[Mesa-dev] [Mesa-stable] [PATCH] swr: fix -march flag for AVX

[Emil Velikov]

On 13 June 2016 at 15:43, Rowley, Timothy O <timothy.o.rowley at intel.com> wrote:
>
>> On Jun 12, 2016, at 3:56 AM, Steven Newbury <steve at snewbury.org.uk> wrote:
>>
>> On Fri, 2016-06-10 at 16:05 -0400, Ilia Mirkin wrote:
>>> On Fri, Jun 10, 2016 at 3:43 PM, Tim Rowley <timothy.o.rowley at intel.c
>>> om> wrote:
>>>>
>>>> Previously used core-avx-i was for ivybridge;
>>>> corei7-avx allows sandybridge.
>>>> ---
>>>>  src/gallium/drivers/swr/Makefile.am | 2 +-
>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/src/gallium/drivers/swr/Makefile.am
>>>> b/src/gallium/drivers/swr/Makefile.am
>>>> index d211f2e..8156cf2 100644
>>>> --- a/src/gallium/drivers/swr/Makefile.am
>>>> +++ b/src/gallium/drivers/swr/Makefile.am
>>>> @@ -124,7 +124,7 @@ COMMON_LDFLAGS = \
>>>>  lib_LTLIBRARIES = libswrAVX.la libswrAVX2.la
>>>>
>>>>  libswrAVX_la_CXXFLAGS = \
>>>> -       -march=core-avx-i \
>>>> +       -march=corei7-avx \
>>> Just wondering if it'd be enough to say like
>>>
>>> -march=x86_64
>>> -mavx
>>>
>>> and add -mavx2 for libswrAVX2.
>>>
>>> I suspect you've iterated through this 20 times and this has some
>>> shortcoming I'm not thinking of, but figured I'd point it out just in
>>> case it helps.
>>>
>>>   -ilia
>>>
>> Maybe I'm the only one who finds it horrible to override -march from
>> within project build systems.

>> It causes no end of problems with LTO,
>> and results in objects being built inappropriately for the target as
>> specified by the builder.
Can you please elaborate on the exact problem ? Is there link where
one can read more on the topic ?

> I agree that the way swr is built can be improved and welcome ideas/discussion for work on the master branch, but for the moment I’m trying to get the 12.0 branch in good shape for swr with non-invasive changes (as we’ve tested the current setup pretty extensively).
>
Fully agree. Getting something more robust and less evasive for 12.0
and work for 'the best' in master. Perhaps attribute((target)) perhaps
folding the multiple binaries into one, or maybe something else ?

>> Why not use function attributes?  Either compile a function
>> specifically for a particular target, or specifically enable AVX though
>> "__attribute__ ((__target__ ("avx")))"
Is this going to produce correct binary if one provides -mno-avx
and/or alike ? I believe for some CPUs one should (or the compiler
does it for them) explicitly disable avx but honestly I've never
looked if cpuid for the said CPUs shows avx as supported.

In the latter case (cpuid listing AVX as supported, while one is
recommended building with -mno-avx) how do you suggest we should
handle things ?

>> (ideally with a fall-back
>> implementation detected at run-time when the instruction set isn't
>> supported).  Even better, have it also protected at build-time by a
>> simple check for __AVX__ so the code can be compiled out entirely where
>> the package builder has specified -march(=native) as a build flag.
The idea here is to have the binary build explicitly with avx/avx2 (if
compiler is capable), even when doing generic builds. The picky part
comes when one has to establish if the user provided one is 'more
generic' or not (as per your example).

Then again, based on my limited 'testing', building mesa with native
and LTO brings us little to no improvements. 'Tested' running glxgears
in small window.

> One problem with the function attribute path is that as far as I know, MSVC does not have a similar feature.
>
Afaict MSVC has optimise pragma [1], just like GCC. Ideally they'll
add an equivalent to attribute((target)) soon ?

Even without that, swr (as seen in mesa) does not build MSVC. When
that cases it should be a matter of factoring the attribute target as
a macro and leaving it empty for the !GCC build ?

TL;DR: IMHO fix is ok for now, but for the future we might want to
explode other options.

-Emil
[1] https://msdn.microsoft.com/en-us/library/chh3fb0k.aspx