DR1213: element access on an array xvalue or prvalue produces an xvalue. In the

latter case, a temporary array object is materialized, and can be
lifetime-extended by binding a reference to the member access. Likewise, in an
array-to-pointer decay, an rvalue array is materialized before being converted
into a pointer.

This caused IR generation to stop treating file-scope array compound literals
as having static storage duration in some cases in C++; that has been rectified
by modeling such a compound literal as an lvalue. This also improves clang's
compatibility with GCC for those cases.

llvm-svn: 288654

clang/lib/AST/ExprClassification.cpp

@@ -141,10 +141,9 @@ static Cl::Kinds ClassifyInternal(ASTContext &Ctx, const Expr *E) {
     return Cl::CL_LValue;
 
     // C99 6.5.2.5p5 says that compound literals are lvalues.
-    // In C++, they're prvalue temporaries.
+    // In C++, they're prvalue temporaries, except for file-scope arrays.
   case Expr::CompoundLiteralExprClass:
-    return Ctx.getLangOpts().CPlusPlus ? ClassifyTemporary(E->getType())
-                                       : Cl::CL_LValue;
+    return !E->isLValue() ? ClassifyTemporary(E->getType()) : Cl::CL_LValue;
 
     // Expressions that are prvalues.
   case Expr::CXXBoolLiteralExprClass:
@@ -196,11 +195,20 @@ static Cl::Kinds ClassifyInternal(ASTContext &Ctx, const Expr *E) {
     return ClassifyInternal(Ctx,
                  cast<SubstNonTypeTemplateParmExpr>(E)->getReplacement());
 
-    // C++ [expr.sub]p1: The result is an lvalue of type "T".
-    // However, subscripting vector types is more like member access.
+    // C, C++98 [expr.sub]p1: The result is an lvalue of type "T".
+    // C++11 (DR1213): in the case of an array operand, the result is an lvalue
+    //                 if that operand is an lvalue and an xvalue otherwise.
+    // Subscripting vector types is more like member access.
   case Expr::ArraySubscriptExprClass:
     if (cast<ArraySubscriptExpr>(E)->getBase()->getType()->isVectorType())
       return ClassifyInternal(Ctx, cast<ArraySubscriptExpr>(E)->getBase());
+    if (Lang.CPlusPlus11) {
+      // Step over the array-to-pointer decay if present, but not over the
+      // temporary materialization.
+      auto *Base = cast<ArraySubscriptExpr>(E)->getBase()->IgnoreImpCasts();
+      if (Base->getType()->isArrayType())
+        return ClassifyInternal(Ctx, Base);
+    }
     return Cl::CL_LValue;
 
     // C++ [expr.prim.general]p3: The result is an lvalue if the entity is a

clang/lib/AST/ExprConstant.cpp

@@ -2907,7 +2907,6 @@ static bool handleLValueToRValueConversion(EvalInfo &Info, const Expr *Conv,
       // In C99, a CompoundLiteralExpr is an lvalue, and we defer evaluating the
       // initializer until now for such expressions. Such an expression can't be
       // an ICE in C, so this only matters for fold.
-      assert(!Info.getLangOpts().CPlusPlus && "lvalue compound literal in c++?");
       if (Type.isVolatileQualified()) {
         Info.FFDiag(Conv);
         return false;
@@ -4711,7 +4710,7 @@ public:
 //  * VarDecl
 //  * FunctionDecl
 // - Literals
-//  * CompoundLiteralExpr in C
+//  * CompoundLiteralExpr in C (and in global scope in C++)
 //  * StringLiteral
 //  * CXXTypeidExpr
 //  * PredefinedExpr
@@ -4906,7 +4905,8 @@ bool LValueExprEvaluator::VisitMaterializeTemporaryExpr(
 
 bool
 LValueExprEvaluator::VisitCompoundLiteralExpr(const CompoundLiteralExpr *E) {
-  assert(!Info.getLangOpts().CPlusPlus && "lvalue compound literal in c++?");
+  assert((!Info.getLangOpts().CPlusPlus || E->isFileScope()) &&
+         "lvalue compound literal in c++?");
   // Defer visiting the literal until the lvalue-to-rvalue conversion. We can
   // only see this when folding in C, so there's no standard to follow here.
   return Success(E);

clang/lib/Sema/Sema.cpp

@@ -390,6 +390,18 @@ ExprResult Sema::ImpCastExprToType(Expr *E, QualType Ty,
   if (ExprTy == TypeTy)
     return E;
 
+  // C++1z [conv.array]: The temporary materialization conversion is applied.
+  // We also use this to fuel C++ DR1213, which applies to C++11 onwards.
+  if (Kind == CK_ArrayToPointerDecay && getLangOpts().CPlusPlus &&
+      E->getValueKind() == VK_RValue) {
+    // The temporary is an lvalue in C++98 and an xvalue otherwise.
+    ExprResult Materialized = CreateMaterializeTemporaryExpr(
+        E->getType(), E, !getLangOpts().CPlusPlus11);
+    if (Materialized.isInvalid())
+      return ExprError();
+    E = Materialized.get();
+  }
+
   if (ImplicitCastExpr *ImpCast = dyn_cast<ImplicitCastExpr>(E)) {
     if (ImpCast->getCastKind() == Kind && (!BasePath || BasePath->empty())) {
       ImpCast->setType(Ty);

clang/lib/Sema/SemaExpr.cpp

@@ -4376,6 +4376,16 @@ Sema::CreateBuiltinArraySubscriptExpr(Expr *Base, SourceLocation LLoc,
   Expr *LHSExp = Base;
   Expr *RHSExp = Idx;
 
+  ExprValueKind VK = VK_LValue;
+  ExprObjectKind OK = OK_Ordinary;
+
+  // Per C++ core issue 1213, the result is an xvalue if either operand is
+  // a non-lvalue array, and an lvalue otherwise.
+  if (getLangOpts().CPlusPlus11 &&
+      ((LHSExp->getType()->isArrayType() && !LHSExp->isLValue()) ||
+       (RHSExp->getType()->isArrayType() && !RHSExp->isLValue())))
+    VK = VK_XValue;
+
   // Perform default conversions.
   if (!LHSExp->getType()->getAs<VectorType>()) {
     ExprResult Result = DefaultFunctionArrayLvalueConversion(LHSExp);
@@ -4389,8 +4399,6 @@ Sema::CreateBuiltinArraySubscriptExpr(Expr *Base, SourceLocation LLoc,
   RHSExp = Result.get();
 
   QualType LHSTy = LHSExp->getType(), RHSTy = RHSExp->getType();
-  ExprValueKind VK = VK_LValue;
-  ExprObjectKind OK = OK_Ordinary;
 
   // C99 6.5.2.1p2: the expression e1[e2] is by definition precisely equivalent
   // to the expression *((e1)+(e2)). This means the array "Base" may actually be
@@ -5587,11 +5595,31 @@ Sema::BuildCompoundLiteralExpr(SourceLocation LParenLoc, TypeSourceInfo *TInfo,
   }
 
   // In C, compound literals are l-values for some reason.
-  ExprValueKind VK = getLangOpts().CPlusPlus ? VK_RValue : VK_LValue;
+  // For GCC compatibility, in C++, file-scope array compound literals with
+  // constant initializers are also l-values, and compound literals are
+  // otherwise prvalues.
+  //
+  // (GCC also treats C++ list-initialized file-scope array prvalues with
+  // constant initializers as l-values, but that's non-conforming, so we don't
+  // follow it there.)
+  //
+  // FIXME: It would be better to handle the lvalue cases as materializing and
+  // lifetime-extending a temporary object, but our materialized temporaries
+  // representation only supports lifetime extension from a variable, not "out
+  // of thin air".
+  // FIXME: For C++, we might want to instead lifetime-extend only if a pointer
+  // is bound to the result of applying array-to-pointer decay to the compound
+  // literal.
+  // FIXME: GCC supports compound literals of reference type, which should
+  // obviously have a value kind derived from the kind of reference involved.
+  ExprValueKind VK =
+      (getLangOpts().CPlusPlus && !(isFileScope && literalType->isArrayType()))
+          ? VK_RValue
+          : VK_LValue;
 
   return MaybeBindToTemporary(
-           new (Context) CompoundLiteralExpr(LParenLoc, TInfo, literalType,
-                                             VK, LiteralExpr, isFileScope));
+      new (Context) CompoundLiteralExpr(LParenLoc, TInfo, literalType,
+                                        VK, LiteralExpr, isFileScope));
 }
 
 ExprResult

clang/lib/Sema/SemaInit.cpp

@@ -5971,9 +5971,10 @@ performReferenceExtension(Expr *Init,
       if (CE->getSubExpr()->isGLValue())
         Init = CE->getSubExpr();
 
-    // FIXME: Per DR1213, subscripting on an array temporary produces an xvalue.
-    // It's unclear if binding a reference to that xvalue extends the array
-    // temporary.
+    // Per the current approach for DR1299, look through array element access
+    // when performing lifetime extension.
+    if (auto *ASE = dyn_cast<ArraySubscriptExpr>(Init))
+      Init = ASE->getBase();
   } while (Init != Old);
 
   if (MaterializeTemporaryExpr *ME = dyn_cast<MaterializeTemporaryExpr>(Init)) {

clang/lib/StaticAnalyzer/Core/ExprEngineC.cpp

@@ -480,15 +480,7 @@ void ExprEngine::VisitCompoundLiteralExpr(const CompoundLiteralExpr *CL,
     Loc CLLoc = State->getLValue(CL, LCtx);
     State = State->bindLoc(CLLoc, V);
 
-    // Compound literal expressions are a GNU extension in C++.
-    // Unlike in C, where CLs are lvalues, in C++ CLs are prvalues,
-    // and like temporary objects created by the functional notation T()
-    // CLs are destroyed at the end of the containing full-expression.
-    // HOWEVER, an rvalue of array type is not something the analyzer can
-    // reason about, since we expect all regions to be wrapped in Locs.
-    // So we treat array CLs as lvalues as well, knowing that they will decay
-    // to pointers as soon as they are used.
-    if (CL->isGLValue() || CL->getType()->isArrayType())
+    if (CL->isGLValue())
       V = CLLoc;
   }
 

clang/test/Analysis/explain-svals.cpp

@@ -76,7 +76,7 @@ void test_4(int x, int y) {
   clang_analyzer_explain(&stat); // expected-warning-re{{{{^pointer to static local variable 'stat'$}}}}
   clang_analyzer_explain(stat_glob); // expected-warning-re{{{{^initial value of global variable 'stat_glob'$}}}}
   clang_analyzer_explain(&stat_glob); // expected-warning-re{{{{^pointer to global variable 'stat_glob'$}}}}
-  clang_analyzer_explain((int[]){1, 2, 3}); // expected-warning-re{{{{^pointer to element of type 'int' with index 0 of compound literal \(int \[3\]\)\{1, 2, 3\}$}}}}
+  clang_analyzer_explain((int[]){1, 2, 3}); // expected-warning-re{{{{^pointer to element of type 'int' with index 0 of temporary object constructed at statement '\(int \[3\]\)\{1, 2, 3\}'$}}}}
 }
 
 namespace {

clang/test/CXX/drs/dr12xx.cpp

@@ -5,6 +5,17 @@
 
 // expected-no-diagnostics
 
+namespace dr1213 { // dr1213: 4.0
+#if __cplusplus >= 201103L
+  using T = int[3];
+  int &&r = T{}[1];
+
+  using T = decltype((T{}));
+  using U = decltype((T{}[2]));
+  using U = int &&;
+#endif
+}
+
 namespace dr1250 {  // dr1250: 3.9
 struct Incomplete;
 

clang/test/CodeGenCXX/compound-literals.cpp

@@ -12,6 +12,9 @@ struct Y {
   X x;
 };
 
+// CHECK: @.compoundliteral = internal global [5 x i32] [i32 1, i32 2, i32 3, i32 4, i32 5], align 4
+// CHECK: @q = global i32* getelementptr inbounds ([5 x i32], [5 x i32]* @.compoundliteral, i32 0, i32 0), align 4
+
 // CHECK-LABEL: define i32 @_Z1fv()
 int f() {
   // CHECK: [[LVALUE:%[a-z0-9.]+]] = alloca
@@ -51,20 +54,27 @@ int *p = (Z){ {1, 2, 3} }.i;
 // CHECK: store i32* %{{.*}}, i32** @p
 
 int *q = (int [5]){1, 2, 3, 4, 5};
-// CHECK-LABEL: define {{.*}}__cxx_global_var_init.1()
-// CHECK: store i32* getelementptr inbounds ([5 x i32], [5 x i32]* @.compoundliteral, i32 0, i32 0), i32** @q
+// (constant initialization, checked above)
 
-int *PR21912_1 = (int []){};
-// CHECK-LABEL: define {{.*}}__cxx_global_var_init.2()
-// CHECK: store i32* getelementptr inbounds ([0 x i32], [0 x i32]* @.compoundliteral.3, i32 0, i32 0), i32** @PR21912_1
+extern int n;
+int *r = (int [5]){1, 2, 3, 4, 5} + n;
+// CHECK-LABEL: define {{.*}}__cxx_global_var_init.1()
+// CHECK: %[[PTR:.*]] = getelementptr inbounds i32, i32* getelementptr inbounds ([5 x i32], [5 x i32]* @.compoundliteral.2, i32 0, i32 0), i32 %
+// CHECK: store i32* %[[PTR]], i32** @r
+
+int *PR21912_1 = (int []){} + n;
+// CHECK-LABEL: define {{.*}}__cxx_global_var_init.3()
+// CHECK: %[[PTR:.*]] = getelementptr inbounds i32, i32* getelementptr inbounds ([0 x i32], [0 x i32]* @.compoundliteral.4, i32 0, i32 0), i32 %
+// CHECK: store i32* %[[PTR]], i32** @PR21912_1
 
 union PR21912Ty {
   long long l;
   double d;
 };
-union PR21912Ty *PR21912_2 = (union PR21912Ty[]){{.d = 2.0}, {.l = 3}};
-// CHECK-LABEL: define {{.*}}__cxx_global_var_init.4()
-// CHECK: store %union.PR21912Ty* getelementptr inbounds ([2 x %union.PR21912Ty], [2 x %union.PR21912Ty]* bitcast (<{ { double }, %union.PR21912Ty }>* @.compoundliteral.5 to [2 x %union.PR21912Ty]*), i32 0, i32 0), %union.PR21912Ty** @PR21912_2
+union PR21912Ty *PR21912_2 = (union PR21912Ty[]){{.d = 2.0}, {.l = 3}} + n;
+// CHECK-LABEL: define {{.*}}__cxx_global_var_init.5()
+// CHECK: %[[PTR:.*]] = getelementptr inbounds %union.PR21912Ty, %union.PR21912Ty* getelementptr inbounds ([2 x %union.PR21912Ty], [2 x %union.PR21912Ty]* bitcast (<{ { double }, %union.PR21912Ty }>* @.compoundliteral.6 to [2 x %union.PR21912Ty]*), i32 0, i32 0), i32 %
+// CHECK: store %union.PR21912Ty* %[[PTR]], %union.PR21912Ty** @PR21912_2, align 4
 
 // This compound literal should have local scope.
 int computed_with_lambda = [] {

clang/test/CodeGenCXX/stack-reuse.cpp

@@ -132,8 +132,8 @@ void large_auto_object() {
 
 int large_combiner_test(S_large s) {
 // CHECK-LABEL: define i32 @large_combiner_test
-// CHECK: [[T1:%.*]] = alloca %struct.Combiner
 // CHECK: [[T2:%.*]] = alloca %struct.Combiner
+// CHECK: [[T1:%.*]] = alloca %struct.Combiner
 // CHECK: [[T3:%.*]] = call %struct.Combiner* @_ZN8CombinerC1E7S_large(%struct.Combiner* nonnull [[T1]], [9 x i32] %s.coerce)
 // CHECK: call void @_ZN8Combiner1fEv(%struct.Combiner* nonnull sret [[T2]], %struct.Combiner* nonnull [[T1]])
 // CHECK: [[T4:%.*]] = getelementptr inbounds %struct.Combiner, %struct.Combiner* [[T2]], i32 0, i32 0, i32 0, i32 0

clang/test/CodeGenCXX/temporaries.cpp

@@ -779,6 +779,36 @@ namespace MultipleExtension {
   }
 }
 
+namespace ArrayAccess {
+  struct A { A(int); ~A(); };
+  void g();
+  void f() {
+    using T = A[3];
+
+    // CHECK: call void @_ZN11ArrayAccess1AC1Ei({{.*}}, i32 1
+    // CHECK-NOT: @_ZN11ArrayAccess1AD
+    // CHECK: call void @_ZN11ArrayAccess1AC1Ei({{.*}}, i32 2
+    // CHECK-NOT: @_ZN11ArrayAccess1AD
+    // CHECK: call void @_ZN11ArrayAccess1AC1Ei({{.*}}, i32 3
+    // CHECK-NOT: @_ZN11ArrayAccess1AD
+    A &&a = T{1, 2, 3}[1];
+
+    // CHECK: call void @_ZN11ArrayAccess1AC1Ei({{.*}}, i32 4
+    // CHECK-NOT: @_ZN11ArrayAccess1AD
+    // CHECK: call void @_ZN11ArrayAccess1AC1Ei({{.*}}, i32 5
+    // CHECK-NOT: @_ZN11ArrayAccess1AD
+    // CHECK: call void @_ZN11ArrayAccess1AC1Ei({{.*}}, i32 6
+    // CHECK-NOT: @_ZN11ArrayAccess1AD
+    A &&b = 2[T{4, 5, 6}];
+
+    // CHECK: call void @_ZN11ArrayAccess1gEv(
+    g();
+
+    // CHECK: call void @_ZN11ArrayAccess1AD
+    // CHECK: call void @_ZN11ArrayAccess1AD
+  }
+}
+
 namespace PR14130 {
   struct S { S(int); };
   struct U { S &&s; };

clang/test/SemaCXX/constant-expression-cxx11.cpp

@@ -1467,13 +1467,26 @@ namespace VLASizeof {
 }
 
 namespace CompoundLiteral {
-  // FIXME:
-  // We don't model the semantics of this correctly: the compound literal is
-  // represented as a prvalue in the AST, but actually behaves like an lvalue.
-  // We treat the compound literal as a temporary and refuse to produce a
-  // pointer to it. This is OK: we're not required to treat this as a constant
-  // in C++, and in C we model compound literals as lvalues.
-  constexpr int *p = (int*)(int[1]){0}; // expected-warning {{C99}} expected-error {{constant expression}} expected-note 2{{temporary}}
+  // Matching GCC, file-scope array compound literals initialized by constants
+  // are lifetime-extended.
+  constexpr int *p = (int*)(int[1]){3}; // expected-warning {{C99}}
+  static_assert(*p == 3, "");
+  static_assert((int[2]){1, 2}[1] == 2, ""); // expected-warning {{C99}}
+
+  // Other kinds are not.
+  struct X { int a[2]; };
+  constexpr int *n = (X){1, 2}.a; // expected-warning {{C99}} expected-warning {{temporary array}}
+  // expected-error@-1 {{constant expression}}
+  // expected-note@-2 {{pointer to subobject of temporary}}
+  // expected-note@-3 {{temporary created here}}
+
+  void f() {
+    static constexpr int *p = (int*)(int[1]){3}; // expected-warning {{C99}}
+    // expected-error@-1 {{constant expression}}
+    // expected-note@-2 {{pointer to subobject of temporary}}
+    // expected-note@-3 {{temporary created here}}
+    static_assert((int[2]){1, 2}[1] == 2, ""); // expected-warning {{C99}}
+  }
 }
 
 namespace Vector {

clang/www/cxx_dr_status.html

@@ -7093,7 +7093,7 @@ and <I>POD class</I></td>
     <td><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1213">1213</a></td>
     <td>CD3</td>
     <td>Array subscripting and xvalues</td>
-    <td class="none" align="center">Unknown</td>
+    <td class="svn" align="center">SVN</td>
   </tr>
   <tr id="1214">
     <td><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1214">1214</a></td>