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Re: [PATCH] x86_64: memset optimized with AVX512
- From: Andrew Senkevich <andrew dot n dot senkevich at gmail dot com>
- To: "H.J. Lu" <hjl dot tools at gmail dot com>
- Cc: libc-alpha <libc-alpha at sourceware dot org>
- Date: Fri, 11 Dec 2015 16:45:40 +0300
- Subject: Re: [PATCH] x86_64: memset optimized with AVX512
- Authentication-results: sourceware.org; auth=none
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2015-12-11 16:39 GMT+03:00 H.J. Lu <hjl.tools@gmail.com>:
> On Fri, Dec 11, 2015 at 5:26 AM, Andrew Senkevich
> <andrew.n.senkevich@gmail.com> wrote:
>> 2015-12-10 22:34 GMT+03:00 H.J. Lu <hjl.tools@gmail.com>:
>>> On Thu, Dec 10, 2015 at 10:28 AM, Andrew Senkevich
>>> <andrew.n.senkevich@gmail.com> wrote:
>>>>>> END (MEMSET)
>>>>>> +libc_hidden_def (__memset_avx2)
>>>>>
>>>>> Why is this change needed? If it is needed, please submit
>>>>> a separate patch.
>>>>
>>>> We can avoid this change if hide implementation, test and IFUNC branch
>>>> under HAVE_AVX512_ASM_SUPPORT.
>>>>
>>>>> Should __memset_chk_avx512 also be provided?
>>>>
>>>> It will be the same as AVX2 version, is it really needed?
>>>
>>> __memset_chk_avx2 calls __memset_avx2. Don't you want
>>> __memset_chk to call __memset_avx512, instead of __memset_avx2,
>>> on KNL?
>>
>> Oh yes, surely we need it.
>>
>> Is patch below Ok for trunk?
>>
>> 2015-12-11 Andrew Senkevich <andrew.senkevich@intel.com>
>>
>> * sysdeps/x86_64/multiarch/Makefile (sysdep_routines): Added new file.
>> * sysdeps/x86_64/multiarch/ifunc-impl-list.c: Added new tests.
>> * sysdeps/x86_64/multiarch/memset-avx512.S: New file.
>> * sysdeps/x86_64/multiarch/memset.S: Added new IFUNC branch.
>> * sysdeps/x86_64/multiarch/memset_chk.S: Likewise.
>>
>
>> diff --git a/sysdeps/x86_64/multiarch/memset.S
>> b/sysdeps/x86_64/multiarch/memset.S
>> index dbc00d2..9f16b7e 100644
>> --- a/sysdeps/x86_64/multiarch/memset.S
>> +++ b/sysdeps/x86_64/multiarch/memset.S
>> @@ -30,6 +30,13 @@ ENTRY(memset)
>> HAS_ARCH_FEATURE (AVX2_Usable)
>> jz 2f
>> leaq __memset_avx2(%rip), %rax
>> +#ifdef HAVE_AVX512_ASM_SUPPORT
>> + HAS_ARCH_FEATURE (AVX512DQ_Usable)
>> + jnz 2f
>> + HAS_ARCH_FEATURE (AVX512F_Usable)
>> + jz 2f
>> + leaq __memset_avx512(%rip), %rax
>> +#endif
>> 2: ret
>> END(memset)
>> #endif
>> diff --git a/sysdeps/x86_64/multiarch/memset_chk.S
>> b/sysdeps/x86_64/multiarch/memset_chk.S
>> index e2abb15..5115dfb 100644
>> --- a/sysdeps/x86_64/multiarch/memset_chk.S
>> +++ b/sysdeps/x86_64/multiarch/memset_chk.S
>> @@ -30,6 +30,13 @@ ENTRY(__memset_chk)
>> HAS_ARCH_FEATURE (AVX2_Usable)
>> jz 2f
>> leaq __memset_chk_avx2(%rip), %rax
>> +#ifdef HAVE_AVX512_ASM_SUPPORT
>> + HAS_ARCH_FEATURE (AVX512DQ_Usable)
>> + jnz 2f
>> + HAS_ARCH_FEATURE (AVX512F_Usable)
>> + jz 2f
>> + leaq __memset_chk_avx512(%rip), %rax
>> +#endif
>> 2: ret
>> END(__memset_chk)
>>
>
> What is the purpose of checking AVX512DQ_Usable? To
> avoid using it on SKX? Is __memset_avx512 slower than
> __memset_avx2 on SKX?
This is implementation only for KNL because SKX may require vzeroupper
on AVX/SEE transition paths.
This version became slower with vzeroupper on that paths, so limited
to KNL hardware.
--
WBR,
Andrew