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Re: [gomp4] Some progress on #pragma omp simd
- From: Jakub Jelinek <jakub at redhat dot com>
- To: "Iyer, Balaji V" <balaji dot v dot iyer at intel dot com>
- Cc: Aldy Hernandez <aldyh at redhat dot com>, Richard Henderson <rth at redhat dot com>, "gcc-patches at gcc dot gnu dot org" <gcc-patches at gcc dot gnu dot org>
- Date: Wed, 24 Apr 2013 08:01:17 +0200
- Subject: Re: [gomp4] Some progress on #pragma omp simd
- References: <20130419132957 dot GE12880 at tucnak dot redhat dot com> <5175B40F dot 7040709 at redhat dot com> <20130423135455 dot GN12880 at tucnak dot redhat dot com> <BF230D13CA30DD48930C31D40993300032A37760 at FMSMSX101 dot amr dot corp dot intel dot com>
- Reply-to: Jakub Jelinek <jakub at redhat dot com>
On Tue, Apr 23, 2013 at 09:32:29PM +0000, Iyer, Balaji V wrote:
> My apologies if the documentation did not explain this correctly. It
> was written by compiler developers and not language developers. #pragma
> simd is the guarantee the user gives the compiler that the inter-iteration
> dependencies do not matter. So, if the user omits the vectorlength the
> clause then the compiler can, in effect, choose N, where N is the number
> of loop iterations.
The documentation doesn't suggest that. Anyway, so
should be equivalent to
#pragma omp simd
wrt. inter-iteration dependencies, and
#pragma simd vectorlength(a, b, c)
#pragma omp simd safelen(max (a, b, c))
? If so, then the FE could emit OMP_SIMD for #pragma simd, and if
vectorlength is present, add OMP_CLAUSE_SAFELEN with the maximum of the
values in all vectorlength clauses, and keep the vectorlength clauses around
too as CILK_CLAUSE_VECTORLENGTH as hints to the vectorizer?
Also, Aldy said that #pragma simd loops allow != condition, how do you
compute number of iterations in that case if the increment isn't constant?
As conditional depending on whether increment is positive or negative?
!= condition isn't allowed in OpenMP, so there it is always obvious which
direction it should iterate, and the expansion code will assume if it sees
NE_EXPR that it is just folded border test (comparison with maximum or