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From: "richard-gccbugzilla at metafoo dot co.uk" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug c/64843] New: miscompilation of atomic_fetch_add on atomic pointer type Date: Wed, 28 Jan 2015 19:38:00 -0000 [thread overview] Message-ID: <bug-64843-4@http.gcc.gnu.org/bugzilla/> (raw) https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64843 Bug ID: 64843 Summary: miscompilation of atomic_fetch_add on atomic pointer type Product: gcc Version: 4.9.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c Assignee: unassigned at gcc dot gnu.org Reporter: richard-gccbugzilla at metafoo dot co.uk #include <stdatomic.h> int *_Atomic p; void f() { atomic_fetch_add(&p, 1); } gives pushq %rbp movq %rsp, %rbp lock addq $1, p(%rip) popq %rbp ret ... which is wrong; gcc should add 4 to p, not 1. C11's atomic_fetch_add seems very difficult to implement with GCC's current set of builtins (you could in principle use _Generic to detect whether you have an atomic integer type). To this end, Clang adds a __c11_atomic_fetch_add builtin which provides the correct C11 semantics (premultiply by sizeof(*x) for an atomic pointer type) of atomic_fetch_add_explicit. Clang's __c11_... builtins also enforce some of the other C11 rules; for instance, only pointers to _Atomic types are permitted, so they may be valuable to add to GCC independent of this bug.
next reply other threads:[~2015-01-28 19:38 UTC|newest] Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top 2015-01-28 19:38 richard-gccbugzilla at metafoo dot co.uk [this message] 2015-01-28 20:38 ` [Bug c/64843] " rguenth at gcc dot gnu.org 2015-01-28 21:44 ` richard-gccbugzilla at metafoo dot co.uk 2015-01-28 23:20 ` joseph at codesourcery dot com 2015-01-28 23:24 ` joseph at codesourcery dot com 2015-01-29 0:05 ` richard-gccbugzilla at metafoo dot co.uk 2015-01-29 1:04 ` redi at gcc dot gnu.org
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