T>()));
+  }
+
+  /// Returns a pointer to the top object.
+  void *top() { return Chunk ? peek(0) : nullptr; }
+
+  /// Returns the size of the stack in bytes.
+  size_t size() const { return StackSize; }
+
+  /// Clears the stack without calling any destructors.
+  void clear();
+
+private:
+  /// All stack slots are aligned to the native pointer alignment for storage.
+  /// The size of an object is rounded up to a pointer alignment multiple.
+  template <typename T> constexpr size_t aligned_size() const {
+    constexpr size_t PtrAlign = alignof(void *);
+    return ((sizeof(T) + PtrAlign - 1) / PtrAlign) * PtrAlign;
+  }
+
+  /// Grows the stack to accomodate a value and returns a pointer to it.
+  void *grow(size_t Size);
+  /// Returns a pointer from the top of the stack.
+  void *peek(size_t Size);
+  /// Shrinks the stack.
+  void shrink(size_t Size);
+
+  /// Allocate stack space in 1Mb chunks.
+  static constexpr size_t ChunkSize = 1024 * 1024;
+
+  /// Metadata for each stack chunk.
+  ///
+  /// The stack is composed of a linked list of chunks. Whenever an allocation
+  /// is out of bounds, a new chunk is linked. When a chunk becomes empty,
+  /// it is not immediately freed: a chunk is deallocated only when the
+  /// predecessor becomes empty.
+  struct StackChunk {
+    StackChunk *Next;
+    StackChunk *Prev;
+    char *End;
+
+    StackChunk(StackChunk *Prev = nullptr)
+        : Next(nullptr), Prev(Prev), End(reinterpret_cast<char *>(this + 1)) {}
+
+    /// Returns the size of the chunk, minus the header.
+    size_t size() { return End - start(); }
+
+    /// Returns a pointer to the start of the data region.
+    char *start() { return reinterpret_cast<char *>(this + 1); }
+  };
+  static_assert(sizeof(StackChunk) < ChunkSize, "Invalid chunk size");
+
+  /// First chunk on the stack.
+  StackChunk *Chunk = nullptr;
+  /// Total size of the stack.
+  size_t StackSize = 0;
+};
+
+} // namespace interp
+} // namespace clang
+
+#endif

lib/AST/Interp/InterpState.cpp

@@ -0,0 +1,74 @@
+//===--- InterpState.cpp - Interpreter for the constexpr VM -----*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "InterpState.h"
+#include <limits>
+#include "Function.h"
+#include "InterpFrame.h"
+#include "InterpStack.h"
+#include "Opcode.h"
+#include "PrimType.h"
+#include "Program.h"
+#include "State.h"
+
+using namespace clang;
+using namespace clang::interp;
+
+using APSInt = llvm::APSInt;
+
+InterpState::InterpState(State &Parent, Program &P, InterpStack &Stk,
+                         Context &Ctx, SourceMapper *M)
+    : Parent(Parent), M(M), P(P), Stk(Stk), Ctx(Ctx), Current(nullptr),
+      CallStackDepth(Parent.getCallStackDepth() + 1) {}
+
+InterpState::~InterpState() {
+  while (Current) {
+    InterpFrame *Next = Current->Caller;
+    delete Current;
+    Current = Next;
+  }
+
+  while (DeadBlocks) {
+    DeadBlock *Next = DeadBlocks->Next;
+    free(DeadBlocks);
+    DeadBlocks = Next;
+  }
+}
+
+Frame *InterpState::getCurrentFrame() {
+  if (Current && Current->Caller) {
+    return Current;
+  } else {
+    return Parent.getCurrentFrame();
+  }
+}
+
+bool InterpState::reportOverflow(const Expr *E, const llvm::APSInt &Value) {
+  QualType Type = E->getType();
+  CCEDiag(E, diag::note_constexpr_overflow) << Value << Type;
+  return noteUndefinedBehavior();
+}
+
+void InterpState::deallocate(Block *B) {
+  Descriptor *Desc = B->getDescriptor();
+  if (B->hasPointers()) {
+    size_t Size = B->getSize();
+
+    // Allocate a new block, transferring over pointers.
+    char *Memory = reinterpret_cast<char *>(malloc(sizeof(DeadBlock) + Size));
+    auto *D = new (Memory) DeadBlock(DeadBlocks, B);
+
+    // Move data from one block to another.
+    if (Desc->MoveFn)
+      Desc->MoveFn(B, B->data(), D->data(), Desc);
+  } else {
+    // Free storage, if necessary.
+    if (Desc->DtorFn)
+      Desc->DtorFn(B, B->data(), Desc);
+  }
+}

lib/AST/Interp/InterpState.h

@@ -0,0 +1,112 @@
+//===--- InterpState.h - Interpreter state for the constexpr VM -*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Definition of the interpreter state and entry point.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_AST_INTERP_INTERPSTATE_H
+#define LLVM_CLANG_AST_INTERP_INTERPSTATE_H
+
+#include "Context.h"
+#include "Function.h"
+#include "InterpStack.h"
+#include "State.h"
+#include "clang/AST/APValue.h"
+#include "clang/AST/ASTDiagnostic.h"
+#include "clang/AST/Expr.h"
+#include "clang/AST/OptionalDiagnostic.h"
+
+namespace clang {
+namespace interp {
+class Context;
+class Function;
+class InterpStack;
+class InterpFrame;
+class SourceMapper;
+
+/// Interpreter context.
+class InterpState final : public State, public SourceMapper {
+public:
+  InterpState(State &Parent, Program &P, InterpStack &Stk, Context &Ctx,
+              SourceMapper *M = nullptr);
+
+  ~InterpState();
+
+  // Stack frame accessors.
+  Frame *getSplitFrame() { return Parent.getCurrentFrame(); }
+  Frame *getCurrentFrame() override;
+  unsigned getCallStackDepth() override { return CallStackDepth; }
+  const Frame *getBottomFrame() const override {
+    return Parent.getBottomFrame();
+  }
+
+  // Acces objects from the walker context.
+  Expr::EvalStatus &getEvalStatus() const override {
+    return Parent.getEvalStatus();
+  }
+  ASTContext &getCtx() const override { return Parent.getCtx(); }
+
+  // Forward status checks and updates to the walker.
+  bool checkingForUndefinedBehavior() const override {
+    return Parent.checkingForUndefinedBehavior();
+  }
+  bool keepEvaluatingAfterFailure() const override {
+    return Parent.keepEvaluatingAfterFailure();
+  }
+  bool checkingPotentialConstantExpression() const override {
+    return Parent.checkingPotentialConstantExpression();
+  }
+  bool noteUndefinedBehavior() override {
+    return Parent.noteUndefinedBehavior();
+  }
+  bool hasActiveDiagnostic() override { return Parent.hasActiveDiagnostic(); }
+  void setActiveDiagnostic(bool Flag) override {
+    Parent.setActiveDiagnostic(Flag);
+  }
+  void setFoldFailureDiagnostic(bool Flag) override {
+    Parent.setFoldFailureDiagnostic(Flag);
+  }
+  bool hasPriorDiagnostic() override { return Parent.hasPriorDiagnostic(); }
+
+  /// Reports overflow and return true if evaluation should continue.
+  bool reportOverflow(const Expr *E, const llvm::APSInt &Value);
+
+  /// Deallocates a pointer.
+  void deallocate(Block *B);
+
+  /// Delegates source mapping to the mapper.
+  SourceInfo getSource(Function *F, CodePtr PC) const override {
+    return M ? M->getSource(F, PC) : F->getSource(PC);
+  }
+
+private:
+  /// AST Walker state.
+  State &Parent;
+  /// Dead block chain.
+  DeadBlock *DeadBlocks = nullptr;
+  /// Reference to the offset-source mapping.
+  SourceMapper *M;
+
+public:
+  /// Reference to the module containing all bytecode.
+  Program &P;
+  /// Temporary stack.
+  InterpStack &Stk;
+  /// Interpreter Context.
+  Context &Ctx;
+  /// The current frame.
+  InterpFrame *Current = nullptr;
+  /// Call stack depth.
+  unsigned CallStackDepth;
+};
+
+} // namespace interp
+} // namespace clang
+
+#endif

lib/AST/Interp/Opcode.h

@@ -0,0 +1,30 @@
+//===--- Opcode.h - Opcodes for the constexpr VM ----------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Defines all opcodes executed by the VM and emitted by the compiler.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_AST_INTERP_OPCODE_H
+#define LLVM_CLANG_AST_INTERP_OPCODE_H
+
+#include <cstdint>
+
+namespace clang {
+namespace interp {
+
+enum Opcode : uint32_t {
+#define GET_OPCODE_NAMES
+#include "Opcodes.inc"
+#undef GET_OPCODE_NAMES
+};
+
+} // namespace interp
+} // namespace clang
+
+#endif

lib/AST/Interp/Opcodes.td

@@ -0,0 +1,422 @@
+//===--- Opcodes.td - Opcode defitions for the constexpr VM -----*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Helper file used to generate opcodes, the interpreter and the disassembler.
+//
+//===----------------------------------------------------------------------===//
+
+
+//===----------------------------------------------------------------------===//
+// Types evaluated by the interpreter.
+//===----------------------------------------------------------------------===//
+
+class Type;
+def Bool : Type;
+def Sint8 : Type;
+def Uint8 : Type;
+def Sint16 : Type;
+def Uint16 : Type;
+def Sint32 : Type;
+def Uint32 : Type;
+def Sint64 : Type;
+def Uint64 : Type;
+def Ptr : Type;
+
+//===----------------------------------------------------------------------===//
+// Types transferred to the interpreter.
+//===----------------------------------------------------------------------===//
+
+class ArgType { string Name = ?; }
+def ArgSint8 : ArgType { let Name = "int8_t"; }
+def ArgUint8 : ArgType { let Name = "uint8_t"; }
+def ArgSint16 : ArgType { let Name = "int16_t"; }
+def ArgUint16 : ArgType { let Name = "uint16_t"; }
+def ArgSint32 : ArgType { let Name = "int32_t"; }
+def ArgUint32 : ArgType { let Name = "uint32_t"; }
+def ArgSint64 : ArgType { let Name = "int64_t"; }
+def ArgUint64 : ArgType { let Name = "uint64_t"; }
+def ArgBool : ArgType { let Name = "bool"; }
+
+def ArgFunction : ArgType { let Name = "Function *"; }
+def ArgRecord : ArgType { let Name = "Record *"; }
+
+def ArgSema : ArgType { let Name = "const fltSemantics *"; }
+
+def ArgExpr : ArgType { let Name = "const Expr *"; }
+def ArgFloatingLiteral : ArgType { let Name = "const FloatingLiteral *"; }
+def ArgCXXMethodDecl : ArgType { let Name = "const CXXMethodDecl *"; }
+def ArgFunctionDecl : ArgType { let Name = "const FunctionDecl *"; }
+def ArgRecordDecl : ArgType { let Name = "const RecordDecl *"; }
+def ArgCXXRecordDecl : ArgType { let Name = "const CXXRecordDecl *"; }
+def ArgValueDecl : ArgType { let Name = "const ValueDecl *"; }
+def ArgRecordField : ArgType { let Name = "const Record::Field *"; }
+
+//===----------------------------------------------------------------------===//
+// Classes of types intructions operate on.
+//===----------------------------------------------------------------------===//
+
+class TypeClass {
+  list<Type> Types;
+}
+
+def AluTypeClass : TypeClass {
+  let Types = [Sint8, Uint8, Sint16, Uint16, Sint32,
+               Uint32, Sint64, Uint64, Bool];
+}
+
+def PtrTypeClass : TypeClass {
+  let Types = [Ptr];
+}
+
+def AllTypeClass : TypeClass {
+  let Types = !listconcat(AluTypeClass.Types, PtrTypeClass.Types);
+}
+
+def ComparableTypeClass : TypeClass {
+  let Types = !listconcat(AluTypeClass.Types, [Ptr]);
+}
+
+class SingletonTypeClass<Type Ty> : TypeClass {
+  let Types = [Ty];
+}
+
+//===----------------------------------------------------------------------===//
+// Record describing all opcodes.
+//===----------------------------------------------------------------------===//
+
+class Opcode {
+  list<TypeClass> Types = [];
+  list<ArgType> Args = [];
+  string Name = "";
+  bit CanReturn = 0;
+  bit ChangesPC = 0;
+  bit HasCustomLink = 0;
+  bit HasCustomEval = 0;
+  bit HasGroup = 0;
+}
+
+class AluOpcode : Opcode {
+  let Types = [AluTypeClass];
+  let HasGroup = 1;
+}
+
+//===----------------------------------------------------------------------===//
+// Jump opcodes
+//===----------------------------------------------------------------------===//
+
+class JumpOpcode : Opcode {
+  let Args = [ArgSint32];
+  let ChangesPC = 1;
+  let HasCustomEval = 1;
+}
+
+// [] -> []
+def Jmp : JumpOpcode;
+// [Bool] -> [], jumps if true.
+def Jt : JumpOpcode;
+// [Bool] -> [], jumps if false.
+def Jf : JumpOpcode;
+
+//===----------------------------------------------------------------------===//
+// Returns
+//===----------------------------------------------------------------------===//
+
+// [Value] -> []
+def Ret : Opcode {
+  let Types = [AllTypeClass];
+  let ChangesPC = 1;
+  let CanReturn = 1;
+  let HasGroup = 1;
+  let HasCustomEval = 1;
+}
+// [] -> []
+def RetVoid : Opcode {
+  let CanReturn = 1;
+  let ChangesPC = 1;
+  let HasCustomEval = 1;
+}
+// [Value] -> []
+def RetValue : Opcode {
+  let CanReturn = 1;
+  let ChangesPC = 1;
+  let HasCustomEval = 1;
+}
+// [] -> EXIT
+def NoRet : Opcode {}
+
+//===----------------------------------------------------------------------===//
+// Frame management
+//===----------------------------------------------------------------------===//
+
+// [] -> []
+def Destroy : Opcode {
+  let Args = [ArgUint32];
+  let HasCustomEval = 1;
+}
+
+//===----------------------------------------------------------------------===//
+// Constants
+//===----------------------------------------------------------------------===//
+
+class ConstOpcode<Type Ty, ArgType ArgTy> : Opcode {
+  let Types = [SingletonTypeClass<Ty>];
+  let Args = [ArgTy];
+  let Name = "Const";
+}
+
+// [] -> [Integer]
+def ConstSint8 : ConstOpcode<Sint8, ArgSint8>;
+def ConstUint8 : ConstOpcode<Uint8, ArgUint8>;
+def ConstSint16 : ConstOpcode<Sint16, ArgSint16>;
+def ConstUint16 : ConstOpcode<Uint16, ArgUint16>;
+def ConstSint32 : ConstOpcode<Sint32, ArgSint32>;
+def ConstUint32 : ConstOpcode<Uint32, ArgUint32>;
+def ConstSint64 : ConstOpcode<Sint64, ArgSint64>;
+def ConstUint64 : ConstOpcode<Uint64, ArgUint64>;
+def ConstBool : ConstOpcode<Bool, ArgBool>;
+
+// [] -> [Integer]
+def Zero : Opcode {
+  let Types = [AluTypeClass];
+}
+
+// [] -> [Pointer]
+def Null : Opcode {
+  let Types = [PtrTypeClass];
+}
+
+//===----------------------------------------------------------------------===//
+// Pointer generation
+//===----------------------------------------------------------------------===//
+
+// [] -> [Pointer]
+def GetPtrLocal : Opcode {
+  // Offset of local.
+  let Args = [ArgUint32];
+  bit HasCustomEval = 1;
+}
+// [] -> [Pointer]
+def GetPtrParam : Opcode {
+  // Offset of parameter.
+  let Args = [ArgUint32];
+}
+// [] -> [Pointer]
+def GetPtrGlobal : Opcode {
+  // Index of global.
+  let Args = [ArgUint32];
+}
+// [Pointer] -> [Pointer]
+def GetPtrField : Opcode {
+  // Offset of field.
+  let Args = [ArgUint32];
+}
+// [Pointer] -> [Pointer]
+def GetPtrActiveField : Opcode {
+  // Offset of field.
+  let Args = [ArgUint32];
+}
+// [] -> [Pointer]
+def GetPtrActiveThisField : Opcode {
+  // Offset of field.
+  let Args = [ArgUint32];
+}
+// [] -> [Pointer]
+def GetPtrThisField : Opcode {
+  // Offset of field.
+  let Args = [ArgUint32];
+}
+// [Pointer] -> [Pointer]
+def GetPtrBase : Opcode {
+  // Offset of field, which is a base.
+  let Args = [ArgUint32];
+}
+// [Pointer] -> [Pointer]
+def GetPtrVirtBase : Opcode {
+  // RecordDecl of base class.
+  let Args = [ArgRecordDecl];
+}
+// [] -> [Pointer]
+def GetPtrThisBase : Opcode {
+  // Offset of field, which is a base.
+  let Args = [ArgUint32];
+}
+// [] -> [Pointer]
+def GetPtrThisVirtBase : Opcode {
+  // RecordDecl of base class.
+  let Args = [ArgRecordDecl];
+}
+// [] -> [Pointer]
+def This : Opcode;
+
+// [Pointer] -> [Pointer]
+def NarrowPtr : Opcode;
+// [Pointer] -> [Pointer]
+def ExpandPtr : Opcode;
+
+//===----------------------------------------------------------------------===//
+// Direct field accessors
+//===----------------------------------------------------------------------===//
+
+class AccessOpcode : Opcode {
+  let Types = [AllTypeClass];
+  let Args = [ArgUint32];
+  let HasGroup = 1;
+}
+
+class BitFieldOpcode : Opcode {
+  let Types = [AluTypeClass];
+  let Args = [ArgRecordField];
+  let HasGroup = 1;
+}
+
+// [] -> [Pointer]
+def GetLocal : AccessOpcode { let HasCustomEval = 1; }
+// [] -> [Pointer]
+def SetLocal : AccessOpcode { let HasCustomEval = 1; }
+
+// [] -> [Value]
+def GetGlobal : AccessOpcode;
+// [Value] -> []
+def InitGlobal : AccessOpcode;
+// [Value] -> []
+def SetGlobal : AccessOpcode;
+
+// [] -> [Value]
+def GetParam : AccessOpcode;
+// [Value] -> []
+def SetParam : AccessOpcode;
+
+// [Pointer] -> [Pointer, Value]
+def GetField : AccessOpcode;
+// [Pointer] -> [Value]
+def GetFieldPop : AccessOpcode;
+// [] -> [Value]
+def GetThisField : AccessOpcode;
+
+// [Pointer, Value] -> [Pointer]
+def SetField : AccessOpcode;
+// [Value] -> []
+def SetThisField : AccessOpcode;
+
+// [Value] -> []
+def InitThisField : AccessOpcode;
+// [Value] -> []
+def InitThisFieldActive : AccessOpcode;
+// [Value] -> []
+def InitThisBitField : BitFieldOpcode;
+// [Pointer, Value] -> []
+def InitField : AccessOpcode;
+// [Pointer, Value] -> []
+def InitBitField : BitFieldOpcode;
+// [Pointer, Value] -> []
+def InitFieldActive : AccessOpcode;
+
+//===----------------------------------------------------------------------===//
+// Pointer access
+//===----------------------------------------------------------------------===//
+
+class LoadOpcode : Opcode {
+  let Types = [AllTypeClass];
+  let HasGroup = 1;
+}
+
+// [Pointer] -> [Pointer, Value]
+def Load : LoadOpcode {}
+// [Pointer] -> [Value]
+def LoadPop : LoadOpcode {}
+
+class StoreOpcode : Opcode {
+  let Types = [AllTypeClass];
+  let HasGroup = 1;
+}
+
+class StoreBitFieldOpcode : Opcode {
+  let Types = [AluTypeClass];
+  let HasGroup = 1;
+}
+
+// [Pointer, Value] -> [Pointer]
+def Store : StoreOpcode {}
+// [Pointer, Value] -> []
+def StorePop : StoreOpcode {}
+
+// [Pointer, Value] -> [Pointer]
+def StoreBitField : StoreBitFieldOpcode {}
+// [Pointer, Value] -> []
+def StoreBitFieldPop : StoreBitFieldOpcode {}
+
+// [Pointer, Value] -> []
+def InitPop : StoreOpcode {}
+// [Pointer, Value] -> [Pointer]
+def InitElem : Opcode {
+  let Types = [AllTypeClass];
+  let Args = [ArgUint32];
+  let HasGroup = 1;
+}
+// [Pointer, Value] -> []
+def InitElemPop : Opcode {
+  let Types = [AllTypeClass];
+  let Args = [ArgUint32];
+  let HasGroup = 1;
+}
+
+//===----------------------------------------------------------------------===//
+// Pointer arithmetic.
+//===----------------------------------------------------------------------===//
+
+// [Pointer, Integral] -> [Pointer]
+def AddOffset : AluOpcode;
+// [Pointer, Integral] -> [Pointer]
+def SubOffset : AluOpcode;
+
+//===----------------------------------------------------------------------===//
+// Binary operators.
+//===----------------------------------------------------------------------===//
+
+// [Real, Real] -> [Real]
+def Sub : AluOpcode;
+def Add : AluOpcode;
+def Mul : AluOpcode;
+
+//===----------------------------------------------------------------------===//
+// Comparison opcodes.
+//===----------------------------------------------------------------------===//
+
+class EqualityOpcode : Opcode {
+  let Types = [AllTypeClass];
+  let HasGroup = 1;
+}
+
+def EQ : EqualityOpcode;
+def NE : EqualityOpcode;
+
+class ComparisonOpcode : Opcode {
+  let Types = [ComparableTypeClass];
+  let HasGroup = 1;
+}
+
+def LT : ComparisonOpcode;
+def LE : ComparisonOpcode;
+def GT : ComparisonOpcode;
+def GE : ComparisonOpcode;
+
+//===----------------------------------------------------------------------===//
+// Stack management.
+//===----------------------------------------------------------------------===//
+
+// [Value] -> []
+def Pop : Opcode {
+  let Types = [AllTypeClass];
+  let HasGroup = 1;
+}
+
+// [Value] -> [Value, Value]
+def Dup : Opcode {
+  let Types = [AllTypeClass];
+  let HasGroup = 1;
+}

lib/AST/Interp/Pointer.cpp

@@ -0,0 +1,193 @@
+//===--- Pointer.cpp - Types for the constexpr VM ---------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "Pointer.h"
+#include "Block.h"
+#include "Function.h"
+#include "PrimType.h"
+
+using namespace clang;
+using namespace clang::interp;
+
+Pointer::Pointer(Block *Pointee) : Pointer(Pointee, 0, 0) {}
+
+Pointer::Pointer(const Pointer &P) : Pointer(P.Pointee, P.Base, P.Offset) {}
+
+Pointer::Pointer(Pointer &&P)
+    : Pointee(P.Pointee), Base(P.Base), Offset(P.Offset) {
+  if (Pointee)
+    Pointee->movePointer(&P, this);
+}
+
+Pointer::Pointer(Block *Pointee, unsigned Base, unsigned Offset)
+    : Pointee(Pointee), Base(Base), Offset(Offset) {
+  assert((Base == RootPtrMark || Base % alignof(void *) == 0) && "wrong base");
+  if (Pointee)
+    Pointee->addPointer(this);
+}
+
+Pointer::~Pointer() {
+  if (Pointee) {
+    Pointee->removePointer(this);
+    Pointee->cleanup();
+  }
+}
+
+void Pointer::operator=(const Pointer &P) {
+  Block *Old = Pointee;
+
+  if (Pointee)
+    Pointee->removePointer(this);
+
+  Offset = P.Offset;
+  Base = P.Base;
+
+  Pointee = P.Pointee;
+  if (Pointee)
+    Pointee->addPointer(this);
+
+  if (Old)
+    Old->cleanup();
+}
+
+void Pointer::operator=(Pointer &&P) {
+  Block *Old = Pointee;
+
+  if (Pointee)
+    Pointee->removePointer(this);
+
+  Offset = P.Offset;
+  Base = P.Base;
+
+  Pointee = P.Pointee;
+  if (Pointee)
+    Pointee->movePointer(&P, this);
+
+  if (Old)
+    Old->cleanup();
+}
+
+APValue Pointer::toAPValue() const {
+  APValue::LValueBase Base;
+  llvm::SmallVector<APValue::LValuePathEntry, 5> Path;
+  CharUnits Offset;
+  bool IsNullPtr;
+  bool IsOnePastEnd;
+
+  if (isZero()) {
+    Base = static_cast<const Expr *>(nullptr);
+    IsNullPtr = true;
+    IsOnePastEnd = false;
+    Offset = CharUnits::Zero();
+  } else {
+    // Build the lvalue base from the block.
+    Descriptor *Desc = getDeclDesc();
+    if (auto *VD = Desc->asValueDecl())
+      Base = VD;
+    else if (auto *E = Desc->asExpr())
+      Base = E;
+    else
+      llvm_unreachable("Invalid allocation type");
+
+    // Not a null pointer.
+    IsNullPtr = false;
+
+    if (isUnknownSizeArray()) {
+      IsOnePastEnd = false;
+      Offset = CharUnits::Zero();
+    } else {
+      // TODO: compute the offset into the object.
+      Offset = CharUnits::Zero();
+
+      // Build the path into the object.
+      Pointer Ptr = *this;
+      while (Ptr.isField()) {
+        if (Ptr.isArrayElement()) {
+          Path.push_back(APValue::LValuePathEntry::ArrayIndex(Ptr.getIndex()));
+          Ptr = Ptr.getArray();
+        } else {
+          // TODO: figure out if base is virtual
+          bool IsVirtual = false;
+
+          // Create a path entry for the field.
+          Descriptor *Desc = Ptr.getFieldDesc();
+          if (auto *BaseOrMember = Desc->asDecl()) {
+            Path.push_back(APValue::LValuePathEntry({BaseOrMember, IsVirtual}));
+            Ptr = Ptr.getBase();
+            continue;
+          }
+          llvm_unreachable("Invalid field type");
+        }
+      }
+
+      IsOnePastEnd = isOnePastEnd();
+    }
+  }
+
+  return APValue(Base, Offset, Path, IsOnePastEnd, IsNullPtr);
+}
+
+bool Pointer::isInitialized() const {
+  assert(Pointee && "Cannot check if null pointer was initialized");
+  Descriptor *Desc = getFieldDesc();
+  if (Desc->isPrimitiveArray()) {
+    if (Pointee->IsStatic)
+      return true;
+    // Primitive array field are stored in a bitset.
+    InitMap *Map = getInitMap();
+    if (!Map)
+      return false;
+    if (Map == (InitMap *)-1)
+      return true;
+    return Map->isInitialized(getIndex());
+  } else {
+    // Field has its bit in an inline descriptor.
+    return Base == 0 || getInlineDesc()->IsInitialized;
+  }
+}
+
+void Pointer::initialize() const {
+  assert(Pointee && "Cannot initialize null pointer");
+  Descriptor *Desc = getFieldDesc();
+  if (Desc->isPrimitiveArray()) {
+    if (!Pointee->IsStatic) {
+      // Primitive array initializer.
+      InitMap *&Map = getInitMap();
+      if (Map == (InitMap *)-1)
+        return;
+      if (Map == nullptr)
+        Map = InitMap::allocate(Desc->getNumElems());
+      if (Map->initialize(getIndex())) {
+        free(Map);
+        Map = (InitMap *)-1;
+      }
+    }
+  } else {
+    // Field has its bit in an inline descriptor.
+    assert(Base != 0 && "Only composite fields can be initialised");
+    getInlineDesc()->IsInitialized = true;
+  }
+}
+
+void Pointer::activate() const {
+  // Field has its bit in an inline descriptor.
+  assert(Base != 0 && "Only composite fields can be initialised");
+  getInlineDesc()->IsActive = true;
+}
+
+void Pointer::deactivate() const {
+  // TODO: this only appears in constructors, so nothing to deactivate.
+}
+
+bool Pointer::hasSameBase(const Pointer &A, const Pointer &B) {
+  return A.Pointee == B.Pointee;
+}
+
+bool Pointer::hasSameArray(const Pointer &A, const Pointer &B) {
+  return A.Base == B.Base && A.getFieldDesc()->IsArray;
+}

lib/AST/Interp/Pointer.h

@@ -0,0 +1,353 @@
+//===--- Pointer.h - Types for the constexpr VM -----------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Defines the classes responsible for pointer tracking.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_AST_INTERP_POINTER_H
+#define LLVM_CLANG_AST_INTERP_POINTER_H
+
+#include "Block.h"
+#include "Descriptor.h"
+#include "clang/AST/Decl.h"
+#include "clang/AST/DeclCXX.h"
+#include "clang/AST/Expr.h"
+#include "clang/AST/ComparisonCategories.h"
+#include "llvm/ADT/PointerUnion.h"
+#include "llvm/Support/raw_ostream.h"
+
+namespace clang {
+namespace interp {
+class Block;
+class DeadBlock;
+class Context;
+class InterpState;
+class Pointer;
+class Function;
+enum PrimType : unsigned;
+
+/// A pointer to a memory block, live or dead.
+///
+/// This object can be allocated into interpreter stack frames. If pointing to
+/// a live block, it is a link in the chain of pointers pointing to the block.
+class Pointer {
+private:
+  static constexpr unsigned PastEndMark = (unsigned)-1;
+  static constexpr unsigned RootPtrMark = (unsigned)-1;
+
+public:
+  Pointer() {}
+  Pointer(Block *B);
+  Pointer(const Pointer &P);
+  Pointer(Pointer &&P);
+  ~Pointer();
+
+  void operator=(const Pointer &P);
+  void operator=(Pointer &&P);
+
+  /// Converts the pointer to an APValue.
+  APValue toAPValue() const;
+
+  /// Offsets a pointer inside an array.
+  Pointer atIndex(unsigned Idx) const {
+    if (Base == RootPtrMark)
+      return Pointer(Pointee, RootPtrMark, getDeclDesc()->getSize());
+    unsigned Off = Idx * elemSize();
+    if (getFieldDesc()->ElemDesc)
+      Off += sizeof(InlineDescriptor);
+    else
+      Off += sizeof(InitMap *);
+    return Pointer(Pointee, Base, Base + Off);
+  }
+
+  /// Creates a pointer to a field.
+  Pointer atField(unsigned Off) const {
+    unsigned Field = Offset + Off;
+    return Pointer(Pointee, Field, Field);
+  }
+
+  /// Restricts the scope of an array element pointer.
+  Pointer narrow() const {
+    // Null pointers cannot be narrowed.
+    if (isZero() || isUnknownSizeArray())
+      return *this;
+
+    // Pointer to an array of base types - enter block.
+    if (Base == RootPtrMark)
+      return Pointer(Pointee, 0, Offset == 0 ? Offset : PastEndMark);
+
+    // Pointer is one past end - magic offset marks that.
+    if (isOnePastEnd())
+      return Pointer(Pointee, Base, PastEndMark);
+
+    // Primitive arrays are a bit special since they do not have inline
+    // descriptors. If Offset != Base, then the pointer already points to
+    // an element and there is nothing to do. Otherwise, the pointer is
+    // adjusted to the first element of the array.
+    if (inPrimitiveArray()) {
+      if (Offset != Base)
+        return *this;
+      return Pointer(Pointee, Base, Offset + sizeof(InitMap *));
+    }
+
+    // Pointer is to a field or array element - enter it.
+    if (Offset != Base)
+      return Pointer(Pointee, Offset, Offset);
+
+    // Enter the first element of an array.
+    if (!getFieldDesc()->isArray())
+      return *this;
+
+    const unsigned NewBase = Base + sizeof(InlineDescriptor);
+    return Pointer(Pointee, NewBase, NewBase);
+  }
+
+  /// Expands a pointer to the containing array, undoing narrowing.
+  Pointer expand() const {
+    if (isElementPastEnd()) {
+      // Revert to an outer one-past-end pointer.
+      unsigned Adjust;
+      if (inPrimitiveArray())
+        Adjust = sizeof(InitMap *);
+      else
+        Adjust = sizeof(InlineDescriptor);
+      return Pointer(Pointee, Base, Base + getSize() + Adjust);
+    }
+
+    // Do not step out of array elements.
+    if (Base != Offset)
+      return *this;
+
+    // If at base, point to an array of base types.
+    if (Base == 0)
+      return Pointer(Pointee, RootPtrMark, 0);
+
+    // Step into the containing array, if inside one.
+    unsigned Next = Base - getInlineDesc()->Offset;
+    Descriptor *Desc = Next == 0 ? getDeclDesc() : getDescriptor(Next)->Desc;
+    if (!Desc->IsArray)
+      return *this;
+    return Pointer(Pointee, Next, Offset);
+  }
+
+  /// Checks if the pointer is null.
+  bool isZero() const { return Pointee == nullptr; }
+  /// Checks if the pointer is live.
+  bool isLive() const { return Pointee && !Pointee->IsDead; }
+  /// Checks if the item is a field in an object.
+  bool isField() const { return Base != 0 && Base != RootPtrMark; }
+
+  /// Accessor for information about the declaration site.
+  Descriptor *getDeclDesc() const { return Pointee->Desc; }
+  SourceLocation getDeclLoc() const { return getDeclDesc()->getLocation(); }
+
+  /// Returns a pointer to the object of which this pointer is a field.
+  Pointer getBase() const {
+    if (Base == RootPtrMark) {
+      assert(Offset == PastEndMark && "cannot get base of a block");
+      return Pointer(Pointee, Base, 0);
+    }
+    assert(Offset == Base && "not an inner field");
+    unsigned NewBase = Base - getInlineDesc()->Offset;
+    return Pointer(Pointee, NewBase, NewBase);
+  }
+  /// Returns the parent array.
+  Pointer getArray() const {
+    if (Base == RootPtrMark) {
+      assert(Offset != 0 && Offset != PastEndMark && "not an array element");
+      return Pointer(Pointee, Base, 0);
+    }
+    assert(Offset != Base && "not an array element");
+    return Pointer(Pointee, Base, Base);
+  }
+
+  /// Accessors for information about the innermost field.
+  Descriptor *getFieldDesc() const {
+    if (Base == 0 || Base == RootPtrMark)
+      return getDeclDesc();
+    return getInlineDesc()->Desc;
+  }
+
+  /// Returns the type of the innermost field.
+  QualType getType() const { return getFieldDesc()->getType(); }
+
+  /// Returns the element size of the innermost field.
+  size_t elemSize() const {
+    if (Base == RootPtrMark)
+      return getDeclDesc()->getSize();
+    return getFieldDesc()->getElemSize();
+  }
+  /// Returns the total size of the innermost field.
+  size_t getSize() const { return getFieldDesc()->getSize(); }
+
+  /// Returns the offset into an array.
+  unsigned getOffset() const {
+    assert(Offset != PastEndMark && "invalid offset");
+    if (Base == RootPtrMark)
+      return Offset;
+
+    unsigned Adjust = 0;
+    if (Offset != Base) {
+      if (getFieldDesc()->ElemDesc)
+        Adjust = sizeof(InlineDescriptor);
+      else
+        Adjust = sizeof(InitMap *);
+    }
+    return Offset - Base - Adjust;
+  }
+
+  /// Checks if the innermost field is an array.
+  bool inArray() const { return getFieldDesc()->IsArray; }
+  /// Checks if the structure is a primitive array.
+  bool inPrimitiveArray() const { return getFieldDesc()->isPrimitiveArray(); }
+  /// Checks if the structure is an array of unknown size.
+  bool isUnknownSizeArray() const {
+    return getFieldDesc()->isUnknownSizeArray();
+  }
+  /// Checks if the pointer points to an array.
+  bool isArrayElement() const { return Base != Offset; }
+  /// Pointer points directly to a block.
+  bool isRoot() const {
+    return (Base == 0 || Base == RootPtrMark) && Offset == 0;
+  }
+
+  /// Returns the record descriptor of a class.
+  Record *getRecord() const { return getFieldDesc()->ElemRecord; }
+  /// Returns the field information.
+  const FieldDecl *getField() const { return getFieldDesc()->asFieldDecl(); }
+
+  /// Checks if the object is a union.
+  bool isUnion() const;
+
+  /// Checks if the storage is extern.
+  bool isExtern() const { return Pointee->isExtern(); }
+  /// Checks if the storage is static.
+  bool isStatic() const { return Pointee->isStatic(); }
+  /// Checks if the storage is temporary.
+  bool isTemporary() const { return Pointee->isTemporary(); }
+  /// Checks if the storage is a static temporary.
+  bool isStaticTemporary() const { return isStatic() && isTemporary(); }
+
+  /// Checks if the field is mutable.
+  bool isMutable() const { return Base != 0 && getInlineDesc()->IsMutable; }
+  /// Checks if an object was initialized.
+  bool isInitialized() const;
+  /// Checks if the object is active.
+  bool isActive() const { return Base == 0 || getInlineDesc()->IsActive; }
+  /// Checks if a structure is a base class.
+  bool isBaseClass() const { return isField() && getInlineDesc()->IsBase; }
+
+  /// Checks if an object or a subfield is mutable.
+  bool isConst() const {
+    return Base == 0 ? getDeclDesc()->IsConst : getInlineDesc()->IsConst;
+  }
+
+  /// Returns the declaration ID.
+  llvm::Optional<unsigned> getDeclID() const { return Pointee->getDeclID(); }
+
+  /// Returns the byte offset from the start.
+  unsigned getByteOffset() const {
+    return Offset;
+  }
+
+  /// Returns the number of elements.
+  unsigned getNumElems() const { return getSize() / elemSize(); }
+
+  /// Returns the index into an array.
+  int64_t getIndex() const {
+    if (isElementPastEnd())
+      return 1;
+    if (auto ElemSize = elemSize())
+      return getOffset() / ElemSize;
+    return 0;
+  }
+
+  /// Checks if the index is one past end.
+  bool isOnePastEnd() const {
+    return isElementPastEnd() || getSize() == getOffset();
+  }
+
+  /// Checks if the pointer is an out-of-bounds element pointer.
+  bool isElementPastEnd() const { return Offset == PastEndMark; }
+
+  /// Dereferences the pointer, if it's live.
+  template <typename T> T &deref() const {
+    assert(isLive() && "Invalid pointer");
+    return *reinterpret_cast<T *>(Pointee->data() + Offset);
+  }
+
+  /// Dereferences a primitive element.
+  template <typename T> T &elem(unsigned I) const {
+    return reinterpret_cast<T *>(Pointee->data())[I];
+  }
+
+  /// Initializes a field.
+  void initialize() const;
+  /// Activats a field.
+  void activate() const;
+  /// Deactivates an entire strurcutre.
+  void deactivate() const;
+
+  /// Checks if two pointers are comparable.
+  static bool hasSameBase(const Pointer &A, const Pointer &B);
+  /// Checks if two pointers can be subtracted.
+  static bool hasSameArray(const Pointer &A, const Pointer &B);
+
+  /// Prints the pointer.
+  void print(llvm::raw_ostream &OS) const {
+    OS << "{" << Base << ", " << Offset << ", ";
+    if (Pointee)
+      OS << Pointee->getSize();
+    else
+      OS << "nullptr";
+    OS << "}";
+  }
+
+private:
+  friend class Block;
+  friend class DeadBlock;
+
+  Pointer(Block *Pointee, unsigned Base, unsigned Offset);
+
+  /// Returns the embedded descriptor preceding a field.
+  InlineDescriptor *getInlineDesc() const { return getDescriptor(Base); }
+
+  /// Returns a descriptor at a given offset.
+  InlineDescriptor *getDescriptor(unsigned Offset) const {
+    assert(Offset != 0 && "Not a nested pointer");
+    return reinterpret_cast<InlineDescriptor *>(Pointee->data() + Offset) - 1;
+  }
+
+  /// Returns a reference to the pointer which stores the initialization map.
+  InitMap *&getInitMap() const {
+    return *reinterpret_cast<InitMap **>(Pointee->data() + Base);
+  }
+
+  /// The block the pointer is pointing to.
+  Block *Pointee = nullptr;
+  /// Start of the current subfield.
+  unsigned Base = 0;
+  /// Offset into the block.
+  unsigned Offset = 0;
+
+  /// Previous link in the pointer chain.
+  Pointer *Prev = nullptr;
+  /// Next link in the pointer chain.
+  Pointer *Next = nullptr;
+};
+
+inline llvm::raw_ostream &operator<<(llvm::raw_ostream &OS, const Pointer &P) {
+  P.print(OS);
+  return OS;
+}
+
+} // namespace interp
+} // namespace clang
+
+#endif

lib/AST/Interp/PrimType.cpp

@@ -0,0 +1,23 @@
+//===--- Type.cpp - Types for the constexpr VM ------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "PrimType.h"
+
+using namespace clang;
+using namespace clang::interp;
+
+namespace clang {
+namespace interp {
+
+size_t primSize(PrimType Type) {
+  TYPE_SWITCH(Type, return sizeof(T));
+  llvm_unreachable("not a primitive type");
+}
+
+} // namespace interp
+} // namespace clang

lib/AST/Interp/PrimType.h

@@ -0,0 +1,115 @@
+//===--- PrimType.h - Types for the constexpr VM --------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Defines the VM types and helpers operating on types.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_AST_INTERP_TYPE_H
+#define LLVM_CLANG_AST_INTERP_TYPE_H
+
+#include <climits>
+#include <cstddef>
+#include <cstdint>
+#include "Boolean.h"
+#include "Integral.h"
+#include "Pointer.h"
+
+namespace clang {
+namespace interp {
+
+/// Enumeration of the primitive types of the VM.
+enum PrimType : unsigned {
+  PT_Sint8,
+  PT_Uint8,
+  PT_Sint16,
+  PT_Uint16,
+  PT_Sint32,
+  PT_Uint32,
+  PT_Sint64,
+  PT_Uint64,
+  PT_Bool,
+  PT_Ptr,
+};
+
+/// Mapping from primitive types to their representation.
+template <PrimType T> struct PrimConv;
+template <> struct PrimConv<PT_Sint8> { using T = Integral<8, true>; };
+template <> struct PrimConv<PT_Uint8> { using T = Integral<8, false>; };
+template <> struct PrimConv<PT_Sint16> { using T = Integral<16, true>; };
+template <> struct PrimConv<PT_Uint16> { using T = Integral<16, false>; };
+template <> struct PrimConv<PT_Sint32> { using T = Integral<32, true>; };
+template <> struct PrimConv<PT_Uint32> { using T = Integral<32, false>; };
+template <> struct PrimConv<PT_Sint64> { using T = Integral<64, true>; };
+template <> struct PrimConv<PT_Uint64> { using T = Integral<64, false>; };
+template <> struct PrimConv<PT_Bool> { using T = Boolean; };
+template <> struct PrimConv<PT_Ptr> { using T = Pointer; };
+
+/// Returns the size of a primitive type in bytes.
+size_t primSize(PrimType Type);
+
+/// Aligns a size to the pointer alignment.
+constexpr size_t align(size_t Size) {
+  return ((Size + alignof(void *) - 1) / alignof(void *)) * alignof(void *);
+}
+
+inline bool isPrimitiveIntegral(PrimType Type) {
+  switch (Type) {
+  case PT_Bool:
+  case PT_Sint8:
+  case PT_Uint8:
+  case PT_Sint16:
+  case PT_Uint16:
+  case PT_Sint32:
+  case PT_Uint32:
+  case PT_Sint64:
+  case PT_Uint64:
+    return true;
+  default:
+    return false;
+  }
+}
+
+} // namespace interp
+} // namespace clang
+
+/// Helper macro to simplify type switches.
+/// The macro implicitly exposes a type T in the scope of the inner block.
+#define TYPE_SWITCH_CASE(Name, B) \
+  case Name: { using T = PrimConv<Name>::T; do {B;} while(0); break; }
+#define TYPE_SWITCH(Expr, B)                                                   \
+  switch (Expr) {                                                              \
+    TYPE_SWITCH_CASE(PT_Sint8, B)                                              \
+    TYPE_SWITCH_CASE(PT_Uint8, B)                                              \
+    TYPE_SWITCH_CASE(PT_Sint16, B)                                             \
+    TYPE_SWITCH_CASE(PT_Uint16, B)                                             \
+    TYPE_SWITCH_CASE(PT_Sint32, B)                                             \
+    TYPE_SWITCH_CASE(PT_Uint32, B)                                             \
+    TYPE_SWITCH_CASE(PT_Sint64, B)                                             \
+    TYPE_SWITCH_CASE(PT_Uint64, B)                                             \
+    TYPE_SWITCH_CASE(PT_Bool, B)                                               \
+    TYPE_SWITCH_CASE(PT_Ptr, B)                                                \
+  }
+#define COMPOSITE_TYPE_SWITCH(Expr, B, D)                                      \
+  switch (Expr) {                                                              \
+    TYPE_SWITCH_CASE(PT_Ptr, B)                                                \
+    default: do { D; } while(0); break;                                        \
+  }
+#define INT_TYPE_SWITCH(Expr, B)                                               \
+  switch (Expr) {                                                              \
+    TYPE_SWITCH_CASE(PT_Sint8, B)                                              \
+    TYPE_SWITCH_CASE(PT_Uint8, B)                                              \
+    TYPE_SWITCH_CASE(PT_Sint16, B)                                             \
+    TYPE_SWITCH_CASE(PT_Uint16, B)                                             \
+    TYPE_SWITCH_CASE(PT_Sint32, B)                                             \
+    TYPE_SWITCH_CASE(PT_Uint32, B)                                             \
+    TYPE_SWITCH_CASE(PT_Sint64, B)                                             \
+    TYPE_SWITCH_CASE(PT_Uint64, B)                                             \
+    default: llvm_unreachable("not an integer");                               \
+  }
+#endif

lib/AST/Interp/Program.cpp

@@ -0,0 +1,364 @@
+//===--- Program.cpp - Bytecode for the constexpr VM ------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "Program.h"
+#include "ByteCodeStmtGen.h"
+#include "Context.h"
+#include "Function.h"
+#include "Opcode.h"
+#include "PrimType.h"
+#include "clang/AST/Decl.h"
+#include "clang/AST/DeclCXX.h"
+
+using namespace clang;
+using namespace clang::interp;
+
+unsigned Program::createGlobalString(const StringLiteral *S) {
+  const size_t CharWidth = S->getCharByteWidth();
+  const size_t BitWidth = CharWidth * Ctx.getCharBit();
+
+  PrimType CharType;
+  switch (CharWidth) {
+  case 1:
+    CharType = PT_Sint8;
+    break;
+  case 2:
+    CharType = PT_Uint16;
+    break;
+  case 4:
+    CharType = PT_Uint32;
+    break;
+  default:
+    llvm_unreachable("unsupported character width");
+  }
+
+  // Create a descriptor for the string.
+  Descriptor *Desc = allocateDescriptor(S, CharType, S->getLength() + 1,
+                                        /*isConst=*/true,
+                                        /*isTemporary=*/false,
+                                        /*isMutable=*/false);
+
+  // Allocate storage for the string.
+  // The byte length does not include the null terminator.
+  unsigned I = Globals.size();
+  unsigned Sz = Desc->getAllocSize();
+  auto *G = new (Allocator, Sz) Global(Desc, /*isStatic=*/true,
+                                       /*isExtern=*/false);
+  Globals.push_back(G);
+
+  // Construct the string in storage.
+  const Pointer Ptr(G->block());
+  for (unsigned I = 0, N = S->getLength(); I <= N; ++I) {
+    Pointer Field = Ptr.atIndex(I).narrow();
+    const uint32_t CodePoint = I == N ? 0 : S->getCodeUnit(I);
+    switch (CharType) {
+      case PT_Sint8: {
+        using T = PrimConv<PT_Sint8>::T;
+        Field.deref<T>() = T::from(CodePoint, BitWidth);
+        break;
+      }
+      case PT_Uint16: {
+        using T = PrimConv<PT_Uint16>::T;
+        Field.deref<T>() = T::from(CodePoint, BitWidth);
+        break;
+      }
+      case PT_Uint32: {
+        using T = PrimConv<PT_Uint32>::T;
+        Field.deref<T>() = T::from(CodePoint, BitWidth);
+        break;
+      }
+      default:
+        llvm_unreachable("unsupported character type");
+    }
+  }
+  return I;
+}
+
+Pointer Program::getPtrGlobal(unsigned Idx) {
+  assert(Idx < Globals.size());
+  return Pointer(Globals[Idx]->block());
+}
+
+llvm::Optional<unsigned> Program::getGlobal(const ValueDecl *VD) {
+  auto It = GlobalIndices.find(VD);
+  if (It != GlobalIndices.end())
+    return It->second;
+
+  // Find any previous declarations which were aleady evaluated.
+  llvm::Optional<unsigned> Index;
+  for (const Decl *P = VD; P; P = P->getPreviousDecl()) {
+    auto It = GlobalIndices.find(P);
+    if (It != GlobalIndices.end()) {
+      Index = It->second;
+      break;
+    }
+  }
+
+  // Map the decl to the existing index.
+  if (Index) {
+    GlobalIndices[VD] = *Index;
+    return {};
+  }
+
+  return Index;
+}
+
+llvm::Optional<unsigned> Program::getOrCreateGlobal(const ValueDecl *VD) {
+  if (auto Idx = getGlobal(VD))
+    return Idx;
+
+  if (auto Idx = createGlobal(VD)) {
+    GlobalIndices[VD] = *Idx;
+    return Idx;
+  }
+  return {};
+}
+
+llvm::Optional<unsigned> Program::getOrCreateDummy(const ParmVarDecl *PD) {
+  auto &ASTCtx = Ctx.getASTContext();
+
+  // Create a pointer to an incomplete array of the specified elements.
+  QualType ElemTy = PD->getType()->getAs<PointerType>()->getPointeeType();
+  QualType Ty = ASTCtx.getIncompleteArrayType(ElemTy, ArrayType::Normal, 0);
+
+  // Dedup blocks since they are immutable and pointers cannot be compared.
+  auto It = DummyParams.find(PD);
+  if (It != DummyParams.end())
+    return It->second;
+
+  if (auto Idx = createGlobal(PD, Ty, /*isStatic=*/true, /*isExtern=*/true)) {
+    DummyParams[PD] = *Idx;
+    return Idx;
+  }
+  return {};
+}
+
+llvm::Optional<unsigned> Program::createGlobal(const ValueDecl *VD) {
+  bool IsStatic, IsExtern;
+  if (auto *Var = dyn_cast<VarDecl>(VD)) {
+    IsStatic = !Var->hasLocalStorage();
+    IsExtern = !Var->getAnyInitializer();
+  } else {
+    IsStatic = false;
+    IsExtern = true;
+  }
+  if (auto Idx = createGlobal(VD, VD->getType(), IsStatic, IsExtern)) {
+    for (const Decl *P = VD; P; P = P->getPreviousDecl())
+      GlobalIndices[P] = *Idx;
+    return *Idx;
+  }
+  return {};
+}
+
+llvm::Optional<unsigned> Program::createGlobal(const Expr *E) {
+  return createGlobal(E, E->getType(), /*isStatic=*/true, /*isExtern=*/false);
+}
+
+llvm::Optional<unsigned> Program::createGlobal(const DeclTy &D, QualType Ty,
+                                               bool IsStatic, bool IsExtern) {
+  // Create a descriptor for the global.
+  Descriptor *Desc;
+  const bool IsConst = Ty.isConstQualified();
+  const bool IsTemporary = D.dyn_cast<const Expr *>();
+  if (auto T = Ctx.classify(Ty)) {
+    Desc = createDescriptor(D, *T, IsConst, IsTemporary);
+  } else {
+    Desc = createDescriptor(D, Ty.getTypePtr(), IsConst, IsTemporary);
+  }
+  if (!Desc)
+    return {};
+
+  // Allocate a block for storage.
+  unsigned I = Globals.size();
+
+  auto *G = new (Allocator, Desc->getAllocSize())
+      Global(getCurrentDecl(), Desc, IsStatic, IsExtern);
+  G->block()->invokeCtor();
+
+  Globals.push_back(G);
+
+  return I;
+}
+
+Function *Program::getFunction(const FunctionDecl *F) {
+  F = F->getDefinition();
+  auto It = Funcs.find(F);
+  return It == Funcs.end() ? nullptr : It->second.get();
+}
+
+llvm::Expected<Function *> Program::getOrCreateFunction(const FunctionDecl *F) {
+  if (Function *Func = getFunction(F)) {
+    return Func;
+  }
+
+  // Try to compile the function if it wasn't compiled yet.
+  if (const FunctionDecl *FD = F->getDefinition())
+    return ByteCodeStmtGen<ByteCodeEmitter>(Ctx, *this).compileFunc(FD);
+
+  // A relocation which traps if not resolved.
+  return nullptr;
+}
+
+Record *Program::getOrCreateRecord(const RecordDecl *RD) {
+  // Use the actual definition as a key.
+  RD = RD->getDefinition();
+  if (!RD)
+    return nullptr;
+
+  // Deduplicate records.
+  auto It = Records.find(RD);
+  if (It != Records.end()) {
+    return It->second;
+  }
+
+  // Number of bytes required by fields and base classes.
+  unsigned Size = 0;
+  // Number of bytes required by virtual base.
+  unsigned VirtSize = 0;
+
+  // Helper to get a base descriptor.
+  auto GetBaseDesc = [this](const RecordDecl *BD, Record *BR) -> Descriptor * {
+    if (!BR)
+      return nullptr;
+    return allocateDescriptor(BD, BR, /*isConst=*/false,
+                              /*isTemporary=*/false,
+                              /*isMutable=*/false);
+  };
+
+  // Reserve space for base classes.
+  Record::BaseList Bases;
+  Record::VirtualBaseList VirtBases;
+  if (auto *CD = dyn_cast<CXXRecordDecl>(RD)) {
+    for (const CXXBaseSpecifier &Spec : CD->bases()) {
+      if (Spec.isVirtual())
+        continue;
+
+      const RecordDecl *BD = Spec.getType()->getAs<RecordType>()->getDecl();
+      Record *BR = getOrCreateRecord(BD);
+      if (Descriptor *Desc = GetBaseDesc(BD, BR)) {
+        Size += align(sizeof(InlineDescriptor));
+        Bases.push_back({BD, Size, Desc, BR});
+        Size += align(BR->getSize());
+        continue;
+      }
+      return nullptr;
+    }
+
+    for (const CXXBaseSpecifier &Spec : CD->vbases()) {
+      const RecordDecl *BD = Spec.getType()->getAs<RecordType>()->getDecl();
+      Record *BR = getOrCreateRecord(BD);
+
+      if (Descriptor *Desc = GetBaseDesc(BD, BR)) {
+        VirtSize += align(sizeof(InlineDescriptor));
+        VirtBases.push_back({BD, VirtSize, Desc, BR});
+        VirtSize += align(BR->getSize());
+        continue;
+      }
+      return nullptr;
+    }
+  }
+
+  // Reserve space for fields.
+  Record::FieldList Fields;
+  for (const FieldDecl *FD : RD->fields()) {
+    // Reserve space for the field's descriptor and the offset.
+    Size += align(sizeof(InlineDescriptor));
+
+    // Classify the field and add its metadata.
+    QualType FT = FD->getType();
+    const bool IsConst = FT.isConstQualified();
+    const bool IsMutable = FD->isMutable();
+    Descriptor *Desc;
+    if (llvm::Optional<PrimType> T = Ctx.classify(FT)) {
+      Desc = createDescriptor(FD, *T, IsConst, /*isTemporary=*/false,
+                              IsMutable);
+    } else {
+      Desc = createDescriptor(FD, FT.getTypePtr(), IsConst,
+                              /*isTemporary=*/false, IsMutable);
+    }
+    if (!Desc)
+      return nullptr;
+    Fields.push_back({FD, Size, Desc});
+    Size += align(Desc->getAllocSize());
+  }
+
+  Record *R = new (Allocator) Record(RD, std::move(Bases), std::move(Fields),
+                                     std::move(VirtBases), VirtSize, Size);
+  Records.insert({RD, R});
+  return R;
+}
+
+Descriptor *Program::createDescriptor(const DeclTy &D, const Type *Ty,
+                                      bool IsConst, bool IsTemporary,
+                                      bool IsMutable) {
+  // Classes and structures.
+  if (auto *RT = Ty->getAs<RecordType>()) {
+    if (auto *Record = getOrCreateRecord(RT->getDecl()))
+      return allocateDescriptor(D, Record, IsConst, IsTemporary, IsMutable);
+  }
+
+  // Arrays.
+  if (auto ArrayType = Ty->getAsArrayTypeUnsafe()) {
+    QualType ElemTy = ArrayType->getElementType();
+    // Array of well-known bounds.
+    if (auto CAT = dyn_cast<ConstantArrayType>(ArrayType)) {
+      size_t NumElems = CAT->getSize().getZExtValue();
+      if (llvm::Optional<PrimType> T = Ctx.classify(ElemTy)) {
+        // Arrays of primitives.
+        unsigned ElemSize = primSize(*T);
+        if (std::numeric_limits<unsigned>::max() / ElemSize <= NumElems) {
+          return {};
+        }
+        return allocateDescriptor(D, *T, NumElems, IsConst, IsTemporary,
+                                  IsMutable);
+      } else {
+        // Arrays of composites. In this case, the array is a list of pointers,
+        // followed by the actual elements.
+        Descriptor *Desc =
+            createDescriptor(D, ElemTy.getTypePtr(), IsConst, IsTemporary);
+        if (!Desc)
+          return nullptr;
+        InterpSize ElemSize = Desc->getAllocSize() + sizeof(InlineDescriptor);
+        if (std::numeric_limits<unsigned>::max() / ElemSize <= NumElems)
+          return {};
+        return allocateDescriptor(D, Desc, NumElems, IsConst, IsTemporary,
+                                  IsMutable);
+      }
+    }
+
+    // Array of unknown bounds - cannot be accessed and pointer arithmetic
+    // is forbidden on pointers to such objects.
+    if (isa<IncompleteArrayType>(ArrayType)) {
+      if (llvm::Optional<PrimType> T = Ctx.classify(ElemTy)) {
+        return allocateDescriptor(D, *T, IsTemporary,
+                                  Descriptor::UnknownSize{});
+      } else {
+        Descriptor *Desc =
+            createDescriptor(D, ElemTy.getTypePtr(), IsConst, IsTemporary);
+        if (!Desc)
+          return nullptr;
+        return allocateDescriptor(D, Desc, IsTemporary,
+                                  Descriptor::UnknownSize{});
+      }
+    }
+  }
+
+  // Atomic types.
+  if (auto *AT = Ty->getAs<AtomicType>()) {
+    const Type *InnerTy = AT->getValueType().getTypePtr();
+    return createDescriptor(D, InnerTy, IsConst, IsTemporary, IsMutable);
+  }
+
+  // Complex types - represented as arrays of elements.
+  if (auto *CT = Ty->getAs<ComplexType>()) {
+    PrimType ElemTy = *Ctx.classify(CT->getElementType());
+    return allocateDescriptor(D, ElemTy, 2, IsConst, IsTemporary, IsMutable);
+  }
+
+  return nullptr;
+}

lib/AST/Interp/Program.h

@@ -0,0 +1,220 @@
+//===--- Program.h - Bytecode for the constexpr VM --------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Defines a program which organises and links multiple bytecode functions.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_AST_INTERP_PROGRAM_H
+#define LLVM_CLANG_AST_INTERP_PROGRAM_H
+
+#include <map>
+#include <vector>
+#include "Function.h"
+#include "Pointer.h"
+#include "PrimType.h"
+#include "Record.h"
+#include "Source.h"
+#include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/PointerUnion.h"
+#include "llvm/ADT/StringRef.h"
+#include "llvm/Support/Allocator.h"
+
+namespace clang {
+class RecordDecl;
+class Expr;
+class FunctionDecl;
+class Stmt;
+class StringLiteral;
+class VarDecl;
+
+namespace interp {
+class Context;
+class State;
+class Record;
+class Scope;
+
+/// The program contains and links the bytecode for all functions.
+class Program {
+public:
+  Program(Context &Ctx) : Ctx(Ctx) {}
+
+  /// Emits a string literal among global data.
+  unsigned createGlobalString(const StringLiteral *S);
+
+  /// Returns a pointer to a global.
+  Pointer getPtrGlobal(unsigned Idx);
+
+  /// Returns the value of a global.
+  Block *getGlobal(unsigned Idx) {
+    assert(Idx < Globals.size());
+    return Globals[Idx]->block();
+  }
+
+  /// Finds a global's index.
+  llvm::Optional<unsigned> getGlobal(const ValueDecl *VD);
+
+  /// Returns or creates a global an creates an index to it.
+  llvm::Optional<unsigned> getOrCreateGlobal(const ValueDecl *VD);
+
+  /// Returns or creates a dummy value for parameters.
+  llvm::Optional<unsigned> getOrCreateDummy(const ParmVarDecl *PD);
+
+  /// Creates a global and returns its index.
+  llvm::Optional<unsigned> createGlobal(const ValueDecl *VD);
+
+  /// Creates a global from a lifetime-extended temporary.
+  llvm::Optional<unsigned> createGlobal(const Expr *E);
+
+  /// Creates a new function from a code range.
+  template <typename... Ts>
+  Function *createFunction(const FunctionDecl *Def, Ts &&... Args) {
+    auto *Func = new Function(*this, Def, std::forward<Ts>(Args)...);
+    Funcs.insert({Def, std::unique_ptr<Function>(Func)});
+    return Func;
+  }
+  /// Creates an anonymous function.
+  template <typename... Ts>
+  Function *createFunction(Ts &&... Args) {
+    auto *Func = new Function(*this, std::forward<Ts>(Args)...);
+    AnonFuncs.emplace_back(Func);
+    return Func;
+  }
+
+  /// Returns a function.
+  Function *getFunction(const FunctionDecl *F);
+
+  /// Returns a pointer to a function if it exists and can be compiled.
+  /// If a function couldn't be compiled, an error is returned.
+  /// If a function was not yet defined, a null pointer is returned.
+  llvm::Expected<Function *> getOrCreateFunction(const FunctionDecl *F);
+
+  /// Returns a record or creates one if it does not exist.
+  Record *getOrCreateRecord(const RecordDecl *RD);
+
+  /// Creates a descriptor for a primitive type.
+  Descriptor *createDescriptor(const DeclTy &D, PrimType Type,
+                               bool IsConst = false,
+                               bool IsTemporary = false,
+                               bool IsMutable = false) {
+    return allocateDescriptor(D, Type, IsConst, IsTemporary, IsMutable);
+  }
+
+  /// Creates a descriptor for a composite type.
+  Descriptor *createDescriptor(const DeclTy &D, const Type *Ty,
+                               bool IsConst = false, bool IsTemporary = false,
+                               bool IsMutable = false);
+
+  /// Context to manage declaration lifetimes.
+  class DeclScope {
+  public:
+    DeclScope(Program &P, const VarDecl *VD) : P(P) { P.startDeclaration(VD); }
+    ~DeclScope() { P.endDeclaration(); }
+
+  private:
+    Program &P;
+  };
+
+  /// Returns the current declaration ID.
+  llvm::Optional<unsigned> getCurrentDecl() const {
+    if (CurrentDeclaration == NoDeclaration)
+      return llvm::Optional<unsigned>{};
+    return LastDeclaration;
+  }
+
+private:
+  friend class DeclScope;
+
+  llvm::Optional<unsigned> createGlobal(const DeclTy &D, QualType Ty,
+                                        bool IsStatic, bool IsExtern);
+
+  /// Reference to the VM context.
+  Context &Ctx;
+  /// Mapping from decls to cached bytecode functions.
+  llvm::DenseMap<const FunctionDecl *, std::unique_ptr<Function>> Funcs;
+  /// List of anonymous functions.
+  std::vector<std::unique_ptr<Function>> AnonFuncs;
+
+  /// Function relocation locations.
+  llvm::DenseMap<const FunctionDecl *, std::vector<unsigned>> Relocs;
+
+  /// Custom allocator for global storage.
+  using PoolAllocTy = llvm::BumpPtrAllocatorImpl<llvm::MallocAllocator>;
+
+  /// Descriptor + storage for a global object.
+  ///
+  /// Global objects never go out of scope, thus they do not track pointers.
+  class Global {
+  public:
+    /// Create a global descriptor for string literals.
+    template <typename... Tys>
+    Global(Tys... Args) : B(std::forward<Tys>(Args)...) {}
+
+    /// Allocates the global in the pool, reserving storate for data.
+    void *operator new(size_t Meta, PoolAllocTy &Alloc, size_t Data) {
+      return Alloc.Allocate(Meta + Data, alignof(void *));
+    }
+
+    /// Return a pointer to the data.
+    char *data() { return B.data(); }
+    /// Return a pointer to the block.
+    Block *block() { return &B; }
+
+  private:
+    /// Required metadata - does not actually track pointers.
+    Block B;
+  };
+
+  /// Allocator for globals.
+  PoolAllocTy Allocator;
+
+  /// Global objects.
+  std::vector<Global *> Globals;
+  /// Cached global indices.
+  llvm::DenseMap<const void *, unsigned> GlobalIndices;
+
+  /// Mapping from decls to record metadata.
+  llvm::DenseMap<const RecordDecl *, Record *> Records;
+
+  /// Dummy parameter to generate pointers from.
+  llvm::DenseMap<const ParmVarDecl *, unsigned> DummyParams;
+
+  /// Creates a new descriptor.
+  template <typename... Ts>
+  Descriptor *allocateDescriptor(Ts &&... Args) {
+    return new (Allocator) Descriptor(std::forward<Ts>(Args)...);
+  }
+
+  /// No declaration ID.
+  static constexpr unsigned NoDeclaration = (unsigned)-1;
+  /// Last declaration ID.
+  unsigned LastDeclaration = 0;
+  /// Current declaration ID.
+  unsigned CurrentDeclaration = NoDeclaration;
+
+  /// Starts evaluating a declaration.
+  void startDeclaration(const VarDecl *Decl) {
+    LastDeclaration += 1;
+    CurrentDeclaration = LastDeclaration;
+  }
+
+  /// Ends a global declaration.
+  void endDeclaration() {
+    CurrentDeclaration = NoDeclaration;
+  }
+
+public:
+  /// Dumps the disassembled bytecode to \c llvm::errs().
+  void dump() const;
+  void dump(llvm::raw_ostream &OS) const;
+};
+
+} // namespace interp
+} // namespace clang
+
+#endif

lib/AST/Interp/Record.cpp

@@ -0,0 +1,46 @@
+//===--- Record.cpp - struct and class metadata for the VM ------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "Record.h"
+
+using namespace clang;
+using namespace clang::interp;
+
+Record::Record(const RecordDecl *Decl, BaseList &&SrcBases,
+               FieldList &&SrcFields, VirtualBaseList &&SrcVirtualBases,
+               unsigned VirtualSize, unsigned BaseSize)
+    : Decl(Decl), Bases(std::move(SrcBases)), Fields(std::move(SrcFields)),
+      BaseSize(BaseSize), VirtualSize(VirtualSize) {
+  for (Base &V : SrcVirtualBases)
+    VirtualBases.push_back({ V.Decl, V.Offset + BaseSize, V.Desc, V.R });
+
+  for (Base &B : Bases)
+    BaseMap[B.Decl] = &B;
+  for (Field &F : Fields)
+    FieldMap[F.Decl] = &F;
+  for (Base &V : VirtualBases)
+    VirtualBaseMap[V.Decl] = &V;
+}
+
+const Record::Field *Record::getField(const FieldDecl *FD) const {
+  auto It = FieldMap.find(FD);
+  assert(It != FieldMap.end() && "Missing field");
+  return It->second;
+}
+
+const Record::Base *Record::getBase(const RecordDecl *FD) const {
+  auto It = BaseMap.find(FD);
+  assert(It != BaseMap.end() && "Missing base");
+  return It->second;
+}
+
+const Record::Base *Record::getVirtualBase(const RecordDecl *FD) const {
+  auto It = VirtualBaseMap.find(FD);
+  assert(It != VirtualBaseMap.end() && "Missing virtual base");
+  return It->second;
+}

lib/AST/Interp/Record.h

@@ -0,0 +1,121 @@
+//===--- Record.h - struct and class metadata for the VM --------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// A record is part of a program to describe the layout and methods of a struct.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_AST_INTERP_RECORD_H
+#define LLVM_CLANG_AST_INTERP_RECORD_H
+
+#include "Pointer.h"
+
+namespace clang {
+namespace interp {
+class Program;
+
+/// Structure/Class descriptor.
+class Record {
+public:
+  /// Describes a record field.
+  struct Field {
+    const FieldDecl *Decl;
+    unsigned Offset;
+    Descriptor *Desc;
+  };
+
+  /// Describes a base class.
+  struct Base {
+    const RecordDecl *Decl;
+    unsigned Offset;
+    Descriptor *Desc;
+    Record *R;
+  };
+
+  /// Mapping from identifiers to field descriptors.
+  using FieldList = llvm::SmallVector<Field, 8>;
+  /// Mapping from identifiers to base classes.
+  using BaseList = llvm::SmallVector<Base, 8>;
+  /// List of virtual base classes.
+  using VirtualBaseList = llvm::SmallVector<Base, 2>;
+
+public:
+  /// Returns the underlying declaration.
+  const RecordDecl *getDecl() const { return Decl; }
+  /// Checks if the record is a union.
+  bool isUnion() const { return getDecl()->isUnion(); }
+  /// Returns the size of the record.
+  unsigned getSize() const { return BaseSize; }
+  /// Returns the full size of the record, including records.
+  unsigned getFullSize() const { return BaseSize + VirtualSize; }
+  /// Returns a field.
+  const Field *getField(const FieldDecl *FD) const;
+  /// Returns a base descriptor.
+  const Base *getBase(const RecordDecl *FD) const;
+  /// Returns a virtual base descriptor.
+  const Base *getVirtualBase(const RecordDecl *RD) const;
+
+  using const_field_iter = FieldList::const_iterator;
+  llvm::iterator_range<const_field_iter> fields() const {
+    return llvm::make_range(Fields.begin(), Fields.end());
+  }
+
+  unsigned getNumFields() { return Fields.size(); }
+  Field *getField(unsigned I) { return &Fields[I]; }
+
+  using const_base_iter = BaseList::const_iterator;
+  llvm::iterator_range<const_base_iter> bases() const {
+    return llvm::make_range(Bases.begin(), Bases.end());
+  }
+
+  unsigned getNumBases() { return Bases.size(); }
+  Base *getBase(unsigned I) { return &Bases[I]; }
+
+  using const_virtual_iter = VirtualBaseList::const_iterator;
+  llvm::iterator_range<const_virtual_iter> virtual_bases() const {
+    return llvm::make_range(VirtualBases.begin(), VirtualBases.end());
+  }
+
+  unsigned getNumVirtualBases() { return VirtualBases.size(); }
+  Base *getVirtualBase(unsigned I) { return &VirtualBases[I]; }
+
+private:
+  /// Constructor used by Program to create record descriptors.
+  Record(const RecordDecl *, BaseList &&Bases, FieldList &&Fields,
+         VirtualBaseList &&VirtualBases, unsigned VirtualSize,
+         unsigned BaseSize);
+
+private:
+  friend class Program;
+
+  /// Original declaration.
+  const RecordDecl *Decl;
+  /// List of all base classes.
+  BaseList Bases;
+  /// List of all the fields in the record.
+  FieldList Fields;
+  /// List o fall virtual bases.
+  VirtualBaseList VirtualBases;
+
+  /// Mapping from declarations to bases.
+  llvm::DenseMap<const RecordDecl *, Base *> BaseMap;
+  /// Mapping from field identifiers to descriptors.
+  llvm::DenseMap<const FieldDecl *, Field *> FieldMap;
+  /// Mapping from declarations to virtual bases.
+  llvm::DenseMap<const RecordDecl *, Base *> VirtualBaseMap;
+  /// Mapping from
+  /// Size of the structure.
+  unsigned BaseSize;
+  /// Size of all virtual bases.
+  unsigned VirtualSize;
+};
+
+} // namespace interp
+} // namespace clang
+
+#endif

lib/AST/Interp/Source.cpp

@@ -0,0 +1,39 @@
+//===--- Source.cpp - Source expression tracking ----------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "Source.h"
+#include "clang/AST/Expr.h"
+
+using namespace clang;
+using namespace clang::interp;
+
+SourceLocation SourceInfo::getLoc() const {
+  if (const Expr *E = asExpr())
+    return E->getExprLoc();
+  if (const Stmt *S = asStmt())
+    return S->getBeginLoc();
+  if (const Decl *D = asDecl())
+    return D->getBeginLoc();
+  return SourceLocation();
+}
+
+const Expr *SourceInfo::asExpr() const {
+  if (auto *S = Source.dyn_cast<const Stmt *>())
+    return dyn_cast<Expr>(S);
+  return nullptr;
+}
+
+const Expr *SourceMapper::getExpr(Function *F, CodePtr PC) const {
+  if (const Expr *E = getSource(F, PC).asExpr())
+    return E;
+  llvm::report_fatal_error("missing source expression");
+}
+
+SourceLocation SourceMapper::getLocation(Function *F, CodePtr PC) const {
+  return getSource(F, PC).getLoc();
+}

lib/AST/Interp/Source.h

@@ -0,0 +1,118 @@
+//===--- Source.h - Source location provider for the VM  --------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Defines a program which organises and links multiple bytecode functions.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_AST_INTERP_SOURCE_H
+#define LLVM_CLANG_AST_INTERP_SOURCE_H
+
+#include "clang/AST/Decl.h"
+#include "clang/AST/Stmt.h"
+#include "llvm/Support/Endian.h"
+
+namespace clang {
+namespace interp {
+class Function;
+
+/// Pointer into the code segment.
+class CodePtr {
+public:
+  CodePtr() : Ptr(nullptr) {}
+
+  CodePtr &operator+=(int32_t Offset) {
+    Ptr += Offset;
+    return *this;
+  }
+
+  int32_t operator-(const CodePtr &RHS) const {
+    assert(Ptr != nullptr && RHS.Ptr != nullptr && "Invalid code pointer");
+    return Ptr - RHS.Ptr;
+  }
+
+  CodePtr operator-(size_t RHS) const {
+    assert(Ptr != nullptr && "Invalid code pointer");
+    return CodePtr(Ptr - RHS);
+  }
+
+  bool operator!=(const CodePtr &RHS) const { return Ptr != RHS.Ptr; }
+
+  /// Reads data and advances the pointer.
+  template <typename T> T read() {
+    T Value = ReadHelper<T>(Ptr);
+    Ptr += sizeof(T);
+    return Value;
+  }
+
+private:
+  /// Constructor used by Function to generate pointers.
+  CodePtr(const char *Ptr) : Ptr(Ptr) {}
+
+  /// Helper to decode a value or a pointer.
+  template <typename T>
+  static typename std::enable_if<!std::is_pointer<T>::value, T>::type
+  ReadHelper(const char *Ptr) {
+    using namespace llvm::support;
+    return endian::read<T, endianness::native, 1>(Ptr);
+  }
+
+  template <typename T>
+  static typename std::enable_if<std::is_pointer<T>::value, T>::type
+  ReadHelper(const char *Ptr) {
+    using namespace llvm::support;
+    auto Punned = endian::read<uintptr_t, endianness::native, 1>(Ptr);
+    return reinterpret_cast<T>(Punned);
+  }
+
+private:
+  friend class Function;
+
+  /// Pointer into the code owned by a function.
+  const char *Ptr;
+};
+
+/// Describes the statement/declaration an opcode was generated from.
+class SourceInfo {
+public:
+  SourceInfo() {}
+  SourceInfo(const Stmt *E) : Source(E) {}
+  SourceInfo(const Decl *D) : Source(D) {}
+
+  SourceLocation getLoc() const;
+
+  const Stmt *asStmt() const { return Source.dyn_cast<const Stmt *>(); }
+  const Decl *asDecl() const { return Source.dyn_cast<const Decl *>(); }
+  const Expr *asExpr() const;
+
+  operator bool() const { return !Source.isNull(); }
+
+private:
+  llvm::PointerUnion<const Decl *, const Stmt *> Source;
+};
+
+using SourceMap = std::vector<std::pair<unsigned, SourceInfo>>;
+
+/// Interface for classes which map locations to sources.
+class SourceMapper {
+public:
+  virtual ~SourceMapper() {}
+
+  /// Returns source information for a given PC in a function.
+  virtual SourceInfo getSource(Function *F, CodePtr PC) const = 0;
+
+  /// Returns the expression if an opcode belongs to one, null otherwise.
+  const Expr *getExpr(Function *F, CodePtr PC) const;
+  /// Returns the location from which an opcode originates.
+  SourceLocation getLocation(Function *F, CodePtr PC) const;
+};
+
+} // namespace interp
+} // namespace clang
+
+#endif

lib/AST/Interp/State.cpp

@@ -0,0 +1,158 @@
+//===--- State.cpp - State chain for the VM and AST Walker ------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "State.h"
+#include "Frame.h"
+#include "Program.h"
+#include "clang/AST/ASTContext.h"
+#include "clang/AST/CXXInheritance.h"
+
+using namespace clang;
+using namespace clang::interp;
+
+State::~State() {}
+
+OptionalDiagnostic State::FFDiag(SourceLocation Loc, diag::kind DiagId,
+                                 unsigned ExtraNotes) {
+  return diag(Loc, DiagId, ExtraNotes, false);
+}
+
+OptionalDiagnostic State::FFDiag(const Expr *E, diag::kind DiagId,
+                                 unsigned ExtraNotes) {
+  if (getEvalStatus().Diag)
+    return diag(E->getExprLoc(), DiagId, ExtraNotes, false);
+  setActiveDiagnostic(false);
+  return OptionalDiagnostic();
+}
+
+OptionalDiagnostic State::FFDiag(const SourceInfo &SI, diag::kind DiagId,
+                                 unsigned ExtraNotes) {
+  if (getEvalStatus().Diag)
+    return diag(SI.getLoc(), DiagId, ExtraNotes, false);
+  setActiveDiagnostic(false);
+  return OptionalDiagnostic();
+}
+
+OptionalDiagnostic State::CCEDiag(SourceLocation Loc, diag::kind DiagId,
+                                  unsigned ExtraNotes) {
+  // Don't override a previous diagnostic. Don't bother collecting
+  // diagnostics if we're evaluating for overflow.
+  if (!getEvalStatus().Diag || !getEvalStatus().Diag->empty()) {
+    setActiveDiagnostic(false);
+    return OptionalDiagnostic();
+  }
+  return diag(Loc, DiagId, ExtraNotes, true);
+}
+
+OptionalDiagnostic State::CCEDiag(const Expr *E, diag::kind DiagId,
+                                  unsigned ExtraNotes) {
+  return CCEDiag(E->getExprLoc(), DiagId, ExtraNotes);
+}
+
+OptionalDiagnostic State::CCEDiag(const SourceInfo &SI, diag::kind DiagId,
+                                  unsigned ExtraNotes) {
+  return CCEDiag(SI.getLoc(), DiagId, ExtraNotes);
+}
+
+OptionalDiagnostic State::Note(SourceLocation Loc, diag::kind DiagId) {
+  if (!hasActiveDiagnostic())
+    return OptionalDiagnostic();
+  return OptionalDiagnostic(&addDiag(Loc, DiagId));
+}
+
+void State::addNotes(ArrayRef<PartialDiagnosticAt> Diags) {
+  if (hasActiveDiagnostic()) {
+    getEvalStatus().Diag->insert(getEvalStatus().Diag->end(), Diags.begin(),
+                                 Diags.end());
+  }
+}
+
+DiagnosticBuilder State::report(SourceLocation Loc, diag::kind DiagId) {
+  return getCtx().getDiagnostics().Report(Loc, DiagId);
+}
+
+/// Add a diagnostic to the diagnostics list.
+PartialDiagnostic &State::addDiag(SourceLocation Loc, diag::kind DiagId) {
+  PartialDiagnostic PD(DiagId, getCtx().getDiagAllocator());
+  getEvalStatus().Diag->push_back(std::make_pair(Loc, PD));
+  return getEvalStatus().Diag->back().second;
+}
+
+OptionalDiagnostic State::diag(SourceLocation Loc, diag::kind DiagId,
+                               unsigned ExtraNotes, bool IsCCEDiag) {
+  Expr::EvalStatus &EvalStatus = getEvalStatus();
+  if (EvalStatus.Diag) {
+    if (hasPriorDiagnostic()) {
+      return OptionalDiagnostic();
+    }
+
+    unsigned CallStackNotes = getCallStackDepth() - 1;
+    unsigned Limit = getCtx().getDiagnostics().getConstexprBacktraceLimit();
+    if (Limit)
+      CallStackNotes = std::min(CallStackNotes, Limit + 1);
+    if (checkingPotentialConstantExpression())
+      CallStackNotes = 0;
+
+    setActiveDiagnostic(true);
+    setFoldFailureDiagnostic(!IsCCEDiag);
+    EvalStatus.Diag->clear();
+    EvalStatus.Diag->reserve(1 + ExtraNotes + CallStackNotes);
+    addDiag(Loc, DiagId);
+    if (!checkingPotentialConstantExpression()) {
+      addCallStack(Limit);
+    }
+    return OptionalDiagnostic(&(*EvalStatus.Diag)[0].second);
+  }
+  setActiveDiagnostic(false);
+  return OptionalDiagnostic();
+}
+
+const LangOptions &State::getLangOpts() const { return getCtx().getLangOpts(); }
+
+void State::addCallStack(unsigned Limit) {
+  // Determine which calls to skip, if any.
+  unsigned ActiveCalls = getCallStackDepth() - 1;
+  unsigned SkipStart = ActiveCalls, SkipEnd = SkipStart;
+  if (Limit && Limit < ActiveCalls) {
+    SkipStart = Limit / 2 + Limit % 2;
+    SkipEnd = ActiveCalls - Limit / 2;
+  }
+
+  // Walk the call stack and add the diagnostics.
+  unsigned CallIdx = 0;
+  Frame *Top = getCurrentFrame();
+  const Frame *Bottom = getBottomFrame();
+  for (Frame *F = Top; F != Bottom; F = F->getCaller(), ++CallIdx) {
+    SourceLocation CallLocation = F->getCallLocation();
+
+    // Skip this call?
+    if (CallIdx >= SkipStart && CallIdx < SkipEnd) {
+      if (CallIdx == SkipStart) {
+        // Note that we're skipping calls.
+        addDiag(CallLocation, diag::note_constexpr_calls_suppressed)
+            << unsigned(ActiveCalls - Limit);
+      }
+      continue;
+    }
+
+    // Use a different note for an inheriting constructor, because from the
+    // user's perspective it's not really a function at all.
+    if (auto *CD = dyn_cast_or_null<CXXConstructorDecl>(F->getCallee())) {
+      if (CD->isInheritingConstructor()) {
+        addDiag(CallLocation, diag::note_constexpr_inherited_ctor_call_here)
+            << CD->getParent();
+        continue;
+      }
+    }
+
+    SmallVector<char, 128> Buffer;
+    llvm::raw_svector_ostream Out(Buffer);
+    F->describe(Out);
+    addDiag(CallLocation, diag::note_constexpr_call_here) << Out.str();
+  }
+}

lib/AST/Interp/State.h

@@ -0,0 +1,130 @@
+//===--- State.h - State chain for the VM and AST Walker --------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Defines the base class of the interpreter and evaluator state.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_AST_INTERP_STATE_H
+#define LLVM_CLANG_AST_INTERP_STATE_H
+
+#include "clang/AST/ASTDiagnostic.h"
+#include "clang/AST/Expr.h"
+#include "clang/AST/OptionalDiagnostic.h"
+
+namespace clang {
+
+/// Kinds of access we can perform on an object, for diagnostics. Note that
+/// we consider a member function call to be a kind of access, even though
+/// it is not formally an access of the object, because it has (largely) the
+/// same set of semantic restrictions.
+enum AccessKinds {
+  AK_Read,
+  AK_Assign,
+  AK_Increment,
+  AK_Decrement,
+  AK_MemberCall,
+  AK_DynamicCast,
+  AK_TypeId,
+};
+
+// The order of this enum is important for diagnostics.
+enum CheckSubobjectKind {
+  CSK_Base,
+  CSK_Derived,
+  CSK_Field,
+  CSK_ArrayToPointer,
+  CSK_ArrayIndex,
+  CSK_Real,
+  CSK_Imag
+};
+
+namespace interp {
+class Frame;
+class SourceInfo;
+
+/// Interface for the VM to interact with the AST walker's context.
+class State {
+public:
+  virtual ~State();
+
+  virtual bool checkingForUndefinedBehavior() const = 0;
+  virtual bool checkingPotentialConstantExpression() const = 0;
+  virtual bool noteUndefinedBehavior() = 0;
+  virtual bool keepEvaluatingAfterFailure() const = 0;
+  virtual Frame *getCurrentFrame() = 0;
+  virtual const Frame *getBottomFrame() const = 0;
+  virtual bool hasActiveDiagnostic() = 0;
+  virtual void setActiveDiagnostic(bool Flag) = 0;
+  virtual void setFoldFailureDiagnostic(bool Flag) = 0;
+  virtual Expr::EvalStatus &getEvalStatus() const = 0;
+  virtual ASTContext &getCtx() const = 0;
+  virtual bool hasPriorDiagnostic() = 0;
+  virtual unsigned getCallStackDepth() = 0;
+
+public:
+  // Diagnose that the evaluation could not be folded (FF => FoldFailure)
+  OptionalDiagnostic
+  FFDiag(SourceLocation Loc,
+         diag::kind DiagId = diag::note_invalid_subexpr_in_const_expr,
+         unsigned ExtraNotes = 0);
+
+  OptionalDiagnostic
+  FFDiag(const Expr *E,
+         diag::kind DiagId = diag::note_invalid_subexpr_in_const_expr,
+         unsigned ExtraNotes = 0);
+
+  OptionalDiagnostic
+  FFDiag(const SourceInfo &SI,
+         diag::kind DiagId = diag::note_invalid_subexpr_in_const_expr,
+         unsigned ExtraNotes = 0);
+
+  /// Diagnose that the evaluation does not produce a C++11 core constant
+  /// expression.
+  ///
+  /// FIXME: Stop evaluating if we're in EM_ConstantExpression or
+  /// EM_PotentialConstantExpression mode and we produce one of these.
+  OptionalDiagnostic
+  CCEDiag(SourceLocation Loc,
+          diag::kind DiagId = diag::note_invalid_subexpr_in_const_expr,
+          unsigned ExtraNotes = 0);
+
+  OptionalDiagnostic
+  CCEDiag(const Expr *E,
+          diag::kind DiagId = diag::note_invalid_subexpr_in_const_expr,
+          unsigned ExtraNotes = 0);
+
+  OptionalDiagnostic
+  CCEDiag(const SourceInfo &SI,
+          diag::kind DiagId = diag::note_invalid_subexpr_in_const_expr,
+          unsigned ExtraNotes = 0);
+
+  /// Add a note to a prior diagnostic.
+  OptionalDiagnostic Note(SourceLocation Loc, diag::kind DiagId);
+
+  /// Add a stack of notes to a prior diagnostic.
+  void addNotes(ArrayRef<PartialDiagnosticAt> Diags);
+
+  /// Directly reports a diagnostic message.
+  DiagnosticBuilder report(SourceLocation Loc, diag::kind DiagId);
+
+  const LangOptions &getLangOpts() const;
+
+private:
+  void addCallStack(unsigned Limit);
+
+  PartialDiagnostic &addDiag(SourceLocation Loc, diag::kind DiagId);
+
+  OptionalDiagnostic diag(SourceLocation Loc, diag::kind DiagId,
+                          unsigned ExtraNotes, bool IsCCEDiag);
+};
+
+} // namespace interp
+} // namespace clang
+
+#endif

lib/Driver/ToolChains/Clang.cpp

@@ -4493,6 +4493,12 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
     CmdArgs.push_back(A->getValue());
   }
 
+  if (Args.hasArg(options::OPT_fexperimental_new_constant_interpreter))
+    CmdArgs.push_back("-fexperimental-new-constant-interpreter");
+
+  if (Args.hasArg(options::OPT_fforce_experimental_new_constant_interpreter))
+    CmdArgs.push_back("-fforce-experimental-new-constant-interpreter");
+
   if (Arg *A = Args.getLastArg(options::OPT_fbracket_depth_EQ)) {
     CmdArgs.push_back("-fbracket-depth");
     CmdArgs.push_back(A->getValue());

lib/Frontend/CompilerInvocation.cpp

@@ -2794,6 +2794,10 @@ static void ParseLangArgs(LangOptions &Opts, ArgList &Args, InputKind IK,
       getLastArgIntValue(Args, OPT_fconstexpr_depth, 512, Diags);
   Opts.ConstexprStepLimit =
       getLastArgIntValue(Args, OPT_fconstexpr_steps, 1048576, Diags);
+  Opts.EnableNewConstInterp =
+      Args.hasArg(OPT_fexperimental_new_constant_interpreter);
+  Opts.ForceNewConstInterp =
+      Args.hasArg(OPT_fforce_experimental_new_constant_interpreter);
   Opts.BracketDepth = getLastArgIntValue(Args, OPT_fbracket_depth, 256, Diags);
   Opts.DelayedTemplateParsing = Args.hasArg(OPT_fdelayed_template_parsing);
   Opts.NumLargeByValueCopy =

test/AST/Interp/cond.cpp

@@ -0,0 +1,11 @@
+// RUN: %clang_cc1 -std=c++17 -fsyntax-only -fforce-experimental-new-constant-interpreter %s -verify
+// RUN: %clang_cc1 -std=c++17 -fsyntax-only %s -verify
+// expected-no-diagnostics
+
+constexpr int cond_then_else(int a, int b) {
+  if (a < b) {
+    return b - a;
+  } else {
+    return a - b;
+  }
+}

utils/TableGen/CMakeLists.txt

@@ -8,6 +8,7 @@ add_tablegen(clang-tblgen CLANG
   ClangCommentHTMLTagsEmitter.cpp
   ClangDataCollectorsEmitter.cpp
   ClangDiagnosticsEmitter.cpp
+  ClangOpcodesEmitter.cpp
   ClangOpenCLBuiltinEmitter.cpp
   ClangOptionDocEmitter.cpp
   ClangSACheckersEmitter.cpp

utils/TableGen/ClangOpcodesEmitter.cpp

@@ -0,0 +1,360 @@
+//=== ClangOpcodesEmitter.cpp - constexpr interpreter opcodes ---*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// These tablegen backends emit Clang AST node tables
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/TableGen/Error.h"
+#include "llvm/TableGen/Record.h"
+#include "llvm/TableGen/StringMatcher.h"
+#include "llvm/TableGen/TableGenBackend.h"
+
+using namespace llvm;
+
+namespace {
+class ClangOpcodesEmitter {
+  RecordKeeper &Records;
+  Record Root;
+  unsigned NumTypes;
+
+public:
+  ClangOpcodesEmitter(RecordKeeper &R)
+    : Records(R), Root("Opcode", SMLoc(), R),
+      NumTypes(Records.getAllDerivedDefinitions("Type").size()) {}
+
+  void run(raw_ostream &OS);
+
+private:
+  /// Emits the opcode name for the opcode enum.
+  /// The name is obtained by concatenating the name with the list of types.
+  void EmitEnum(raw_ostream &OS, StringRef N, Record *R);
+
+  /// Emits the switch case and the invocation in the interpreter.
+  void EmitInterp(raw_ostream &OS, StringRef N, Record *R);
+
+  /// Emits the disassembler.
+  void EmitDisasm(raw_ostream &OS, StringRef N, Record *R);
+
+  /// Emits the byte code emitter method.
+  void EmitEmitter(raw_ostream &OS, StringRef N, Record *R);
+
+  /// Emits the prototype.
+  void EmitProto(raw_ostream &OS, StringRef N, Record *R);
+
+  /// Emits the prototype to dispatch from a type.
+  void EmitGroup(raw_ostream &OS, StringRef N, Record *R);
+
+  /// Emits the evaluator method.
+  void EmitEval(raw_ostream &OS, StringRef N, Record *R);
+
+  void PrintTypes(raw_ostream &OS, ArrayRef<Record *> Types);
+};
+
+void Enumerate(const Record *R,
+               StringRef N,
+               std::function<void(ArrayRef<Record *>, Twine)> &&F) {
+  llvm::SmallVector<Record *, 2> TypePath;
+  auto *Types = R->getValueAsListInit("Types");
+
+  std::function<void(size_t, const Twine &)> Rec;
+  Rec = [&TypePath, Types, &Rec, &F](size_t I, const Twine &ID) {
+    if (I >= Types->size()) {
+      F(TypePath, ID);
+      return;
+    }
+
+    if (auto *TypeClass = dyn_cast<DefInit>(Types->getElement(I))) {
+      for (auto *Type : TypeClass->getDef()->getValueAsListOfDefs("Types")) {
+        TypePath.push_back(Type);
+        Rec(I + 1, ID + Type->getName());
+        TypePath.pop_back();
+      }
+    } else {
+      PrintFatalError("Expected a type class");
+    }
+  };
+  Rec(0, N);
+}
+
+} // namespace
+
+void ClangOpcodesEmitter::run(raw_ostream &OS) {
+  for (auto *Opcode : Records.getAllDerivedDefinitions(Root.getName())) {
+    // The name is the record name, unless overriden.
+    StringRef N = Opcode->getValueAsString("Name");
+    if (N.empty())
+      N = Opcode->getName();
+
+    EmitEnum(OS, N, Opcode);
+    EmitInterp(OS, N, Opcode);
+    EmitDisasm(OS, N, Opcode);
+    EmitProto(OS, N, Opcode);
+    EmitGroup(OS, N, Opcode);
+    EmitEmitter(OS, N, Opcode);
+    EmitEval(OS, N, Opcode);
+  }
+}
+
+void ClangOpcodesEmitter::EmitEnum(raw_ostream &OS, StringRef N, Record *R) {
+  OS << "#ifdef GET_OPCODE_NAMES\n";
+  Enumerate(R, N, [&OS](ArrayRef<Record *>, const Twine &ID) {
+    OS << "OP_" << ID << ",\n";
+  });
+  OS << "#endif\n";
+}
+
+void ClangOpcodesEmitter::EmitInterp(raw_ostream &OS, StringRef N, Record *R) {
+  OS << "#ifdef GET_INTERP\n";
+
+  Enumerate(R, N, [this, R, &OS, &N](ArrayRef<Record *> TS, const Twine &ID) {
+    bool CanReturn = R->getValueAsBit("CanReturn");
+    bool ChangesPC = R->getValueAsBit("ChangesPC");
+    auto Args = R->getValueAsListOfDefs("Args");
+
+    OS << "case OP_" << ID << ": {\n";
+
+    // Emit calls to read arguments.
+    for (size_t I = 0, N = Args.size(); I < N; ++I) {
+      OS << "\tauto V" << I;
+      OS << " = ";
+      OS << "PC.read<" << Args[I]->getValueAsString("Name") << ">();\n";
+    }
+
+    // Emit a call to the template method and pass arguments.
+    OS << "\tif (!" << N;
+    PrintTypes(OS, TS);
+    OS << "(S";
+    if (ChangesPC)
+      OS << ", PC";
+    else
+      OS << ", OpPC";
+    if (CanReturn)
+      OS << ", Result";
+    for (size_t I = 0, N = Args.size(); I < N; ++I)
+      OS << ", V" << I;
+    OS << "))\n";
+    OS << "\t\treturn false;\n";
+
+    // Bail out if interpreter returned.
+    if (CanReturn) {
+      OS << "\tif (!S.Current || S.Current->isRoot())\n";
+      OS << "\t\treturn true;\n";
+    }
+
+    OS << "\tcontinue;\n";
+    OS << "}\n";
+  });
+  OS << "#endif\n";
+}
+
+void ClangOpcodesEmitter::EmitDisasm(raw_ostream &OS, StringRef N, Record *R) {
+  OS << "#ifdef GET_DISASM\n";
+  Enumerate(R, N, [R, &OS](ArrayRef<Record *>, const Twine &ID) {
+    OS << "case OP_" << ID << ":\n";
+    OS << "\tPrintName(\"" << ID << "\");\n";
+    OS << "\tOS << \"\\t\"";
+
+    for (auto *Arg : R->getValueAsListOfDefs("Args"))
+      OS << " << PC.read<" << Arg->getValueAsString("Name") << ">() << \" \"";
+
+    OS << "<< \"\\n\";\n";
+    OS << "\tcontinue;\n";
+  });
+  OS << "#endif\n";
+}
+
+void ClangOpcodesEmitter::EmitEmitter(raw_ostream &OS, StringRef N, Record *R) {
+  if (R->getValueAsBit("HasCustomLink"))
+    return;
+
+  OS << "#ifdef GET_LINK_IMPL\n";
+  Enumerate(R, N, [R, &OS](ArrayRef<Record *>, const Twine &ID) {
+    auto Args = R->getValueAsListOfDefs("Args");
+
+    // Emit the list of arguments.
+    OS << "bool ByteCodeEmitter::emit" << ID << "(";
+    for (size_t I = 0, N = Args.size(); I < N; ++I)
+      OS << Args[I]->getValueAsString("Name") << " A" << I << ",";
+    OS << "const SourceInfo &L) {\n";
+
+    // Emit a call to write the opcodes.
+    OS << "\treturn emitOp<";
+    for (size_t I = 0, N = Args.size(); I < N; ++I) {
+      if (I != 0)
+        OS << ", ";
+      OS << Args[I]->getValueAsString("Name");
+    }
+    OS << ">(OP_" << ID;
+    for (size_t I = 0, N = Args.size(); I < N; ++I)
+      OS << ", A" << I;
+    OS << ", L);\n";
+    OS << "}\n";
+  });
+  OS << "#endif\n";
+}
+
+void ClangOpcodesEmitter::EmitProto(raw_ostream &OS, StringRef N, Record *R) {
+  OS << "#if defined(GET_EVAL_PROTO) || defined(GET_LINK_PROTO)\n";
+  auto Args = R->getValueAsListOfDefs("Args");
+  Enumerate(R, N, [&OS, &Args](ArrayRef<Record *> TS, const Twine &ID) {
+    OS << "bool emit" << ID << "(";
+    for (auto *Arg : Args)
+      OS << Arg->getValueAsString("Name") << ", ";
+    OS << "const SourceInfo &);\n";
+  });
+
+  // Emit a template method for custom emitters to have less to implement.
+  auto TypeCount = R->getValueAsListInit("Types")->size();
+  if (R->getValueAsBit("HasCustomEval") && TypeCount) {
+    OS << "#if defined(GET_EVAL_PROTO)\n";
+    OS << "template<";
+    for (size_t I = 0; I < TypeCount; ++I) {
+      if (I != 0)
+        OS << ", ";
+      OS << "PrimType";
+    }
+    OS << ">\n";
+    OS << "bool emit" << N << "(";
+    for (auto *Arg : Args)
+      OS << Arg->getValueAsString("Name") << ", ";
+    OS << "const SourceInfo &);\n";
+    OS << "#endif\n";
+  }
+
+  OS << "#endif\n";
+}
+
+void ClangOpcodesEmitter::EmitGroup(raw_ostream &OS, StringRef N, Record *R) {
+  if (!R->getValueAsBit("HasGroup"))
+    return;
+
+  auto *Types = R->getValueAsListInit("Types");
+  auto Args = R->getValueAsListOfDefs("Args");
+
+  // Emit the prototype of the group emitter in the header.
+  OS << "#if defined(GET_EVAL_PROTO) || defined(GET_LINK_PROTO)\n";
+  OS << "bool emit" << N << "(";
+  for (size_t I = 0, N = Types->size(); I < N; ++I)
+    OS << "PrimType, ";
+  for (auto *Arg : Args)
+    OS << Arg->getValueAsString("Name") << ", ";
+  OS << "const SourceInfo &I);\n";
+  OS << "#endif\n";
+
+  // Emit the dispatch implementation in the source.
+  OS << "#if defined(GET_EVAL_IMPL) || defined(GET_LINK_IMPL)\n";
+  OS << "bool \n";
+  OS << "#if defined(GET_EVAL_IMPL)\n";
+  OS << "EvalEmitter\n";
+  OS << "#else\n";
+  OS << "ByteCodeEmitter\n";
+  OS << "#endif\n";
+  OS << "::emit" << N << "(";
+  for (size_t I = 0, N = Types->size(); I < N; ++I)
+    OS << "PrimType T" << I << ", ";
+  for (size_t I = 0, N = Args.size(); I < N; ++I)
+    OS << Args[I]->getValueAsString("Name") << " A" << I << ", ";
+  OS << "const SourceInfo &I) {\n";
+
+  std::function<void(size_t, const Twine &)> Rec;
+  llvm::SmallVector<Record *, 2> TS;
+  Rec = [this, &Rec, &OS, Types, &Args, R, &TS, N](size_t I, const Twine &ID) {
+    if (I >= Types->size()) {
+      // Print a call to the emitter method.
+      // Custom evaluator methods dispatch to template methods.
+      if (R->getValueAsBit("HasCustomEval")) {
+        OS << "#ifdef GET_LINK_IMPL\n";
+        OS << "return emit" << ID << "\n";
+        OS << "#else\n";
+        OS << "return emit" << N;
+        PrintTypes(OS, TS);
+        OS << "\n#endif\n";
+      } else {
+        OS << "return emit" << ID;
+      }
+
+      OS << "(";
+      for (size_t I = 0; I < Args.size(); ++I) {
+        OS << "A" << I << ", ";
+      }
+      OS << "I);\n";
+      return;
+    }
+
+    // Print a switch statement selecting T.
+    if (auto *TypeClass = dyn_cast<DefInit>(Types->getElement(I))) {
+      OS << "switch (T" << I << "){\n";
+      auto Cases = TypeClass->getDef()->getValueAsListOfDefs("Types");
+      for (auto *Case : Cases) {
+        OS << "case PT_" << Case->getName() << ":\n";
+        TS.push_back(Case);
+        Rec(I + 1, ID + Case->getName());
+        TS.pop_back();
+      }
+      // Emit a default case if not all types are present.
+      if (Cases.size() < NumTypes)
+        OS << "default: llvm_unreachable(\"invalid type\");\n";
+      OS << "}\n";
+      OS << "llvm_unreachable(\"invalid enum value\");\n";
+    } else {
+      PrintFatalError("Expected a type class");
+    }
+  };
+  Rec(0, N);
+
+  OS << "}\n";
+  OS << "#endif\n";
+}
+
+void ClangOpcodesEmitter::EmitEval(raw_ostream &OS, StringRef N, Record *R) {
+  if (R->getValueAsBit("HasCustomEval"))
+    return;
+
+  OS << "#ifdef GET_EVAL_IMPL\n";
+  Enumerate(R, N, [this, R, &N, &OS](ArrayRef<Record *> TS, const Twine &ID) {
+    auto Args = R->getValueAsListOfDefs("Args");
+
+    OS << "bool EvalEmitter::emit" << ID << "(";
+    for (size_t I = 0, N = Args.size(); I < N; ++I)
+      OS << Args[I]->getValueAsString("Name") << " A" << I << ",";
+    OS << "const SourceInfo &L) {\n";
+    OS << "if (!isActive()) return true;\n";
+    OS << "CurrentSource = L;\n";
+
+    OS << "return " << N;
+    PrintTypes(OS, TS);
+    OS << "(S, OpPC";
+    for (size_t I = 0, N = Args.size(); I < N; ++I)
+      OS << ", A" << I;
+    OS << ");\n";
+    OS << "}\n";
+  });
+
+  OS << "#endif\n";
+}
+
+void ClangOpcodesEmitter::PrintTypes(raw_ostream &OS, ArrayRef<Record *> Types) {
+  if (Types.empty())
+    return;
+  OS << "<";
+  for (size_t I = 0, N = Types.size(); I < N; ++I) {
+    if (I != 0)
+      OS << ", ";
+    OS << "PT_" << Types[I]->getName();
+  }
+  OS << ">";
+}
+
+namespace clang {
+
+void EmitClangOpcodes(RecordKeeper &Records, raw_ostream &OS) {
+  ClangOpcodesEmitter(Records).run(OS);
+}
+
+} // end namespace clang

utils/TableGen/TableGen.cpp

@@ -47,6 +47,7 @@ enum ActionType {
   GenClangCommentNodes,
   GenClangDeclNodes,
   GenClangStmtNodes,
+  GenClangOpcodes,
   GenClangSACheckers,
   GenClangCommentHTMLTags,
   GenClangCommentHTMLTagsProperties,
@@ -129,6 +130,8 @@ cl::opt<ActionType> Action(
                    "Generate Clang AST declaration nodes"),
         clEnumValN(GenClangStmtNodes, "gen-clang-stmt-nodes",
                    "Generate Clang AST statement nodes"),
+        clEnumValN(GenClangOpcodes, "gen-clang-opcodes",
+                   "Generate Clang constexpr interpreter opcodes"),
         clEnumValN(GenClangSACheckers, "gen-clang-sa-checkers",
                    "Generate Clang Static Analyzer checkers"),
         clEnumValN(GenClangCommentHTMLTags, "gen-clang-comment-html-tags",
@@ -251,6 +254,9 @@ bool ClangTableGenMain(raw_ostream &OS, RecordKeeper &Records) {
   case GenClangStmtNodes:
     EmitClangASTNodes(Records, OS, "Stmt", "");
     break;
+  case GenClangOpcodes:
+    EmitClangOpcodes(Records, OS);
+    break;
   case GenClangSACheckers:
     EmitClangSACheckers(Records, OS);
     break;

utils/TableGen/TableGenBackends.h

@@ -77,6 +77,7 @@ void EmitClangCommentCommandInfo(llvm::RecordKeeper &Records,
                                  llvm::raw_ostream &OS);
 void EmitClangCommentCommandList(llvm::RecordKeeper &Records,
                                  llvm::raw_ostream &OS);
+void EmitClangOpcodes(llvm::RecordKeeper &Records, llvm::raw_ostream &OS);
 
 void EmitNeon(llvm::RecordKeeper &Records, llvm::raw_ostream &OS);
 void EmitFP16(llvm::RecordKeeper &Records, llvm::raw_ostream &OS);