Re: [PATCH] x86_64: Exclude SSE, AVX and FMA4 variants in libm multiarch

[Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>]

On 20/02/24 15:54, H.J. Lu wrote:
> On Tue, Feb 20, 2024 at 10:48 AM Adhemerval Zanella Netto
> <adhemerval.zanella@linaro.org> wrote:
>>
>>
>>
>> On 20/02/24 15:36, H.J. Lu wrote:
>>> On Tue, Feb 20, 2024 at 10:32 AM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
>>>>
>>>> On Tue, Feb 20, 2024 at 6:28 PM H.J. Lu <hjl.tools@gmail.com> wrote:
>>>>>
>>>>> On Tue, Feb 20, 2024 at 10:19 AM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
>>>>>>
>>>>>> On Tue, Feb 20, 2024 at 6:14 PM H.J. Lu <hjl.tools@gmail.com> wrote:
>>>>>>>
>>>>>>> On Tue, Feb 20, 2024 at 10:07 AM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
>>>>>>>>
>>>>>>>> On Tue, Feb 20, 2024 at 6:05 PM H.J. Lu <hjl.tools@gmail.com> wrote:
>>>>>>>>>
>>>>>>>>> On Tue, Feb 20, 2024 at 9:56 AM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
>>>>>>>>>>
>>>>>>>>>> On Tue, Feb 20, 2024 at 5:51 PM Sunil Pandey <skpgkp2@gmail.com> wrote:
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> On Tue, Feb 20, 2024 at 9:34 AM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
>>>>>>>>>>>>
>>>>>>>>>>>> On Tue, Feb 20, 2024 at 4:58 PM Sunil K Pandey <skpgkp2@gmail.com> wrote:
>>>>>>>>>>>>>
>>>>>>>>>>>>> When glibc is built with FMA and AVX2 enabled by default, the resulting
>>>>>>>>>>>>> glibc binaries won't run on SSE or FMA4 processors.  Exclude SSE, AVX and
>>>>>>>>>>>>> FMA4 variants in libm multiarch when both FMA and AVX2 are enabled by
>>>>>>>>>>>>> default.  Disallow glibc build with only AVX2 or FMA enabled as all AVX2
>>>>>>>>>>>>> processors, including VMs, should also support FMA and vice versa.
>>>>>>>>>>>>>
>>>>>>>>>>>>> When glibc is built with SSE4.1 enabled by default, only keep SSE4.1
>>>>>>>>>>>>> variant.
>>>>>>>>>>>> Not avx2 + FMA as well?
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> Correct. Logic is as follows
>>>>>>>>>>> If (build with AVX2+FMA): Keep AVX2+FMA variants only.
>>>>>>>>>>> else if (build with SSE4.1): Keep SSE4.1 variants only.
>>>>>>>>>> What if someone builds with sse4.1 as a minimum but then
>>>>>>>>>> runs on avx2+ machines?
>>>>>>>>>
>>>>>>>>> Only SSE4.1 variant will be used in this case.   Both SSE4.1
>>>>>>>>> and AVX versions only have a single instruction.  This matches
>>>>>>>>> the compiler builtin function of SS4.1 and AVX.
>>>>>>>>
>>>>>>>> if they are all the same, whats the rationale for having an
>>>>>>>> avx version at all?
>>>>>>>
>>>>>>> They aren't the same.  For ceil, it is
>>>>>>>
>>>>>>> roundsd $10, %xmm0, %xmm0
>>>>>>> ret
>>>>>>>
>>>>>>> vs
>>>>>>>
>>>>>>> vroundsd $10, %xmm0, %xmm0, %xmm0
>>>>>>> ret
>>>>>>>
>>>>>>> You get the same things with
>>>>>>>
>>>>>>> return __builtin_ceil (x);
>>>>>>
>>>>>> I mean if they are equal quality sse4.1 / avx,
>>>>>> why not just remove the avx impls are using sse4.1 impls
>>>>>> on avx targets?
>>>>>
>>>>> If glibc is compiled with AVX, we should use the AVX version if
>>>>> appropriate.   Since the minimum GCC for glibc build can't inline
>>>>>  __builtin_ceil, we inline  __builtin_ceil by hand.
>>>> if compiled with avx, but for generic target do we need to hold
>>>> onto avx versions for any reason?
>>>
>>> I don't understand what you were asking.   This patch leads to the same
>>> assembly code generated from
>>>
>>> double
>>> __ceil (double x)
>>> {
>>>   return __builtin_ceil (x);
>>> }
>>
>> Wouldn't make sense to follow the already define x86_64 ABI versions and
>> provided the ifunc variants based on the ABI uses?
> 
> There are no conflicts here.  For these math functions, ISA level 2 == SSE4.1
> and ISA level 3 == AVX2 + FMA.   If glibc is built with ISA level N, this patch
> will exclude ISA level N-1 or older variants in IFUNC selection.
> 

I mean, why not use the MINIMUM_X86_ISA_LEVEL to define whether to provide/build
the variants instead of adding two new configure checks?