PR28394: For compatibility with c++11 and c++14, if a static constexpr data

member is redundantly redeclared outside the class definition in code built in
c++17 mode, ensure we emit a non-discardable definition of the data member for
c++11 and c++14 compilations to use.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@274416 91177308-0d34-0410-b5e6-96231b3b80d8

include/clang/AST/ASTContext.h

@@ -2528,7 +2528,21 @@ public:
   /// \brief Returns true if this is an inline-initialized static data member
   /// which is treated as a definition for MSVC compatibility.
   bool isMSStaticDataMemberInlineDefinition(const VarDecl *VD) const;
-  
+
+  enum class InlineVariableDefinitionKind {
+    None,        ///< Not an inline variable.
+    Weak,        ///< Weak definition of inline variable.
+    WeakUnknown, ///< Weak for now, might become strong later in this TU.
+    Strong       ///< Strong definition.
+  };
+  /// \brief Determine whether a definition of this inline variable should
+  /// be treated as a weak or strong definition. For compatibility with
+  /// C++14 and before, for a constexpr static data member, if there is an
+  /// out-of-line declaration of the member, we may promote it from weak to
+  /// strong.
+  InlineVariableDefinitionKind
+  getInlineVariableDefinitionKind(const VarDecl *VD) const;
+
 private:
   const ASTRecordLayout &
   getObjCLayout(const ObjCInterfaceDecl *D,

lib/AST/ASTContext.cpp

@@ -5146,6 +5146,27 @@ bool ASTContext::isMSStaticDataMemberInlineDefinition(const VarDecl *VD) const {
          !VD->getFirstDecl()->isOutOfLine() && VD->getFirstDecl()->hasInit();
 }
 
+ASTContext::InlineVariableDefinitionKind
+ASTContext::getInlineVariableDefinitionKind(const VarDecl *VD) const {
+  if (!VD->isInline())
+    return InlineVariableDefinitionKind::None;
+
+  // In almost all cases, it's a weak definition.
+  auto *First = VD->getFirstDecl();
+  if (!First->isConstexpr() || First->isInlineSpecified() ||
+      !VD->isStaticDataMember())
+    return InlineVariableDefinitionKind::Weak;
+
+  // If there's a file-context declaration in this translation unit, it's a
+  // non-discardable definition.
+  for (auto *D : VD->redecls())
+    if (D->getLexicalDeclContext()->isFileContext())
+      return InlineVariableDefinitionKind::Strong;
+
+  // If we've not seen one yet, we don't know.
+  return InlineVariableDefinitionKind::WeakUnknown;
+}
+
 static inline 
 std::string charUnitsToString(const CharUnits &CU) {
   return llvm::itostr(CU.getQuantity());
@@ -8494,9 +8515,21 @@ static GVALinkage basicGVALinkageForVariable(const ASTContext &Context,
   if (Context.isMSStaticDataMemberInlineDefinition(VD))
     return GVA_DiscardableODR;
 
-  GVALinkage StrongLinkage = GVA_StrongExternal;
-  if (VD->isInline())
+  // Most non-template variables have strong linkage; inline variables are
+  // linkonce_odr or (occasionally, for compatibility) weak_odr.
+  GVALinkage StrongLinkage;
+  switch (Context.getInlineVariableDefinitionKind(VD)) {
+  case ASTContext::InlineVariableDefinitionKind::None:
+    StrongLinkage = GVA_StrongExternal;
+    break;
+  case ASTContext::InlineVariableDefinitionKind::Weak:
+  case ASTContext::InlineVariableDefinitionKind::WeakUnknown:
     StrongLinkage = GVA_DiscardableODR;
+    break;
+  case ASTContext::InlineVariableDefinitionKind::Strong:
+    StrongLinkage = GVA_StrongODR;
+    break;
+  }
 
   switch (VD->getTemplateSpecializationKind()) {
   case TSK_Undeclared:

lib/CodeGen/CodeGenModule.cpp

@@ -1447,6 +1447,12 @@ bool CodeGenModule::MayBeEmittedEagerly(const ValueDecl *Global) {
       // Implicit template instantiations may change linkage if they are later
       // explicitly instantiated, so they should not be emitted eagerly.
       return false;
+  if (const auto *VD = dyn_cast<VarDecl>(Global))
+    if (Context.getInlineVariableDefinitionKind(VD) ==
+        ASTContext::InlineVariableDefinitionKind::WeakUnknown)
+      // A definition of an inline constexpr static data member may change
+      // linkage later if it's redeclared outside the class.
+      return false;
   // If OpenMP is enabled and threadprivates must be generated like TLS, delay
   // codegen for global variables, because they may be marked as threadprivate.
   if (LangOpts.OpenMP && LangOpts.OpenMPUseTLS &&
@@ -1595,8 +1601,14 @@ void CodeGenModule::EmitGlobal(GlobalDecl GD) {
                             VD->hasAttr<CUDADeviceAttr>());
     if (!MustEmitForCuda &&
         VD->isThisDeclarationADefinition() != VarDecl::Definition &&
-        !Context.isMSStaticDataMemberInlineDefinition(VD))
+        !Context.isMSStaticDataMemberInlineDefinition(VD)) {
+      // If this declaration may have caused an inline variable definition to
+      // change linkage, make sure that it's emitted.
+      if (Context.getInlineVariableDefinitionKind(VD) ==
+          ASTContext::InlineVariableDefinitionKind::Strong)
+        GetAddrOfGlobalVar(VD);
       return;
+    }
   }
 
   // Defer code generation to first use when possible, e.g. if this is an inline

test/CodeGenCXX/cxx1z-inline-variables.cpp

@@ -24,6 +24,26 @@ int a = f();
 inline int b = f();
 int c = f();
 
+// For compatibility with C++11 and C++14, an out-of-line declaration of a
+// static constexpr local variable promotes the variable to weak_odr.
+struct compat {
+  static constexpr int a = 1;
+  static constexpr int b = 2;
+  static constexpr int c = 3;
+  static inline constexpr int d = 4;
+};
+const int &compat_use_before_redecl = compat::b;
+const int compat::a;
+const int compat::b;
+const int compat::c;
+const int compat::d;
+const int &compat_use_after_redecl1 = compat::c;
+const int &compat_use_after_redecl2 = compat::d;
+// CHECK: @_ZN6compat1bE = weak_odr constant i32 2
+// CHECK: @_ZN6compat1aE = weak_odr constant i32 1
+// CHECK: @_ZN6compat1cE = weak_odr constant i32 3
+// CHECK: @_ZN6compat1dE = linkonce_odr constant i32 4
+
 template<typename T> struct X {
   static int a;
   static inline int b;