PR23833, DR2140: an lvalue-to-rvalue conversion on a glvalue of type

nullptr_t does not access memory.

We now reuse CK_NullToPointer to represent a conversion from a glvalue
of type nullptr_t to a prvalue of nullptr_t where necessary.

This reinstates r345562, reverted in r346065, now that CodeGen's
handling of non-odr-used variables has been fixed.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@363337 91177308-0d34-0410-b5e6-96231b3b80d8

lib/AST/Expr.cpp

@@ -1885,6 +1885,11 @@ ImplicitCastExpr *ImplicitCastExpr::Create(const ASTContext &C, QualType T,
                                            ExprValueKind VK) {
   unsigned PathSize = (BasePath ? BasePath->size() : 0);
   void *Buffer = C.Allocate(totalSizeToAlloc<CXXBaseSpecifier *>(PathSize));
+  // Per C++ [conv.lval]p3, lvalue-to-rvalue conversions on class and
+  // std::nullptr_t have special semantics not captured by CK_LValueToRValue.
+  assert((Kind != CK_LValueToRValue ||
+          !(T->isNullPtrType() || T->getAsCXXRecordDecl())) &&
+         "invalid type for lvalue-to-rvalue conversion");
   ImplicitCastExpr *E =
     new (Buffer) ImplicitCastExpr(T, Kind, Operand, PathSize, VK);
   if (PathSize)

lib/CodeGen/CGExprAgg.cpp

@@ -1352,7 +1352,8 @@ static bool isSimpleZero(const Expr *E, CodeGenFunction &CGF) {
   // (int*)0 - Null pointer expressions.
   if (const CastExpr *ICE = dyn_cast<CastExpr>(E))
     return ICE->getCastKind() == CK_NullToPointer &&
-        CGF.getTypes().isPointerZeroInitializable(E->getType());
+           CGF.getTypes().isPointerZeroInitializable(E->getType()) &&
+           !E->HasSideEffects(CGF.getContext());
   // '\0'
   if (const CharacterLiteral *CL = dyn_cast<CharacterLiteral>(E))
     return CL->getValue() == 0;

lib/CodeGen/CGExprScalar.cpp

@@ -2148,14 +2148,14 @@ Value *ScalarExprEmitter::VisitCastExpr(CastExpr *CE) {
 
   case CK_NullToPointer:
     if (MustVisitNullValue(E))
-      (void) Visit(E);
+      CGF.EmitIgnoredExpr(E);
 
     return CGF.CGM.getNullPointer(cast<llvm::PointerType>(ConvertType(DestTy)),
                               DestTy);
 
   case CK_NullToMemberPointer: {
     if (MustVisitNullValue(E))
-      (void) Visit(E);
+      CGF.EmitIgnoredExpr(E);
 
     const MemberPointerType *MPT = CE->getType()->getAs<MemberPointerType>();
     return CGF.CGM.getCXXABI().EmitNullMemberPointer(MPT);

lib/Sema/SemaExpr.cpp

@@ -635,8 +635,10 @@ ExprResult Sema::DefaultLvalueConversion(Expr *E) {
   if (E->getType().getObjCLifetime() == Qualifiers::OCL_Weak)
     Cleanup.setExprNeedsCleanups(true);
 
-  Res = ImplicitCastExpr::Create(Context, T, CK_LValueToRValue, E, nullptr,
-                                 VK_RValue);
+  // C++ [conv.lval]p3:
+  //   If T is cv std::nullptr_t, the result is a null pointer constant.
+  CastKind CK = T->isNullPtrType() ? CK_NullToPointer : CK_LValueToRValue;
+  Res = ImplicitCastExpr::Create(Context, T, CK, E, nullptr, VK_RValue);
 
   // C11 6.3.2.1p2:
   //   ... if the lvalue has atomic type, the value has the non-atomic version

lib/Sema/SemaInit.cpp

@@ -7779,9 +7779,13 @@ ExprResult InitializationSequence::Perform(Sema &S,
 
     case SK_LValueToRValue: {
       assert(CurInit.get()->isGLValue() && "cannot load from a prvalue");
-      CurInit = ImplicitCastExpr::Create(S.Context, Step->Type,
-                                         CK_LValueToRValue, CurInit.get(),
-                                         /*BasePath=*/nullptr, VK_RValue);
+      // C++ [conv.lval]p3:
+      //   If T is cv std::nullptr_t, the result is a null pointer constant.
+      CastKind CK =
+          Step->Type->isNullPtrType() ? CK_NullToPointer : CK_LValueToRValue;
+      CurInit =
+          ImplicitCastExpr::Create(S.Context, Step->Type, CK, CurInit.get(),
+                                   /*BasePath=*/nullptr, VK_RValue);
       break;
     }
 

lib/StaticAnalyzer/Core/ExprEngineC.cpp

@@ -378,7 +378,6 @@ void ExprEngine::VisitCast(const CastExpr *CastE, const Expr *Ex,
       case CK_BitCast:
       case CK_AddressSpaceConversion:
       case CK_BooleanToSignedIntegral:
-      case CK_NullToPointer:
       case CK_IntegralToPointer:
       case CK_PointerToIntegral: {
         SVal V = state->getSVal(Ex, LCtx);
@@ -503,6 +502,12 @@ void ExprEngine::VisitCast(const CastExpr *CastE, const Expr *Ex,
         Bldr.generateNode(CastE, Pred, state);
         continue;
       }
+      case CK_NullToPointer: {
+        SVal V = svalBuilder.makeNull();
+        state = state->BindExpr(CastE, LCtx, V);
+        Bldr.generateNode(CastE, Pred, state);
+        continue;
+      }
       case CK_NullToMemberPointer: {
         SVal V = svalBuilder.getMemberPointer(nullptr);
         state = state->BindExpr(CastE, LCtx, V);

test/Analysis/nullptr.cpp

@@ -126,20 +126,12 @@ struct Type {
 
 void shouldNotCrash() {
   decltype(nullptr) p; // expected-note{{'p' declared without an initial value}}
-  if (getSymbol()) // expected-note   {{Assuming the condition is false}}
-                   // expected-note@-1{{Taking false branch}}
-                   // expected-note@-2{{Assuming the condition is false}}
-                   // expected-note@-3{{Taking false branch}}
-                   // expected-note@-4{{Assuming the condition is true}}
-                   // expected-note@-5{{Taking true branch}}
-    invokeF(p); // expected-warning{{1st function call argument is an uninitialized value}}
-                // expected-note@-1{{1st function call argument is an uninitialized value}}
   if (getSymbol()) // expected-note   {{Assuming the condition is false}}
                    // expected-note@-1{{Taking false branch}}
                    // expected-note@-2{{Assuming the condition is true}}
                    // expected-note@-3{{Taking true branch}}
-    invokeF(nullptr); // expected-note   {{Calling 'invokeF'}}
-                      // expected-note@-1{{Passing null pointer value via 1st parameter 'x'}}
+    invokeF(p);    // expected-note   {{Calling 'invokeF'}}
+                   // expected-note@-1{{Passing null pointer value via 1st parameter 'x'}}
   if (getSymbol()) {  // expected-note  {{Assuming the condition is true}}
                       // expected-note@-1{{Taking true branch}}
     X *xx = Type().x; // expected-note   {{Null pointer value stored to field 'x'}}

test/CXX/drs/dr21xx.cpp

@@ -32,6 +32,16 @@ namespace dr2120 { // dr2120: 7
   static_assert(!__is_standard_layout(E), "");
 }
 
+namespace dr2140 { // dr2140: 9
+#if __cplusplus >= 201103L
+  union U { int a; decltype(nullptr) b; };
+  constexpr int *test(U u) {
+    return u.b;
+  }
+  static_assert(!test({123}), "u.b should be valid even when b is inactive");
+#endif
+}
+
 namespace dr2170 { // dr2170: 9
 #if __cplusplus >= 201103L
   void f() {

test/CodeGenCXX/nullptr.cpp

@@ -22,3 +22,50 @@ void g() {
 const std::type_info& f2() {
   return typeid(nullptr_t);
 }
+
+union U {
+  int n;
+  nullptr_t b;
+};
+// CHECK-LABEL: define {{.*}}pr23833_a
+// CHECK: store
+// CHECK: load
+// CHECK-NOT: load
+// CHECK: ret i1 false
+bool pr23833_a(U &u) { return u.b; }
+
+// CHECK-LABEL: define {{.*}}pr23833_b
+// CHECK: store
+// CHECK: load
+// CHECK-NOT: load
+// CHECK: ret i8* null
+nullptr_t pr23833_b(nullptr_t &n) { return n; }
+
+struct X1 { operator int*(); };
+struct X2 { operator const nullptr_t&(); };
+
+// CHECK-LABEL: define {{.*}}pr23833_c
+// CHECK: call {{.*}}X1
+// CHECK: call {{.*}}X2
+// CHECK-NOT: load
+// CHECK: ret i32
+int pr23833_c() {
+  return X1() != X2();
+}
+
+// CHECK-LABEL: define {{.*}}pr23833_d
+// CHECK: call {{.*}}X2
+// CHECK-NOT: load
+// CHECK: store
+// CHECK: load
+// CHECK: ret i32*
+int *pr23833_d() {
+  int *p = X2();
+  return p;
+}
+
+namespace PR39528 {
+  constexpr nullptr_t null = nullptr;
+  void f(nullptr_t);
+  void g() { f(null); }
+}

www/cxx_dr_status.html

@@ -12655,7 +12655,7 @@ and <I>POD class</I></td>
     <td><a href="http://wg21.link/cwg2140">2140</a></td>
     <td>CD4</td>
     <td>Lvalue-to-rvalue conversion of <TT>std::nullptr_t</TT></td>
-    <td class="none" align="center">Unknown</td>
+    <td class="svn" align="center">SVN</td>
   </tr>
   <tr id="2141">
     <td><a href="http://wg21.link/cwg2141">2141</a></td>