[Bug c/64843] New: miscompilation of atomic_fetch_add on atomic pointer type

["richard-gccbugzilla at metafoo dot co.uk" <gcc-bugzilla at gcc dot gnu dot org>]

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.