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Re: gcc Digest 26 Dec 2014 16:51:42 -0000 Issue 7953


On Mon, Jan 12, 2015 at 07:38:10PM +0300, Andrew Senkevich wrote:
> > during work on addition vector math functions to Glibc and discussions
> > with community was found an issue with meaning of â#pragma omp declare
> > simdâ (which will appear in math.h).
> >
> > Issue is there are no working way to specify ISA of vector function
> > in GCC 5.0, and hence no way to determine exact vector function name.
> >
> > Here is description of exact meaning of â#pragma omp declare simdâ for x86_64.
> >
> > This is proposed as agreement between compilers supporting OpenMP.
> >
> > *************** OpenMP vector function ABI for x86_64 ***************
> >
> > Name of vector math function is based on Intel Vector Function ABI
> > (http://www.cilkplus.org/sites/default/files/open_specifications/Intel-ABI-Vector-Function-2012-v0.9.5.pdf)
> > with a little difference in part of name specifying ISA â namely
> > letters b, c, d instead of x, y, Y.
> >
> > #pragma omp declare simd notinbranch simdlen(2) for some function
> > âfuncâ means what the name of vector version is:
> >
> > _ZGVbN2v_func (it is SSE4 implementation).
> >
> > #pragma omp declare simd notinbranch simdlen(4) for some function
> > âfuncâ means what the following names are available:
> >
> >  _ZGVcN4v_func (it is AVX implementation)
> > and
> > _ZGVdN4v_func (it is AVX2 implementation).
> >
> > Every vector function should be provided by math library for each
> > supported ISA (currently SSE4, AVX and AVX2).
> > Semantics of those pragmas are independent of the processor for which
> > code is being generated.
> > Those pragmas must not be interpreted as meaning version of other ISA
> > of functions are available even if code is being built for a processor
> > with such ISA support.
> > Any future ABI extension that defines additional vector function
> > versions will also define a different pragma to declare their
> > availability.
> >
> > *********************************************************************
> >
> > Any feedback?
> 
> is this agreement OK?
> 
> Consensus is required to commit x86_64 vector math functions by Glibc
> maintainer.

With the difference that b stands for SSE2, not SSE4, and the fact
that those functions do not use the __regcall calling conventions, but
normal psABI calling conventions after replacing the arguments/return values
with the vectors documented in the 0.9.5 pdf (and/or adding the vector mask
arg) it describes what has been implemented, yes.

	Jakub


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