https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70191
Uroš Bizjak <ubizjak at gmail dot com> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |RESOLVED
Resolution|--- |INVALID
--- Comment #1 from Uroš Bizjak <ubizjak at gmail dot com> ---
(In reply to Alexey Lapshin from comment #0)
> __atomic_always_lock_free reports TRUE for 16-byte data object.
> inlined implementation uses lock-free instruction for load 16-byte atomic
> object.
>
> At the same time __atomic_load_16 routine from libatomic library does not
> use lock-free implementation. This looks incorrect.
>
> -bash-4.1$ uname -a
> SunOS 5.11 11.1 i86pc i386 i86pc
>
>
> -bash-4.1$ cat atomic_load.c
>
> #include <stdatomic.h>
> #include <stdio.h>
>
> typedef struct {
> char a[16];
> } test;
>
> int main()
> {
> test y = {0};
> _Atomic test x = ATOMIC_VAR_INIT(y);
>
> printf("\n __atomic_always_lock_free(sizeof(x), &x) %d ",
> __atomic_always_lock_free(sizeof(x), &x) );
>
> y = atomic_load (&x);
>
> return 0;
> }
>
> -bash-4.1$ gcc -m64 atomic_load.c -latomic -march=nehalem
> -bash-4.1$ a.out
>
> __atomic_always_lock_free(sizeof(x), &x) 1 <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
> -bash-4.1$ gcc -m64 atomic_load.c -latomic -march=nehalem -S
> -bash-4.1$ grep cmpx atomic_load.s
> lock cmpxchg16b (%rsi) <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
> -bash-4.1$
>
>
> If I would compile example without -march=nehalem then a call to libatomic
> __atomic_load_16 library function would be inserted. But that function does
> not use "lock cmpxchg16b" it uses locked implementation.
>
> -bash-4.1$ ldd a.out
> libatomic.so.1 =>
> /gcc/5.2.0/intel-S2/lib/amd64//libatomic.so.1
> libc.so.1 => /lib/64/libc.so.1
> libpthread.so.1 => /lib/64/libpthread.so.1
> libm.so.2 => /lib/64/libm.so.2
>
> So it looks like a bug that libatomic library have non lock-free
> implementation on Solaris.
Not all x86 targets implement cmpxchg16b. You should use appropriate -march= or
-mcx16 if you are sure your code will always run on cmpxchg16b capable target.
