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Re: [PATCH] Bug 201689: Belt-and-suspenders detection of FMA.


On 10/17/2016 11:44 AM, Carlos O'Donell wrote:
> On 10/14/2016 03:12 PM, H.J. Lu wrote:
>> On Fri, Oct 14, 2016 at 11:09 AM, Carlos O'Donell <carlos@redhat.com> wrote:
>>> In the Intel Architecture Instruction Set Extensions Programming reference
>>> the recommended way to test for FMA in section '2.2.1 Detection of FMA'
>>> is:
>>>
>>> "Application Software must identify that hardware supports AVX as explained
>>>  in ... after that it must also detect support for FMA..."
>>>
>>> We don't do that in glibc. We use osxsave to detect the use of xgetbv, and
>>> after that we check for AVX and FMA orthogonally. It is conceivable that
>>> you could have the AVX bit clear and the FMA bit in an undefined state.
>>>
>>> I have never seen a machine with the AVX bit clear and the FMA bit set, but
>>> we should follow the intel specifications and adjust our check as the following
>>> patch works.
>>
>> One can't write a program with FMA nor AVX2 without using AVX
>> instructions.  AVX2/FMA aren't usable if AVX isn't.   We should do
>>
>> if (CPU_FEATURES_CPU_P (cpu_features, AVX))
>>    {
>>       Set AVX available
>>       if (CPU_FEATURES_CPU_P (cpu_features, AVX2))
>>          Set AVX2 available
>>       if (CPU_FEATURES_CPU_P (cpu_features, FMA))
>>          Set FMA available
>>    }
>>
> 
> Agreed. I double checked the manual, here is a new patch.
> 
> Testing on x86_64 and i686.
> 
> OK to commit if the results are clean?
> 
> v2
> - Move FMA and AVX2 check up into AVX check since they depend upon it.
> 
> 2016-10-14  Carlos O'Donell  <carlos@redhat.com>
> 
> 	[BZ #20689]
> 	* sysdeps/x86/cpu-features.c: Only enable FMA and AVX2 if
> 	AVX is usable.

Committed. Thanks.

-- 
Cheers,
Carlos.


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