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Re: [PATCH] Compile AVX libm functions with -mavx
On Tue, Oct 2, 2012 at 1:19 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Tue, Oct 2, 2012 at 12:47 PM, OndÅej BÃlka <neleai@seznam.cz> wrote:
>> On Tue, Oct 02, 2012 at 03:31:50PM -0400, Mike Frysinger wrote:
>>> On Tuesday 02 October 2012 15:20:54 H.J. Lu wrote:
>>> > On Tue, Oct 2, 2012 at 12:02 PM, Mike Frysinger <vapier@gentoo.org> wrote:
>>> > > On Tuesday 02 October 2012 09:53:25 H.J. Lu wrote:
>>> > >> This patch compiles AVX libm functions with -mavx. It reduces text size
>>> > >
>>> > >> of libm.so by about 1%:
>>> > > looks like you're reverting 56f6f6a2403cfa7267cad722597113be35ecf70d.
>>> > > shouldn't you revert all of it and not just change the CFLAGS back ?
>>> >
>>> > Doesn't this patch:
>>> >
>>> > http://sourceware.org/ml/libc-alpha/2012-10/msg00055.html
>>> >
>>> > do that?
>>>
>>> yes, i missed the follow up
>>>
>>> > > it'd be useful to know *why* Ulrich moved away from -mavx, but
>>> > > unfortunately his commit message is useless.
>>> >
>>> > I can only guess:
>>>
>>> might be useful to put some notes (like referring to the older commit) into
>>> the commit message when you do commit things
>>> -mike
>>
>> could it be a 60 cycle penalty when switching between legagy sse and avx
>> state?
>
> This true. We can use -mprefer-avx128 to make sure that only 128bit AVX
> instructions are used.
>
> --
> H.J.
The latency for switching between old SSE and new (AVX-style
3-operand) form is what causes the penalty. What is the purpose of
-mprefer-avx128? I can't find a description of it online.