Re: Question about __builtin_ia32_mfence and memory barriers

[Chung-Ju Wu <jasonwucj@gmail.com>]

2013/6/13 Ian Lance Taylor <iant@google.com>:
> On Wed, Jun 12, 2013 at 1:15 AM, dw <limegreensocks@yahoo.com> wrote:
[deleted]
>> In fact, I *can* generate failure cases if I comment the
>> __builtin_ia32_mfence() call out of _mm_mfence and replace it with something
>> else (like asm("mfence")).  But as soon as I put the __builtin_ia32_mfence
>> call back in, my "failure scenario" clears right up.
>>
>> In short, it looks like __builtin_ia32_mfence *does* generate a barrier.
>> But so do other builtins (like __builtin_ia32_pause).  Does that even seem
>> possible?  It would be weird if every builtin (or even every ia32 builtin)
>> implied a barrier.
[deleted]
>
> So when I say that as far as I know __builtin_ia32_mfence does not
> generate a barrier, what I mean is that as far as I know after it is
> expanded to RTL there is no barrier.  But I could be wrong.
>
> Ian

I just noticed there is a statement "MEM_VOLATILE_P(operands[0]=1"
for mfence pattern in gcc/config/i386/sync.md:

(define_expand "sse2_mfence"
  [(set (match_dup 0)
        (unspec:BLK [(match_dup 0)] UNSPEC_MFENCE))]
  "TARGET_SSE2"
{
  operands[0] = gen_rtx_MEM (BLKmode, gen_rtx_SCRATCH (Pmode));
  MEM_VOLATILE_P (operands[0]) = 1;
})


And so does "pause" pattern in gcc/config/i386/i386.md:

(define_expand "pause"
  [(set (match_dup 0)
        (unspec:BLK [(match_dup 0)] UNSPEC_PAUSE))]
  ""
{
  operands[0] = gen_rtx_MEM (BLKmode, gen_rtx_SCRATCH (Pmode));
  MEM_VOLATILE_P (operands[0]) = 1;
})


According to GCC Internal 10.5, the description says that
"Volatile memory references may not be deleted, reordered or combined."
I think that is why __builtin_ia32_mfence and __builtin_ia32_pause *do*
generate a barrier in dw's experiment.


Best regards,
jasonwucj