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Re: [gomp4] Some progress on #pragma omp simd


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
#pragma simd
should be equivalent to
#pragma omp simd
wrt. inter-iteration dependencies, and
#pragma simd vectorlength(a, b, c)
to
#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
minimum value).

	Jakub


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