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Re: [PATCH] x86_64: memset optimized with AVX512
- From: "H.J. Lu" <hjl dot tools at gmail dot com>
- To: Andrew Senkevich <andrew dot n dot senkevich at gmail dot com>
- Cc: libc-alpha <libc-alpha at sourceware dot org>
- Date: Fri, 11 Dec 2015 05:53:56 -0800
- Subject: Re: [PATCH] x86_64: memset optimized with AVX512
- Authentication-results: sourceware.org; auth=none
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On Fri, Dec 11, 2015 at 5:45 AM, Andrew Senkevich
<andrew.n.senkevich@gmail.com> wrote:
> 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.
>
Please make following changes:
1. Change _avx512 to _avx512_no_vzeroupper.
2. Add a feature, Prefer_No_VZEROUPPER, to cpu-features.h, and set
it for KNL.
3. Check Prefer_No_VZEROUPPER instead of AVX512DQ_Usable,
4. Don't check AVX512DQ_Usable nor Prefer_No_VZEROUPPER in
ifunc-impl-list.c.
--
H.J.