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

[Noah Goldstein <goldstein.w.n@gmail.com>]

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?
>
>
> --
> H.J.