return new (Context) BreakStmt(BreakLoc);
}
-/// Determine whether the given expression is a candidate for
-/// copy elision in either a return statement or a throw expression.
+/// Determine whether the given expression might be move-eligible or
+/// copy-elidable in either a (co_)return statement or throw expression,
+/// without considering function return type, if applicable.
///
-/// \param ReturnType If we're determining the copy elision candidate for
-/// a return statement, this is the return type of the function. If we're
-/// determining the copy elision candidate for a throw expression, this will
-/// be a NULL type.
+/// \param E The expression being returned from the function or block,
+/// being thrown, or being co_returned from a coroutine.
///
-/// \param E The expression being returned from the function or block, or
-/// being thrown.
+/// \param ForceCXX20 Overrides detection of current language mode
+/// and uses the rules for C++20.
///
-/// \param CESK 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,
- CopyElisionSemanticsKind CESK) {
+/// \returns An aggregate which contains the Candidate and isMoveEligible
+/// and isCopyElidable methods. If Candidate is non-null, it means
+/// isMoveEligible() would be true under the most permissive language standard.
+Sema::NamedReturnInfo Sema::getNamedReturnInfo(const Expr *E, bool ForceCXX20) {
+ if (!E)
+ return NamedReturnInfo();
// - in a return statement in a function [where] ...
// ... the expression is the name of a non-volatile automatic object ...
- DeclRefExpr *DR = dyn_cast<DeclRefExpr>(E->IgnoreParens());
+ const auto *DR = dyn_cast<DeclRefExpr>(E->IgnoreParens());
if (!DR || DR->refersToEnclosingVariableOrCapture())
- return nullptr;
- VarDecl *VD = dyn_cast<VarDecl>(DR->getDecl());
+ return NamedReturnInfo();
+ const auto *VD = dyn_cast<VarDecl>(DR->getDecl());
if (!VD)
- return nullptr;
+ return NamedReturnInfo();
+ return getNamedReturnInfo(VD, ForceCXX20);
+}
- if (isCopyElisionCandidate(ReturnType, VD, CESK))
- return VD;
- return nullptr;
+/// Updates the status in the given NamedReturnInfo object to disallow
+/// copy elision, and optionally also implicit move.
+///
+/// \param Info The NamedReturnInfo object to update.
+///
+/// \param CanMove If true, disallow only copy elision.
+/// If false, also disallow implcit move.
+static void disallowNRVO(Sema::NamedReturnInfo &Info, bool CanMove) {
+ Info.S = std::min(Info.S, CanMove ? Sema::NamedReturnInfo::MoveEligible
+ : Sema::NamedReturnInfo::None);
}
-bool Sema::isCopyElisionCandidate(QualType ReturnType, const VarDecl *VD,
- CopyElisionSemanticsKind CESK) {
- QualType VDType = VD->getType();
+/// Determine whether the given NRVO candidate variable is move-eligible or
+/// copy-elidable, without considering function return type.
+///
+/// \param VD The NRVO candidate variable.
+///
+/// \param ForceCXX20 Overrides detection of current language mode
+/// and uses the rules for C++20.
+///
+/// \returns An aggregate which contains the Candidate and isMoveEligible
+/// and isCopyElidable methods. If Candidate is non-null, it means
+/// isMoveEligible() would be true under the most permissive language standard.
+Sema::NamedReturnInfo Sema::getNamedReturnInfo(const VarDecl *VD,
+ bool ForceCXX20) {
+ bool hasCXX11 = getLangOpts().CPlusPlus11 || ForceCXX20;
+ bool hasCXX20 = getLangOpts().CPlusPlus20 || ForceCXX20;
+ NamedReturnInfo Info{VD, NamedReturnInfo::MoveEligibleAndCopyElidable};
+
+ // C++20 [class.copy.elision]p3:
// - in a return statement in a function with ...
- // ... a class return type ...
- if (!ReturnType.isNull() && !ReturnType->isDependentType()) {
- if (!ReturnType->isRecordType())
- return false;
- // ... the same cv-unqualified type as the function return type ...
- // When considering moving this expression out, allow dissimilar types.
- if (!(CESK & CES_AllowDifferentTypes) && !VDType->isDependentType() &&
- !Context.hasSameUnqualifiedType(ReturnType, VDType))
- return false;
- }
+ // (other than a function ... parameter)
+ if (VD->getKind() == Decl::ParmVar)
+ disallowNRVO(Info, hasCXX11);
+ else if (VD->getKind() != Decl::Var)
+ return NamedReturnInfo();
- // ...object (other than a function or catch-clause parameter)...
- if (VD->getKind() != Decl::Var &&
- !((CESK & CES_AllowParameters) && VD->getKind() == Decl::ParmVar))
- return false;
- if (!(CESK & CES_AllowExceptionVariables) && VD->isExceptionVariable())
- return false;
+ // (other than ... a catch-clause parameter)
+ if (VD->isExceptionVariable())
+ disallowNRVO(Info, hasCXX20);
// ...automatic...
- if (!VD->hasLocalStorage()) return false;
+ if (!VD->hasLocalStorage())
+ return NamedReturnInfo();
- // Return false if VD is a __block variable. We don't want to implicitly move
- // out of a __block variable during a return because we cannot assume the
- // variable will no longer be used.
+ // We don't want to implicitly move out of a __block variable during a return
+ // because we cannot assume the variable will no longer be used.
if (VD->hasAttr<BlocksAttr>())
- return false;
+ return NamedReturnInfo();
+ QualType VDType = VD->getType();
if (VDType->isObjectType()) {
// C++17 [class.copy.elision]p3:
// ...non-volatile automatic object...
if (VDType.isVolatileQualified())
- return false;
+ return NamedReturnInfo();
} else if (VDType->isRValueReferenceType()) {
// C++20 [class.copy.elision]p3:
- // ...either a non-volatile object or an rvalue reference to a non-volatile object type...
- if (!(CESK & CES_AllowRValueReferenceType))
- return false;
+ // ...either a non-volatile object or an rvalue reference to a non-volatile
+ // object type...
QualType VDReferencedType = VDType.getNonReferenceType();
- if (VDReferencedType.isVolatileQualified() || !VDReferencedType->isObjectType())
- return false;
+ if (VDReferencedType.isVolatileQualified() ||
+ !VDReferencedType->isObjectType())
+ return NamedReturnInfo();
+ disallowNRVO(Info, hasCXX20);
} else {
- return false;
+ return NamedReturnInfo();
}
- if (CESK & CES_AllowDifferentTypes)
- return true;
-
// Variables with higher required alignment than their type's ABI
// alignment cannot use NRVO.
if (!VDType->isDependentType() && VD->hasAttr<AlignedAttr>() &&
Context.getDeclAlign(VD) > Context.getTypeAlignInChars(VDType))
- return false;
+ disallowNRVO(Info, hasCXX11);
- return true;
+ return Info;
+}
+
+/// Updates given NamedReturnInfo's move-eligible and
+/// copy-elidable statuses, considering the function
+/// return type criteria as applicable to return statements.
+///
+/// \param Info The NamedReturnInfo object to update.
+///
+/// \param ReturnType This is the return type of the function.
+/// \returns The copy elision candidate, in case the initial return expression
+/// was copy elidable, or nullptr otherwise.
+const VarDecl *Sema::getCopyElisionCandidate(NamedReturnInfo &Info,
+ QualType ReturnType) {
+ if (!Info.Candidate)
+ return nullptr;
+
+ auto invalidNRVO = [&] {
+ Info = NamedReturnInfo();
+ return nullptr;
+ };
+
+ // If we got a non-deduced auto ReturnType, we are in a dependent context and
+ // there is no point in allowing copy elision since we won't have it deduced
+ // by the point the VardDecl is instantiated, which is the last chance we have
+ // of deciding if the candidate is really copy elidable.
+ if ((ReturnType->getTypeClass() == Type::TypeClass::Auto &&
+ ReturnType->isCanonicalUnqualified()) ||
+ ReturnType->isSpecificBuiltinType(BuiltinType::Dependent))
+ return invalidNRVO();
+
+ if (!ReturnType->isDependentType()) {
+ // - in a return statement in a function with ...
+ // ... a class return type ...
+ if (!ReturnType->isRecordType())
+ return invalidNRVO();
+
+ QualType VDType = Info.Candidate->getType();
+ // ... the same cv-unqualified type as the function return type ...
+ // When considering moving this expression out, allow dissimilar types.
+ if (!VDType->isDependentType() &&
+ !Context.hasSameUnqualifiedType(ReturnType, VDType))
+ disallowNRVO(Info, getLangOpts().CPlusPlus11);
+ }
+ return Info.isCopyElidable() ? Info.Candidate : nullptr;
}
/// Try to perform the initialization of a potentially-movable value,
/// the selected constructor/operator doesn't match the additional criteria, we
/// need to do the second overload resolution.
static bool TryMoveInitialization(Sema &S, const InitializedEntity &Entity,
- const VarDecl *NRVOCandidate,
- QualType ResultType, Expr *&Value,
+ const VarDecl *NRVOCandidate, Expr *&Value,
bool ConvertingConstructorsOnly,
bool IsDiagnosticsCheck, ExprResult &Res) {
ImplicitCastExpr AsRvalue(ImplicitCastExpr::OnStack, Value->getType(),
/// This routine implements C++20 [class.copy.elision]p3, 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 Sema::PerformMoveOrCopyInitialization(
- const InitializedEntity &Entity, const VarDecl *NRVOCandidate,
- QualType ResultType, Expr *Value, bool AllowNRVO) {
- ExprResult Res = ExprError();
- bool NeedSecondOverloadResolution = true;
-
- if (AllowNRVO) {
- CopyElisionSemanticsKind CESK = CES_Strict;
- if (getLangOpts().CPlusPlus20) {
- CESK = CES_ImplicitlyMovableCXX20;
- } else if (getLangOpts().CPlusPlus11) {
- CESK = CES_ImplicitlyMovableCXX11CXX14CXX17;
- }
-
- if (!NRVOCandidate) {
- NRVOCandidate = getCopyElisionCandidate(ResultType, Value, CESK);
- }
-
- if (NRVOCandidate) {
- NeedSecondOverloadResolution =
- TryMoveInitialization(*this, Entity, NRVOCandidate, ResultType, Value,
- !getLangOpts().CPlusPlus20, false, Res);
+ExprResult
+Sema::PerformMoveOrCopyInitialization(const InitializedEntity &Entity,
+ const NamedReturnInfo &NRInfo,
+ Expr *Value) {
+
+ if (NRInfo.Candidate) {
+ if (NRInfo.isMoveEligible()) {
+ ExprResult Res;
+ if (!TryMoveInitialization(*this, Entity, NRInfo.Candidate, Value,
+ !getLangOpts().CPlusPlus20, false, Res))
+ return Res;
}
-
- if (!getLangOpts().CPlusPlus20 && NeedSecondOverloadResolution &&
- !getDiagnostics().isIgnored(diag::warn_return_std_move,
+ if (!getDiagnostics().isIgnored(diag::warn_return_std_move,
Value->getExprLoc())) {
- const VarDecl *FakeNRVOCandidate = getCopyElisionCandidate(
- QualType(), Value, CES_ImplicitlyMovableCXX20);
- if (FakeNRVOCandidate) {
- QualType QT = FakeNRVOCandidate->getType();
- if (QT->isLValueReferenceType()) {
- // Adding 'std::move' around an lvalue reference variable's name is
- // dangerous. Don't suggest it.
- } else if (QT.getNonReferenceType()
- .getUnqualifiedType()
- .isTriviallyCopyableType(Context)) {
- // Adding 'std::move' around a trivially copyable variable is probably
- // pointless. Don't suggest it.
- } else {
- ExprResult FakeRes = ExprError();
- Expr *FakeValue = Value;
- TryMoveInitialization(*this, Entity, FakeNRVOCandidate, ResultType,
- FakeValue, false, true, FakeRes);
- if (!FakeRes.isInvalid()) {
- bool IsThrow =
- (Entity.getKind() == InitializedEntity::EK_Exception);
- SmallString<32> Str;
- Str += "std::move(";
- Str += FakeNRVOCandidate->getDeclName().getAsString();
- Str += ")";
- Diag(Value->getExprLoc(), diag::warn_return_std_move)
- << Value->getSourceRange()
- << FakeNRVOCandidate->getDeclName() << IsThrow;
- Diag(Value->getExprLoc(), diag::note_add_std_move)
- << FixItHint::CreateReplacement(Value->getSourceRange(), Str);
- }
+ QualType QT = NRInfo.Candidate->getType();
+ if (QT.getNonReferenceType().getUnqualifiedType().isTriviallyCopyableType(
+ Context)) {
+ // Adding 'std::move' around a trivially copyable variable is probably
+ // pointless. Don't suggest it.
+ } else {
+ ExprResult FakeRes = ExprError();
+ Expr *FakeValue = Value;
+ TryMoveInitialization(*this, Entity, NRInfo.Candidate, FakeValue, false,
+ true, FakeRes);
+ if (!FakeRes.isInvalid()) {
+ bool IsThrow = (Entity.getKind() == InitializedEntity::EK_Exception);
+ SmallString<32> Str;
+ Str += "std::move(";
+ Str += NRInfo.Candidate->getDeclName().getAsString();
+ Str += ")";
+ Diag(Value->getExprLoc(), diag::warn_return_std_move)
+ << Value->getSourceRange() << NRInfo.Candidate->getDeclName()
+ << IsThrow;
+ Diag(Value->getExprLoc(), diag::note_add_std_move)
+ << FixItHint::CreateReplacement(Value->getSourceRange(), Str);
}
}
}
// Either we didn't meet the criteria for treating an lvalue as an rvalue,
// above, or overload resolution failed. Either way, we need to try
// (again) now with the return value expression as written.
- if (NeedSecondOverloadResolution)
- Res = PerformCopyInitialization(Entity, SourceLocation(), Value);
-
- return Res;
+ return PerformCopyInitialization(Entity, SourceLocation(), Value);
}
/// Determine whether the declared return type of the specified function
/// ActOnCapScopeReturnStmt - Utility routine to type-check return statements
/// for capturing scopes.
///
-StmtResult
-Sema::ActOnCapScopeReturnStmt(SourceLocation ReturnLoc, Expr *RetValExp) {
+StmtResult Sema::ActOnCapScopeReturnStmt(SourceLocation ReturnLoc,
+ Expr *RetValExp,
+ NamedReturnInfo &NRInfo) {
// If this is the first return we've seen, infer the return type.
// [expr.prim.lambda]p4 in C++11; block literals follow the same rules.
CapturingScopeInfo *CurCap = cast<CapturingScopeInfo>(getCurFunction());
if (CurCap->ReturnType.isNull())
CurCap->ReturnType = FnRetType;
}
- assert(!FnRetType.isNull());
+ const VarDecl *NRVOCandidate = getCopyElisionCandidate(NRInfo, FnRetType);
if (auto *CurBlock = dyn_cast<BlockScopeInfo>(CurCap)) {
if (CurBlock->FunctionType->castAs<FunctionType>()->getNoReturnAttr()) {
// Otherwise, verify that this result type matches the previous one. We are
// pickier with blocks than for normal functions because we don't have GCC
// compatibility to worry about here.
- const VarDecl *NRVOCandidate = nullptr;
if (FnRetType->isDependentType()) {
// Delay processing for now. TODO: there are lots of dependent
// types we can conclusively prove aren't void.
// In C++ the return statement is handled via a copy initialization.
// the C version of which boils down to CheckSingleAssignmentConstraints.
- NRVOCandidate = getCopyElisionCandidate(FnRetType, RetValExp, CES_Strict);
- InitializedEntity Entity = InitializedEntity::InitializeResult(ReturnLoc,
- FnRetType,
- NRVOCandidate != nullptr);
- ExprResult Res = PerformMoveOrCopyInitialization(Entity, NRVOCandidate,
- FnRetType, RetValExp);
+ InitializedEntity Entity = InitializedEntity::InitializeResult(
+ ReturnLoc, FnRetType, NRVOCandidate != nullptr);
+ ExprResult Res = PerformMoveOrCopyInitialization(Entity, NRInfo, RetValExp);
if (Res.isInvalid()) {
// FIXME: Cleanup temporaries here, anyway?
return StmtError();
}
RetValExp = Res.get();
CheckReturnValExpr(RetValExp, FnRetType, ReturnLoc);
- } else {
- NRVOCandidate = getCopyElisionCandidate(FnRetType, RetValExp, CES_Strict);
}
if (RetValExp) {
if (RetValExp && DiagnoseUnexpandedParameterPack(RetValExp))
return StmtError();
+ NamedReturnInfo NRInfo = getNamedReturnInfo(RetValExp);
+
if (isa<CapturingScopeInfo>(getCurFunction()))
- return ActOnCapScopeReturnStmt(ReturnLoc, RetValExp);
+ return ActOnCapScopeReturnStmt(ReturnLoc, RetValExp, NRInfo);
QualType FnRetType;
QualType RelatedRetType;
}
}
}
+ const VarDecl *NRVOCandidate = getCopyElisionCandidate(NRInfo, FnRetType);
bool HasDependentReturnType = FnRetType->isDependentType();
/* NRVOCandidate=*/nullptr);
} else {
assert(RetValExp || HasDependentReturnType);
- const VarDecl *NRVOCandidate = nullptr;
-
QualType RetType = RelatedRetType.isNull() ? FnRetType : RelatedRetType;
// C99 6.8.6.4p3(136): The return statement is not an assignment. The
// In C++ the return statement is handled via a copy initialization,
// the C version of which boils down to CheckSingleAssignmentConstraints.
- if (RetValExp)
- NRVOCandidate = getCopyElisionCandidate(FnRetType, RetValExp, CES_Strict);
if (!HasDependentReturnType && !RetValExp->isTypeDependent()) {
// we have a non-void function with an expression, continue checking
- InitializedEntity Entity = InitializedEntity::InitializeResult(ReturnLoc,
- RetType,
- NRVOCandidate != nullptr);
- ExprResult Res = PerformMoveOrCopyInitialization(Entity, NRVOCandidate,
- RetType, RetValExp);
+ InitializedEntity Entity = InitializedEntity::InitializeResult(
+ ReturnLoc, RetType, NRVOCandidate != nullptr);
+ ExprResult Res =
+ PerformMoveOrCopyInitialization(Entity, NRInfo, RetValExp);
if (Res.isInvalid()) {
// FIXME: Clean up temporaries here anyway?
return StmtError();