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[Clang][Sema]:Fix musttail attribute on a function with not_tail_called attribute has no warning/error #134465

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Merged
merged 9 commits into from
Apr 14, 2025
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[Clang][Sema]:Fix musttail attribute on a function with not_tail_called attribute has no warning/error #134465

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596679a
[llvm][Stmt]:git clang format
MillePlateaux Apr 4, 2025
8999e0f
Add error report
MillePlateaux Apr 8, 2025
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Error report
MillePlateaux Apr 8, 2025
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Report error when a musttail call is made to a function marked with t…
MillePlateaux Apr 10, 2025
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#GH134465 Modified version
MillePlateaux Apr 11, 2025
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Modified version
MillePlateaux Apr 11, 2025
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Modified version-3
MillePlateaux Apr 11, 2025
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Add note version
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not 'here' version
MillePlateaux Apr 14, 2025
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3 changes: 3 additions & 0 deletions clang/docs/ReleaseNotes.rst
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Expand Up @@ -325,6 +325,9 @@ Improvements to Clang's diagnostics
- Now correctly diagnose a tentative definition of an array with static
storage duration in pedantic mode in C. (#GH50661)

- An error is now emitted when a ``musttail`` call is made to a function marked with the ``not_tail_called`` attribute. (#GH133509).


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@erichkeane erichkeane Apr 14, 2025

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There should only be a single -new-line at the end of this list.

Improvements to Clang's time-trace
----------------------------------

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6 changes: 6 additions & 0 deletions clang/lib/Sema/SemaStmt.cpp
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Expand Up @@ -717,6 +717,12 @@ bool Sema::checkMustTailAttr(const Stmt *St, const Attr &MTA) {
return false;
}

if (const FunctionDecl *CalleeDecl = CE->getDirectCallee();
CalleeDecl && CalleeDecl->hasAttr<NotTailCalledAttr>()) {
Diag(St->getBeginLoc(), diag::err_musttail_mismatch) << true << CalleeDecl;
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@erichkeane erichkeane Apr 14, 2025

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Suggested change
Diag(St->getBeginLoc(), diag::err_musttail_mismatch) << true << CalleeDecl;
Diag(St->getBeginLoc(), diag::err_musttail_mismatch) << /*show-function-callee=*/true << CalleeDecl;

Not exactly our policy, but makes it way easier to understand what is going on here.

return false;
}

if (const auto *EWC = dyn_cast<ExprWithCleanups>(E)) {
if (EWC->cleanupsHaveSideEffects()) {
Diag(St->getBeginLoc(), diag::err_musttail_needs_trivial_args) << &MTA;
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17 changes: 17 additions & 0 deletions clang/test/Sema/attr-musttail.cpp
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@@ -0,0 +1,17 @@
// RUN: %clang_cc1 -verify -fsyntax-only %s

int __attribute__((not_tail_called)) foo1(int a) {
return a + 1;
}


int foo2(int a) {
[[clang::musttail]]
return foo1(a); // expected-error {{cannot perform a tail call to function 'foo1' because its signature is incompatible with the calling function}}
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@erichkeane erichkeane Apr 14, 2025

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Ooof... I know @AaronBallman suggested reusing the diagnostic... can we do at least a note (perhaps pointing to the attribute not_tail_called) to show WHY we are emitting this error?

Something like: `note: 'not_tail_called' attribute here prevents being called as a tail call'?

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@MillePlateaux MillePlateaux Apr 14, 2025

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@erichkeane Where do you want this note to be added?

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@erichkeane erichkeane Apr 14, 2025

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Sorry, a 'note' diagnostic. See a bunch of our diagnostics starting with note. Then you just do a second Diag call afterwards. I would think the source-location for it cna be the attribute on the callee.

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@MillePlateaux MillePlateaux Apr 14, 2025

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@erichkeane I still don't understand what you mean. Do I need to report cannot perform a tail call to function 'foo1' because its signature is incompatible with the calling function note: 'not_tail_called' attribute here prevents being called as a tail call' after the function is run?

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@erichkeane erichkeane Apr 14, 2025

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In SemaStmt.cpp right after the line:
Diag(St->getBeginLoc(), diag::err_musttail_mismatch) << true << CalleeDecl;

You can do:
Diag(<get location of not_tail_called_attribute>, diag::note_<your new diag>);

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@AaronBallman AaronBallman Apr 14, 2025

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Good idea Erich! I don't think we need a new diagnostic though, we can reuse note_conflicting_attribute, so I think this will suffice:

Diag(CalleeDecl->getAttr<NotTailCalledAttr>()->getLoc(), diag::note_conflicting_attribute);

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@erichkeane erichkeane Apr 14, 2025

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that attribute is imperfect (and requires reading tea leaves a bit), but probably good enough. I think that's a good suggestion.

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@AaronBallman AaronBallman Apr 14, 2025

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yeah, it'd help if the note mentioned the other attribute that was in conflict with, but I think it probably suffices. It at least gets folks' eyeballs closer to the conflict.

}

int main() {
int result = foo2(10);
return 0;
}