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

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.


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