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 6:37 PM H.J. Lu <hjl.tools@gmail.com> 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);
> }
>
> by a GCC which can inline __builtin_ceil, compiling with -msse4.1 or -mavx.

Ahh, I had a misunderstanding, okay its clear sse4.1 makes sense.
>
> --
> H.J.