[clang] don't mark as Elidable CXXConstruct expressions used in NRVO

See PR51862.

The consumers of the Elidable flag in CXXConstructExpr assume that
an elidable construction just goes through a single copy/move construction,
so that the source object is immediately passed as an argument and is the same
type as the parameter itself.

With the implementation of P2266 and after some adjustments to the
implementation of P1825, we started (correctly, as per standard)
allowing more cases where the copy initialization goes through
user defined conversions.

With this patch we stop using this flag in NRVO contexts, to preserve code
that relies on that assumption.
This causes no known functional changes, we just stop firing some asserts
in a cople of included test cases.

Reviewed By: rsmith

Differential Revision: https://reviews.llvm.org/D109800

Author: mizvekov

clang/include/clang/Sema/Initialization.h

@@ -298,8 +298,8 @@ class alignas(8) InitializedEntity {
 
   /// Create the initialization entity for the result of a function.
   static InitializedEntity InitializeResult(SourceLocation ReturnLoc,
-                                            QualType Type, bool NRVO) {
-    return InitializedEntity(EK_Result, ReturnLoc, Type, NRVO);
+                                            QualType Type) {
+    return InitializedEntity(EK_Result, ReturnLoc, Type);
   }
 
   static InitializedEntity InitializeStmtExprResult(SourceLocation ReturnLoc,
@@ -308,20 +308,20 @@ class alignas(8) InitializedEntity {
   }
 
   static InitializedEntity InitializeBlock(SourceLocation BlockVarLoc,
-                                           QualType Type, bool NRVO) {
-    return InitializedEntity(EK_BlockElement, BlockVarLoc, Type, NRVO);
+                                           QualType Type) {
+    return InitializedEntity(EK_BlockElement, BlockVarLoc, Type);
   }
 
   static InitializedEntity InitializeLambdaToBlock(SourceLocation BlockVarLoc,
-                                                   QualType Type, bool NRVO) {
+                                                   QualType Type) {
     return InitializedEntity(EK_LambdaToBlockConversionBlockElement,
-                             BlockVarLoc, Type, NRVO);
+                             BlockVarLoc, Type);
   }
 
   /// Create the initialization entity for an exception object.
   static InitializedEntity InitializeException(SourceLocation ThrowLoc,
-                                               QualType Type, bool NRVO) {
-    return InitializedEntity(EK_Exception, ThrowLoc, Type, NRVO);
+                                               QualType Type) {
+    return InitializedEntity(EK_Exception, ThrowLoc, Type);
   }
 
   /// Create the initialization entity for an object allocated via new.

clang/lib/AST/ExprConstant.cpp

@@ -9933,10 +9933,19 @@ bool RecordExprEvaluator::VisitCXXConstructExpr(const CXXConstructExpr *E,
     return false;
 
   // Avoid materializing a temporary for an elidable copy/move constructor.
-  if (E->isElidable() && !ZeroInit)
-    if (const MaterializeTemporaryExpr *ME
-          = dyn_cast<MaterializeTemporaryExpr>(E->getArg(0)))
+  if (E->isElidable() && !ZeroInit) {
+    // FIXME: This only handles the simplest case, where the source object
+    //        is passed directly as the first argument to the constructor.
+    //        This should also handle stepping though implicit casts and
+    //        and conversion sequences which involve two steps, with a
+    //        conversion operator followed by a converting constructor.
+    const Expr *SrcObj = E->getArg(0);
+    assert(SrcObj->isTemporaryObject(Info.Ctx, FD->getParent()));
+    assert(Info.Ctx.hasSameUnqualifiedType(E->getType(), SrcObj->getType()));
+    if (const MaterializeTemporaryExpr *ME =
+            dyn_cast<MaterializeTemporaryExpr>(SrcObj))
       return Visit(ME->getSubExpr());
+  }
 
   if (ZeroInit && !ZeroInitialization(E, T))
     return false;

clang/lib/CodeGen/CGExprCXX.cpp

@@ -609,15 +609,18 @@ CodeGenFunction::EmitCXXConstructExpr(const CXXConstructExpr *E,
     return;
 
   // Elide the constructor if we're constructing from a temporary.
-  // The temporary check is required because Sema sets this on NRVO
-  // returns.
   if (getLangOpts().ElideConstructors && E->isElidable()) {
-    assert(getContext().hasSameUnqualifiedType(E->getType(),
-                                               E->getArg(0)->getType()));
-    if (E->getArg(0)->isTemporaryObject(getContext(), CD->getParent())) {
-      EmitAggExpr(E->getArg(0), Dest);
-      return;
-    }
+    // FIXME: This only handles the simplest case, where the source object
+    //        is passed directly as the first argument to the constructor.
+    //        This should also handle stepping though implicit casts and
+    //        conversion sequences which involve two steps, with a
+    //        conversion operator followed by a converting constructor.
+    const Expr *SrcObj = E->getArg(0);
+    assert(SrcObj->isTemporaryObject(getContext(), CD->getParent()));
+    assert(
+        getContext().hasSameUnqualifiedType(E->getType(), SrcObj->getType()));
+    EmitAggExpr(SrcObj, Dest);
+    return;
   }
 
   if (const ArrayType *arrayType

clang/lib/Sema/Sema.cpp

@@ -2022,7 +2022,7 @@ static void checkEscapingByref(VarDecl *VD, Sema &S) {
   Expr *VarRef =
       new (S.Context) DeclRefExpr(S.Context, VD, false, T, VK_LValue, Loc);
   ExprResult Result;
-  auto IE = InitializedEntity::InitializeBlock(Loc, T, false);
+  auto IE = InitializedEntity::InitializeBlock(Loc, T);
   if (S.getLangOpts().CPlusPlus2b) {
     auto *E = ImplicitCastExpr::Create(S.Context, T, CK_NoOp, VarRef, nullptr,
                                        VK_XValue, FPOptionsOverride());

clang/lib/Sema/SemaCoroutine.cpp

@@ -1533,7 +1533,7 @@ bool CoroutineStmtBuilder::makeGroDeclAndReturnStmt() {
   if (GroType->isVoidType()) {
     // Trigger a nice error message.
     InitializedEntity Entity =
-        InitializedEntity::InitializeResult(Loc, FnRetType, false);
+        InitializedEntity::InitializeResult(Loc, FnRetType);
     S.PerformCopyInitialization(Entity, SourceLocation(), ReturnValue);
     noteMemberDeclaredHere(S, ReturnValue, Fn);
     return false;

clang/lib/Sema/SemaDeclCXX.cpp

@@ -15312,8 +15312,17 @@ Sema::BuildCXXConstructExpr(SourceLocation ConstructLoc, QualType DeclInitType,
   //       can be omitted by constructing the temporary object
   //       directly into the target of the omitted copy/move
   if (ConstructKind == CXXConstructExpr::CK_Complete && Constructor &&
+      // FIXME: Converting constructors should also be accepted.
+      // But to fix this, the logic that digs down into a CXXConstructExpr
+      // to find the source object needs to handle it.
+      // Right now it assumes the source object is passed directly as the
+      // first argument.
       Constructor->isCopyOrMoveConstructor() && hasOneRealArgument(ExprArgs)) {
     Expr *SubExpr = ExprArgs[0];
+    // FIXME: Per above, this is also incorrect if we want to accept
+    //        converting constructors, as isTemporaryObject will
+    //        reject temporaries with different type from the
+    //        CXXRecord itself.
     Elidable = SubExpr->isTemporaryObject(
         Context, cast<CXXRecordDecl>(FoundDecl->getDeclContext()));
   }

clang/lib/Sema/SemaExpr.cpp

@@ -15713,7 +15713,7 @@ ExprResult Sema::ActOnBlockStmtExpr(SourceLocation CaretLoc,
         if (!Result.isInvalid()) {
           Result = PerformCopyInitialization(
               InitializedEntity::InitializeBlock(Var->getLocation(),
-                                                 Cap.getCaptureType(), false),
+                                                 Cap.getCaptureType()),
               Loc, Result.get());
         }
 

clang/lib/Sema/SemaExprCXX.cpp

@@ -893,9 +893,8 @@ ExprResult Sema::BuildCXXThrow(SourceLocation OpLoc, Expr *Ex,
     if (CheckCXXThrowOperand(OpLoc, ExceptionObjectTy, Ex))
       return ExprError();
 
-    InitializedEntity Entity = InitializedEntity::InitializeException(
-        OpLoc, ExceptionObjectTy,
-        /*NRVO=*/NRInfo.isCopyElidable());
+    InitializedEntity Entity =
+        InitializedEntity::InitializeException(OpLoc, ExceptionObjectTy);
     ExprResult Res = PerformMoveOrCopyInitialization(Entity, NRInfo, Ex);
     if (Res.isInvalid())
       return ExprError();

clang/lib/Sema/SemaLambda.cpp

@@ -1978,8 +1978,7 @@ ExprResult Sema::BuildBlockForLambdaConversion(SourceLocation CurrentLocation,
   CallOperator->markUsed(Context);
 
   ExprResult Init = PerformCopyInitialization(
-      InitializedEntity::InitializeLambdaToBlock(ConvLocation, Src->getType(),
-                                                 /*NRVO=*/false),
+      InitializedEntity::InitializeLambdaToBlock(ConvLocation, Src->getType()),
       CurrentLocation, Src);
   if (!Init.isInvalid())
     Init = ActOnFinishFullExpr(Init.get(), /*DiscardedValue*/ false);

clang/lib/Sema/SemaObjCProperty.cpp

@@ -1467,8 +1467,7 @@ Decl *Sema::ActOnPropertyImplDecl(Scope *S,
                                       LoadSelfExpr, true, true);
       ExprResult Res = PerformCopyInitialization(
           InitializedEntity::InitializeResult(PropertyDiagLoc,
-                                              getterMethod->getReturnType(),
-                                              /*NRVO=*/false),
+                                              getterMethod->getReturnType()),
           PropertyDiagLoc, IvarRefExpr);
       if (!Res.isInvalid()) {
         Expr *ResExpr = Res.getAs<Expr>();

clang/lib/Sema/SemaStmt.cpp

@@ -3653,8 +3653,8 @@ StmtResult Sema::ActOnCapScopeReturnStmt(SourceLocation ReturnLoc,
 
     // In C++ the return statement is handled via a copy initialization.
     // the C version of which boils down to CheckSingleAssignmentConstraints.
-    InitializedEntity Entity = InitializedEntity::InitializeResult(
-        ReturnLoc, FnRetType, NRVOCandidate != nullptr);
+    InitializedEntity Entity =
+        InitializedEntity::InitializeResult(ReturnLoc, FnRetType);
     ExprResult Res = PerformMoveOrCopyInitialization(
         Entity, NRInfo, RetValExp, SupressSimplerImplicitMoves);
     if (Res.isInvalid()) {
@@ -4085,8 +4085,8 @@ StmtResult Sema::BuildReturnStmt(SourceLocation ReturnLoc, Expr *RetValExp) {
     // the C version of which boils down to CheckSingleAssignmentConstraints.
     if (!HasDependentReturnType && !RetValExp->isTypeDependent()) {
       // we have a non-void function with an expression, continue checking
-      InitializedEntity Entity = InitializedEntity::InitializeResult(
-          ReturnLoc, RetType, NRVOCandidate != nullptr);
+      InitializedEntity Entity =
+          InitializedEntity::InitializeResult(ReturnLoc, RetType);
       ExprResult Res = PerformMoveOrCopyInitialization(
           Entity, NRInfo, RetValExp, SupressSimplerImplicitMoves);
       if (Res.isInvalid()) {

clang/test/CodeGen/nrvo-tracking.cpp

@@ -282,3 +282,40 @@ X t5() {
 }
 
 } // namespace test_alignas
+
+namespace PR51862 {
+
+template <class T> T test() {
+  T a;
+  T b;
+  if (0)
+    return a;
+  return b;
+}
+
+struct A {
+  A();
+  A(A &);
+  A(int);
+  operator int();
+};
+
+// CHECK-LABEL: define{{.*}} void @_ZN7PR518624testINS_1AEEET_v
+// CHECK:       call i32 @_ZN7PR518621AcviEv
+// CHECK-NEXT:  call void @_ZN7PR518621AC1Ei
+// CHECK-NEXT:  call void @llvm.lifetime.end
+template A test<A>();
+
+struct BSub {};
+struct B : BSub {
+  B();
+  B(B &);
+  B(const BSub &);
+};
+
+// CHECK-LABEL: define{{.*}} void @_ZN7PR518624testINS_1BEEET_v
+// CHECK:       call void @_ZN7PR518621BC1ERKNS_4BSubE
+// CHECK-NEXT:  call void @llvm.lifetime.end
+template B test<B>();
+
+} // namespace PR51862

clang/test/CodeGenCXX/copy-elision.cpp

@@ -0,0 +1,34 @@
+// RUN: %clang_cc1 -std=c++14 -triple x86_64-unknown-unknown-gnu -emit-llvm -O1 -fexperimental-new-pass-manager -o - %s | FileCheck %s
+
+template <class T> T test() {
+  return T();
+}
+
+struct A {
+  A();
+  A(A &);
+  A(int);
+  operator int();
+};
+
+// FIXME: There should be copy elision here.
+// CHECK-LABEL: define{{.*}} void @_Z4testI1AET_v
+// CHECK:       call void @_ZN1AC1Ev
+// CHECK-NEXT:  call i32 @_ZN1AcviEv
+// CHECK-NEXT:  call void @_ZN1AC1Ei
+// CHECK-NEXT:  call void @llvm.lifetime.end
+template A test<A>();
+
+struct BSub {};
+struct B : BSub {
+  B();
+  B(B &);
+  B(const BSub &);
+};
+
+// FIXME: There should be copy elision here.
+// CHECK-LABEL: define{{.*}} void @_Z4testI1BET_v
+// CHECK:       call void @_ZN1BC1Ev
+// CHECK:       call void @_ZN1BC1ERK4BSub
+// CHECK-NEXT:  call void @llvm.lifetime.end
+template B test<B>();