On Wed, May 25, 2011 at 12:17 PM, Basile Starynkevitch
<bas...@starynkevitch.net> wrote:
> On Wed, 25 May 2011 11:26:51 +0100
> Andrew Haley <a...@redhat.com> wrote:
>
>> On 05/24/2011 07:28 PM, H.J. Lu wrote:
>>
>> > This patch implements pause intrinsic suggested by Andi. OK
>> > for trunk?
>>
>> What does "full memory barrier" here mean?
>>
>> +@table @code
>> +@item void __builtin_ia32_pause (void)
>> +Generates the @code{pause} machine instruction with full memory barrier.
>> +@end table
>>
>> There a memory clobber, but no barrier instruction AFAICS. The
>> doc needs to explain it a bit better.
>
> Perhaps the doc might explain why is it necessary to have a builtin for
> two independent roles: first, the full compiler memory barrier (which
> probably means to spill all the registers on the stack - definitely a
> task for a compiler); second, to "pause" the processor (which might
> also mean to flush or invalidate some data caches). In particular, I
> would naively imagine that we might have a more generic builtin for the
> compiler memory barrier (which probably could be independent of the
> particular ia32 target), and in that case which can't we just implement
> the pause ia32 builtin as builtin_compiler_barrier(); asm ("pause")?
>
We may need
builtin_compiler_barrier();
asm ("pause");
builtin_compiler_barrier();
--
H.J.
