Diagnose cases of "return x" that should be "return std::move(x)" for efficiency
Summary:
This patch adds two new diagnostics, which are off by default:
**-Wreturn-std-move**
This diagnostic is enabled by `-Wreturn-std-move`, `-Wmove`, or `-Wall`.
Diagnose cases of `return x` or `throw x`, where `x` is the name of a local variable or parameter, in which a copy operation is performed when a move operation would have been available. The user probably expected a move, but they're not getting a move, perhaps because the type of "x" is different from the return type of the function.
A place where this comes up in the wild is `stdext::inplace_function<Sig, N>` which implements conversion via a conversion operator rather than a converting constructor; see https://github.com/WG21-SG14/SG14/issues/125#issue-297201412
Another place where this has come up in the wild, but where the fix ended up being different, was
try { ... } catch (ExceptionType ex) {
throw ex;
}
where the appropriate fix in that case was to replace `throw ex;` with `throw;`, and incidentally to catch by reference instead of by value. (But one could contrive a scenario where the slicing was intentional, in which case throw-by-move would have been the appropriate fix after all.)
Another example (intentional slicing to a base class) is dissected in https://github.com/accuBayArea/Slides/blob/master/slides/2018-03-07.pdf
**-Wreturn-std-move-in-c++11**
This diagnostic is enabled only by the exact spelling `-Wreturn-std-move-in-c++11`.
Diagnose cases of "return x;" or "throw x;" which in this version of Clang *do* produce moves, but which prior to Clang 3.9 / GCC 5.1 produced copies instead. This is useful in codebases which care about portability to those older compilers.
The name "-in-c++11" is not technically correct; what caused the version-to-version change in behavior here was actually CWG 1579, not C++14. I think it's likely that codebases that need portability to GCC 4.9-and-earlier may understand "C++11" as a colloquialism for "older compilers." The wording of this diagnostic is based on feedback from @rsmith.
**Discussion**
Notice that this patch is kind of a negative-space version of Richard Trieu's `-Wpessimizing-move`. That diagnostic warns about cases of `return std::move(x)` that should be `return x` for speed. These diagnostics warn about cases of `return x` that should be `return std::move(x)` for speed. (The two diagnostics' bailiwicks do not overlap: we don't have to worry about a `return` statement flipping between the two states indefinitely.)
I propose to write a paper for San Diego that would relax the implicit-move rules so that in C++2a the user //would// see the moves they expect, and the diagnostic could be re-worded in a later version of Clang to suggest explicit `std::move` only "in C++17 and earlier." But in the meantime (and/or forever if that proposal is not well received), this diagnostic will be useful to detect accidental copy operations.
Reviewers: rtrieu, rsmith
Reviewed By: rsmith
Subscribers: lebedev.ri, Rakete1111, rsmith, cfe-commits
Tags: #clang
Differential Revision: https://reviews.llvm.org/D43322
Patch by Arthur O'Dwyer.
llvm-svn: 329914
diff --git a/clang/lib/Sema/SemaStmt.cpp b/clang/lib/Sema/SemaStmt.cpp
index 8963f4c..c3a627f 100644
--- a/clang/lib/Sema/SemaStmt.cpp
+++ b/clang/lib/Sema/SemaStmt.cpp
@@ -2905,7 +2905,8 @@
if (VD->getKind() != Decl::Var &&
!((CESK & CES_AllowParameters) && VD->getKind() == Decl::ParmVar))
return false;
- if (VD->isExceptionVariable()) return false;
+ if (!(CESK & CES_AllowExceptionVariables) && VD->isExceptionVariable())
+ return false;
// ...automatic...
if (!VD->hasLocalStorage()) return false;
@@ -2937,6 +2938,11 @@
/// returned lvalues as rvalues in certain cases (to prefer move construction),
/// then falls back to treating them as lvalues if that failed.
///
+/// \param ConvertingConstructorsOnly If true, follow [class.copy]p32 and reject
+/// resolutions that find non-constructors, such as derived-to-base conversions
+/// or `operator T()&&` member functions. If false, do consider such
+/// conversion sequences.
+///
/// \param Res We will fill this in if move-initialization was possible.
/// If move-initialization is not possible, such that we must fall back to
/// treating the operand as an lvalue, we will leave Res in its original
@@ -2946,6 +2952,7 @@
const VarDecl *NRVOCandidate,
QualType ResultType,
Expr *&Value,
+ bool ConvertingConstructorsOnly,
ExprResult &Res) {
ImplicitCastExpr AsRvalue(ImplicitCastExpr::OnStack, Value->getType(),
CK_NoOp, Value, VK_XValue);
@@ -2966,22 +2973,39 @@
continue;
FunctionDecl *FD = Step.Function.Function;
- if (isa<CXXConstructorDecl>(FD)) {
- // C++14 [class.copy]p32:
- // [...] 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.
- const RValueReferenceType *RRefType =
- FD->getParamDecl(0)->getType()->getAs<RValueReferenceType>();
- if (!RRefType)
- break;
- if (!S.Context.hasSameUnqualifiedType(RRefType->getPointeeType(),
- NRVOCandidate->getType()))
- break;
+ if (ConvertingConstructorsOnly) {
+ if (isa<CXXConstructorDecl>(FD)) {
+ // C++14 [class.copy]p32:
+ // [...] 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.
+ const RValueReferenceType *RRefType =
+ FD->getParamDecl(0)->getType()->getAs<RValueReferenceType>();
+ if (!RRefType)
+ break;
+ if (!S.Context.hasSameUnqualifiedType(RRefType->getPointeeType(),
+ NRVOCandidate->getType()))
+ break;
+ } else {
+ continue;
+ }
} else {
- continue;
+ if (isa<CXXConstructorDecl>(FD)) {
+ // Check that overload resolution selected a constructor taking an
+ // rvalue reference. If it selected an lvalue reference, then we
+ // didn't need to cast this thing to an rvalue in the first place.
+ if (!isa<RValueReferenceType>(FD->getParamDecl(0)->getType()))
+ break;
+ } else if (isa<CXXMethodDecl>(FD)) {
+ // Check that overload resolution selected a conversion operator
+ // taking an rvalue reference.
+ if (cast<CXXMethodDecl>(FD)->getRefQualifier() != RQ_RValue)
+ break;
+ } else {
+ continue;
+ }
}
// Promote "AsRvalue" to the heap, since we now need this
@@ -3019,13 +3043,82 @@
ExprResult Res = ExprError();
if (AllowNRVO) {
+ bool AffectedByCWG1579 = false;
+
if (!NRVOCandidate) {
NRVOCandidate = getCopyElisionCandidate(ResultType, Value, CES_Default);
+ if (NRVOCandidate &&
+ !getDiagnostics().isIgnored(diag::warn_return_std_move_in_cxx11,
+ Value->getExprLoc())) {
+ const VarDecl *NRVOCandidateInCXX11 =
+ getCopyElisionCandidate(ResultType, Value, CES_FormerDefault);
+ AffectedByCWG1579 = (!NRVOCandidateInCXX11);
+ }
}
if (NRVOCandidate) {
TryMoveInitialization(*this, Entity, NRVOCandidate, ResultType, Value,
- Res);
+ true, Res);
+ }
+
+ if (!Res.isInvalid() && AffectedByCWG1579) {
+ QualType QT = NRVOCandidate->getType();
+ if (QT.getNonReferenceType()
+ .getUnqualifiedType()
+ .isTriviallyCopyableType(Context)) {
+ // Adding 'std::move' around a trivially copyable variable is probably
+ // pointless. Don't suggest it.
+ } else {
+ // Common cases for this are returning unique_ptr<Derived> from a
+ // function of return type unique_ptr<Base>, or returning T from a
+ // function of return type Expected<T>. This is totally fine in a
+ // post-CWG1579 world, but was not fine before.
+ assert(!ResultType.isNull());
+ SmallString<32> Str;
+ Str += "std::move(";
+ Str += NRVOCandidate->getDeclName().getAsString();
+ Str += ")";
+ Diag(Value->getExprLoc(), diag::warn_return_std_move_in_cxx11)
+ << Value->getSourceRange()
+ << NRVOCandidate->getDeclName() << ResultType << QT;
+ Diag(Value->getExprLoc(), diag::note_add_std_move_in_cxx11)
+ << FixItHint::CreateReplacement(Value->getSourceRange(), Str);
+ }
+ } else if (Res.isInvalid() &&
+ !getDiagnostics().isIgnored(diag::warn_return_std_move,
+ Value->getExprLoc())) {
+ const VarDecl *FakeNRVOCandidate =
+ getCopyElisionCandidate(QualType(), Value, CES_AsIfByStdMove);
+ 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, 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);
+ }
+ }
+ }
}
}