From: Ian Lance Taylor <iant@google.com>
To: dw <limegreensocks@yahoo.com>
Cc: "gcc-help@gcc.gnu.org" <gcc-help@gcc.gnu.org>
Subject: Re: Question about __builtin_ia32_mfence and memory barriers
Date: Tue, 04 Jun 2013 23:52:00 -0000 [thread overview]
Message-ID: <CAKOQZ8yxRncKoRjLcnR5rZnkybtOTAtCoLo9f-OJyCFe47JWEw@mail.gmail.com> (raw)
In-Reply-To: <51AE7119.5090000@yahoo.com>
On Tue, Jun 4, 2013 at 3:58 PM, dw <limegreensocks@yahoo.com> wrote:
>
> To create a
> processor fence, you could do something like
>
> __builtin_ia32_mfence();
A better choice these days is __atomic_thread_fence(__ATOMIC_SEQ_CST)
(or __atomic_signal_fence).
> 1) Am I right that __builtin_ia32_mfence() does not generate a memory
> barrier?
That is correct: it does not prevent the compiler from moving loads
and stores across the call to __builtin_ia32_mfence.
> 1) Is this "two statement thing" guaranteed to be safe? Could the optimizer
> re-order instructions moving code in between the two? (Yes, I realize that
> the asm statement doesn't actually generate any output. But given my
> understanding of how the compiler processes code, I believe the question is
> still valid).
It is probably safe, because why would the compiler put anything in
there, but it is not absolutely guaranteed to be safe.
> 2) If it is not guaranteed to be safe, what is the use of
> __builtin_ia32_mfence()? What value is there in preventing the *processor*
> from executing statements out of order if the *compiler* is just going to
> move them around?
__builtin_ia32_mfence exists to support the Intel documented
_mm_mfence intrinsic. I'm not clear on whether _mm_mfence is meant to
be a compiler memory barrier or not. If it is, then I think GCC has a
bug in the way it is implemented. Please feel free to file a bug
report at http://gcc.gnu.org/bugzilla/ , especially if you can come up
with a case that fails.
> I expect this would always work:
>
> asm ("mfence" ::: "memory");
>
> But I would rather use the builtins if possible.
Yes, you should use the builtins. The __atomic builtins, which work
better and are portable across processors.
Ian
next prev parent reply other threads:[~2013-06-04 23:52 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-06-04 22:58 dw
2013-06-04 23:52 ` Ian Lance Taylor [this message]
2013-06-05 2:45 ` dw
2013-06-05 4:30 ` Ian Lance Taylor
2013-06-12 8:15 ` dw
2013-06-12 19:01 ` Ian Lance Taylor
2013-06-13 2:55 ` dw
2013-06-13 3:01 ` Chung-Ju Wu
2013-06-13 3:25 ` Ian Lance Taylor
2013-06-13 3:44 ` dw
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