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[Clang] Make '-Wglobal-constructors` work on the GNU attributes #129917

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Mar 7, 2025
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[Clang] Make '-Wglobal-constructors` work on the GNU attributes #129917

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[Clang] Make '-Wglobal-constructors` work on the GNU attributes
jhuber6 Mar 5, 2025
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jhuber6 Mar 6, 2025
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3 changes: 3 additions & 0 deletions clang/docs/ReleaseNotes.rst
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Expand Up @@ -62,6 +62,9 @@ AST Dumping Potentially Breaking Changes
Clang Frontend Potentially Breaking Changes
-------------------------------------------

- The ``-Wglobal-constructors`` flag now applies to ``[[gnu::constructor]]`` and
``[[gnu::destructor]]`` attributes.

Clang Python Bindings Potentially Breaking Changes
--------------------------------------------------

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3 changes: 3 additions & 0 deletions clang/lib/Sema/SemaDeclAttr.cpp
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Expand Up @@ -2138,6 +2138,8 @@ static void handleConstructorAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
if (AL.getNumArgs() &&
!S.checkUInt32Argument(AL, AL.getArgAsExpr(0), priority))
return;
S.Diag(D->getLocation(), diag::warn_global_constructor)
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@erichkeane erichkeane Mar 6, 2025

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Why is this firing always? I thought this is only supposed to fire on Global and ThreadLocal variables according to the GCC patch summary. But this will fire any time there is a ConstructorAttr or DestructorAttr somewhere.

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@AaronBallman AaronBallman Mar 6, 2025

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How I see it is:

From GCC's docs: "The constructor attribute causes the function to be called automatically before execution enters main ()."

And -Wglobal-constructors is meant to tell you "this has a constructor which will fire before main" which matters for global and thread local variables, so the constructor attribute is sort of morally equivalent.

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@erichkeane erichkeane Mar 6, 2025

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Ah! I missed that about the constructor attribute. Thanks! Makes sense to me.

<< D->getSourceRange();

D->addAttr(::new (S.Context) ConstructorAttr(S.Context, AL, priority));
}
Expand All @@ -2147,6 +2149,7 @@ static void handleDestructorAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
if (AL.getNumArgs() &&
!S.checkUInt32Argument(AL, AL.getArgAsExpr(0), priority))
return;
S.Diag(D->getLocation(), diag::warn_global_destructor) << D->getSourceRange();

D->addAttr(::new (S.Context) DestructorAttr(S.Context, AL, priority));
}
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5 changes: 5 additions & 0 deletions clang/test/SemaCXX/attr-require-constant-initialization.cpp
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Expand Up @@ -337,6 +337,11 @@ constexpr TestCtor<NotC> inval_constexpr(42); // expected-error {{must be initia
ATTR constexpr TestCtor<NotC> inval_constexpr2(42); // expected-error {{must be initialized by a constant expression}}
// expected-note@-1 {{in call to 'TestCtor(42)'}}

[[gnu::constructor]] void ctor() {}
// expected-warning@-1 {{declaration requires a global constructor}}
[[gnu::destructor]] void dtor() {}
// expected-warning@-1 {{declaration requires a global destructor}}

#elif defined(TEST_THREE)
#if defined(__cplusplus)
#error This test requires C
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