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Re: [PATCH 1/N] x86_64 vectorization support: vectorized math functions addition to Glibc

On Wed, Nov 12, 2014 at 09:42:11PM +0400, Andrew Senkevich wrote:
> 2014-09-12 21:08 GMT+04:00 Jakub Jelinek <>:
> > On Fri, Sep 12, 2014 at 05:03:06PM +0000, Joseph S. Myers wrote:
> >> That's what this patch does - but we need a way for the headers to declare
> >> to GCC which vector ISAs have such versions of a given function available,
> >> in a way that works both for (new GCC, old glibc) (GCC knows about newer
> >> vector ISAs without function versions in that glibc, and mustn't try to
> >> generate calls to functions that glibc doesn't have) and (old GCC, new
> >> glibc) (glibc is declaring availability of vector versions the old GCC
> >> doesn't know how to use, so the references to those vector versions need
> >> to be quietly ignored or conditional on GCC version).
> > In Cilk+ there is a way to tell which ISA the elemental function is compiled
> > for.  In OpenMP we've made a GCC ABI decision that on i?86/x86_64 all of
> > SSE2, AVX, AVX2 and AVX-512 passing conventions are used; those can be
> > emitted as aliases, thunks or real functions (have to optimize this on GCC
> > side at some point).  E.g. in Intel ABI (which uses different letters) it
> > always uses just SSE2.
> > When/if AVX-1024 is added, we won't change the ABI, so one will still have
> > to use two AVX-512 calls; but perhaps we can add as OpenMP extension
> > some clause like Cilk+ has to specify ISA.
> As we see processor clause unfortunately not parsed by GCC.
> So no such way to specify ISA for both OpenMP and Cilk+ so far.

Processor clause is parsed for Cilk+.  OpenMP doesn't have anything like
that, and I'm not sure it would be appropriate for the standard, because
the standard is not specific to a single CPU architecture and all parties
would need to agree on the names.  You actually don't care about the
processors anyway, but about the vector ISA.

> GCC 4.9 and 5.0 both can vectorize in SSE2, AVX, AVX2, AVX-512 (with
> -mavx512f generated 2 calls of AVX2 versions).

Indeed, those 3 are what GCC emits for OpenMP, AVX-512 is not emitted,
for Cilk+ it should emit SSE2 unless processor clause is used.


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