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

["H.J. Lu" <hjl.tools@gmail.com>]

On Tue, Feb 20, 2024 at 11:02 AM Adhemerval Zanella Netto
<adhemerval.zanella@linaro.org> wrote:
>
>
>
> 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?

One issue is that the minimum GCC (GCC 6?) doesn't support -march=x86-64-vN.
Another reason is that these math functions don't need the full ISA
level instructions.

-- 
H.J.