[PATCH] x86: Add MathVec_Prefer_No_AVX512 to cpu-features [BZ #21967]

[H.J. Lu]

On Mon, Sep 4, 2017 at 3:16 AM, Senkevich, Andrew
<andrew.senkevich@intel.com> wrote:
>> -----Original Message-----
>> From: Lu, Hongjiu
>> Sent: Friday, September 1, 2017 19:41
>> To: GNU C Library <libc-alpha@sourceware.org>
>> Cc: Senkevich, Andrew <andrew.senkevich@intel.com>
>> Subject: [PATCH] x86: Add MathVec_Prefer_No_AVX512 to cpu-features [BZ
>> #21967]
>>
>> AVX512 functions in mathvec are used on machines with AVX512.  An AVX2
>> wrapper is also provided and it can be used when the AVX512 version isn't
>> profitable.  MathVec_Prefer_No_AVX512 is addded to cpu-features.
>> If glibc.tune.hwcaps=MathVec_Prefer_No_AVX512 is set in GLIBC_TUNABLES
>> environment variable, the AVX2 wrapper will be used.
>>
>> Tested on x86-64 machines with and without AVX512.  Also verified
>> glibc.tune.hwcaps=MathVec_Prefer_No_AVX512 on AVX512 machine.
>>
>> Any comments?
>
> But libc selects AVX2 mem/str on SKX, looks like libmvec should do the same (and backward option MathVec_Prefer_AVX512 needed).
>

AVX2 mem/str  can be called from codes which may use only SSE
or AVX.  But AVX512 functions in mathvec are called only from AVX512
programs.  Do we have benchmarks to show that AVX512 programs
are faster with AVX2 mathvec?

-- 
H.J.