[Sema] Implement C++14's DR1579: Prefer returning by converting move constructor
Fixes PR28096.
Differential Revision: http://reviews.llvm.org/D21619
llvm-svn: 274291
diff --git a/clang/lib/Sema/SemaStmt.cpp b/clang/lib/Sema/SemaStmt.cpp
index 61ef8f3..ed80b08 100644
--- a/clang/lib/Sema/SemaStmt.cpp
+++ b/clang/lib/Sema/SemaStmt.cpp
@@ -2697,16 +2697,16 @@
/// \param E The expression being returned from the function or block, or
/// being thrown.
///
-/// \param AllowFunctionParameter Whether we allow function parameters to
-/// be considered NRVO candidates. C++ prohibits this for NRVO itself, but
-/// we re-use this logic to determine whether we should try to move as part of
-/// a return or throw (which does allow function parameters).
+/// \param AllowParamOrMoveConstructible Whether we allow function parameters or
+/// id-expressions that could be moved out of the function to be considered NRVO
+/// candidates. C++ prohibits these for NRVO itself, but we re-use this logic to
+/// determine whether we should try to move as part of a return or throw (which
+/// does allow function parameters).
///
/// \returns The NRVO candidate variable, if the return statement may use the
/// NRVO, or NULL if there is no such candidate.
-VarDecl *Sema::getCopyElisionCandidate(QualType ReturnType,
- Expr *E,
- bool AllowFunctionParameter) {
+VarDecl *Sema::getCopyElisionCandidate(QualType ReturnType, Expr *E,
+ bool AllowParamOrMoveConstructible) {
if (!getLangOpts().CPlusPlus)
return nullptr;
@@ -2719,13 +2719,13 @@
if (!VD)
return nullptr;
- if (isCopyElisionCandidate(ReturnType, VD, AllowFunctionParameter))
+ if (isCopyElisionCandidate(ReturnType, VD, AllowParamOrMoveConstructible))
return VD;
return nullptr;
}
bool Sema::isCopyElisionCandidate(QualType ReturnType, const VarDecl *VD,
- bool AllowFunctionParameter) {
+ bool AllowParamOrMoveConstructible) {
QualType VDType = VD->getType();
// - in a return statement in a function with ...
// ... a class return type ...
@@ -2733,20 +2733,24 @@
if (!ReturnType->isRecordType())
return false;
// ... the same cv-unqualified type as the function return type ...
- if (!VDType->isDependentType() &&
+ // When considering moving this expression out, allow dissimilar types.
+ if (!AllowParamOrMoveConstructible && !VDType->isDependentType() &&
!Context.hasSameUnqualifiedType(ReturnType, VDType))
return false;
}
// ...object (other than a function or catch-clause parameter)...
if (VD->getKind() != Decl::Var &&
- !(AllowFunctionParameter && VD->getKind() == Decl::ParmVar))
+ !(AllowParamOrMoveConstructible && VD->getKind() == Decl::ParmVar))
return false;
if (VD->isExceptionVariable()) return false;
// ...automatic...
if (!VD->hasLocalStorage()) return false;
+ if (AllowParamOrMoveConstructible)
+ return true;
+
// ...non-volatile...
if (VD->getType().isVolatileQualified()) return false;
@@ -2765,7 +2769,7 @@
/// \brief Perform the initialization of a potentially-movable value, which
/// is the result of return value.
///
-/// This routine implements C++0x [class.copy]p33, which attempts to treat
+/// This routine implements C++14 [class.copy]p32, which attempts to treat
/// returned lvalues as rvalues in certain cases (to prefer move construction),
/// then falls back to treating them as lvalues if that failed.
ExprResult
@@ -2774,52 +2778,59 @@
QualType ResultType,
Expr *Value,
bool AllowNRVO) {
- // C++0x [class.copy]p33:
- // When the criteria for elision of a copy operation are met or would
- // be met save for the fact that the source object is a function
- // parameter, and the object to be copied is designated by an lvalue,
- // overload resolution to select the constructor for the copy is first
- // performed as if the object were designated by an rvalue.
+ // C++14 [class.copy]p32:
+ // When the criteria for elision of a copy/move operation are met, but not for
+ // an exception-declaration, and the object to be copied is designated by an
+ // lvalue, or when the expression in a return statement is a (possibly
+ // parenthesized) id-expression that names an object with automatic storage
+ // duration declared in the body or parameter-declaration-clause of the
+ // innermost enclosing function or lambda-expression, overload resolution to
+ // select the constructor for the copy is first performed as if the object
+ // were designated by an rvalue.
ExprResult Res = ExprError();
- if (AllowNRVO &&
- (NRVOCandidate || getCopyElisionCandidate(ResultType, Value, true))) {
- ImplicitCastExpr AsRvalue(ImplicitCastExpr::OnStack,
- Value->getType(), CK_NoOp, Value, VK_XValue);
+
+ if (AllowNRVO && !NRVOCandidate)
+ NRVOCandidate = getCopyElisionCandidate(ResultType, Value, true);
+
+ if (AllowNRVO && NRVOCandidate) {
+ ImplicitCastExpr AsRvalue(ImplicitCastExpr::OnStack, Value->getType(),
+ CK_NoOp, Value, VK_XValue);
Expr *InitExpr = &AsRvalue;
- InitializationKind Kind
- = InitializationKind::CreateCopy(Value->getLocStart(),
- Value->getLocStart());
- InitializationSequence Seq(*this, Entity, Kind, InitExpr);
- // [...] If overload resolution fails, or if the type of the first
- // parameter of the selected constructor is not an rvalue reference
- // to the object's type (possibly cv-qualified), overload resolution
- // is performed again, considering the object as an lvalue.
+ InitializationKind Kind = InitializationKind::CreateCopy(
+ Value->getLocStart(), Value->getLocStart());
+
+ InitializationSequence Seq(*this, Entity, Kind, InitExpr);
if (Seq) {
- for (InitializationSequence::step_iterator Step = Seq.step_begin(),
- StepEnd = Seq.step_end();
- Step != StepEnd; ++Step) {
- if (Step->Kind != InitializationSequence::SK_ConstructorInitialization)
+ for (const InitializationSequence::Step &Step : Seq.steps()) {
+ if (!(Step.Kind ==
+ InitializationSequence::SK_ConstructorInitialization ||
+ (Step.Kind == InitializationSequence::SK_UserConversion &&
+ isa<CXXConstructorDecl>(Step.Function.Function))))
continue;
- CXXConstructorDecl *Constructor
- = cast<CXXConstructorDecl>(Step->Function.Function);
+ CXXConstructorDecl *Constructor =
+ cast<CXXConstructorDecl>(Step.Function.Function);
const RValueReferenceType *RRefType
= Constructor->getParamDecl(0)->getType()
->getAs<RValueReferenceType>();
- // If we don't meet the criteria, break out now.
+ // [...] If the first overload resolution fails or was not performed, or
+ // if the type of the first parameter of the selected constructor is not
+ // an rvalue reference to the object’s type (possibly cv-qualified),
+ // overload resolution is performed again, considering the object as an
+ // lvalue.
if (!RRefType ||
!Context.hasSameUnqualifiedType(RRefType->getPointeeType(),
- Context.getTypeDeclType(Constructor->getParent())))
+ NRVOCandidate->getType()))
break;
// Promote "AsRvalue" to the heap, since we now need this
// expression node to persist.
- Value = ImplicitCastExpr::Create(Context, Value->getType(),
- CK_NoOp, Value, nullptr, VK_XValue);
+ Value = ImplicitCastExpr::Create(Context, Value->getType(), CK_NoOp,
+ Value, nullptr, VK_XValue);
// Complete type-checking the initialization of the return type
// using the constructor we found.