[Bug target/36793] New: x86-64 does not get __sync_synchronize right

["jfc at mit dot edu" <gcc-bugzilla at gcc dot gnu dot org>]

As I understand __sync_synchronize, the intent is to emit a memory barrier
instruction, at least on multiprocessor systems.  Currently on x86
__sync_synchronize inhibits explicit code motion across the builtin function
call but not processor reordering of memory operations across the builtin
function call.

I think mfence is the right instruction on x86-64, and this or a similar
pattern should be added to sync.md:

(define_insn "memory_barrier"
  [(unspec_volatile [(const_int 0)] UNSPEC_MFENCE)]
  "TARGET_SSE2"
  "mfence")


-- 
           Summary: x86-64 does not get __sync_synchronize right
           Product: gcc
           Version: 4.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: jfc at mit dot edu
 GCC build triplet: x86_64-linux-gnu
  GCC host triplet: x86_64-linux-gnu
GCC target triplet: x86_64-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36793