[Bug c/63368] New: undefined reference to `__sync_val_compare_and_swap_8'

["mathieu.malaterre at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63368

            Bug ID: 63368
           Summary: undefined reference to `__sync_val_compare_and_swap_8'
           Product: gcc
           Version: 4.9.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mathieu.malaterre at gmail dot com

If I am reading the online documentation properly:

https://gcc.gnu.org/onlinedocs/gcc-4.9.1/gcc/_005f_005fsync-Builtins.html#_005f_005fsync-Builtins

__sync_val_compare_and_swap on some targets will result in a function call
(where direct code generation is not available or not yet implemented). 

It seems to be happening in my testcase, however __sync_val_compare_and_swap_8
does not seems to be defined anywhere else.

Steps:

$ cat t.c 
#include <stdint.h>

int main()
{
  int64_t x = 0, y = 1; 
  y = __sync_val_compare_and_swap(&x, x, y);
  return 0;
}
$ gcc t.c
/tmp/cc6JIpFx.o: In function `main':
t.c:(.text+0x64): undefined reference to `__sync_val_compare_and_swap_8'
collect2: ld returned 1 exit status
$ gcc t.c -lgcc_s
/tmp/ccu3ReMZ.o: In function `main':
t.c:(.text+0x64): undefined reference to `__sync_val_compare_and_swap_8'
collect2: ld returned 1 exit status
$ gcc t.c -lgcc
/tmp/ccO8gyeR.o: In function `main':
t.c:(.text+0x64): undefined reference to `__sync_val_compare_and_swap_8'
collect2: ld returned 1 exit status