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Re: Vectorizing HIRLAM 4: complicated access patterns examined.
- From: Dorit Naishlos <DORIT at il dot ibm dot com>
- To: Toon Moene <toon at moene dot indiv dot nluug dot nl>
- Cc: fortran at gcc dot gnu dot org, gcc at gcc dot gnu dot org
- Date: Fri, 21 Oct 2005 17:42:20 +0200
- Subject: Re: Vectorizing HIRLAM 4: complicated access patterns examined.
This one gets vectorized for me, on powerpc-linux:
~/mainline_cvs/bin/gfortran -O3 -ftree-vectorize -maltivec
-ftree-vectorizer-verbose=4 -S hilaram4.f90
hilaram4.f90:4: note: Alignment of access forced using peeling.
hilaram4.f90:4: note: Vectorizing an unaligned access.
hilaram4.f90:4: note: LOOP VECTORIZED.
hilaram4.f90:7: note: vectorized 1 loops in function.
dorit
> L.S.,
>
> This code:
>
> SUBROUTINE S(N)
> DIMENSION A(N), B(N)
> READ*,ISTART,ISTOP,B
> DO I = ISTART, ISTOP
> A(I) = B(I)
> ENDDO
> PRINT*,A
> END
>
> when compiled thusly:
>
> $ gfortran -g -S -O3 -ftree-vectorize -ftree-vectorizer-verbose=2 -
> msse2 vect4.f
>
> draws the following "not vectorized" message:
>
> vect4.f:4: note: not vectorized: complicated access pattern.
> vect4.f:4: note: vectorized 0 loops in function.
>
> This sort of coding is rather prevalent in HIRLAM, especially in the
> interesting inner loops. One needs this when not all of the inner area
> is accessible / computable by the code at hand (for instance because
there
> are inaccessible "halo's" around the subarea that have to be treated
> specially).
>
> The funny thing is that replacing ISTART with a constant makes this loop
> vectorizable. So what's the fundamental difference (as far as
vectorizing
> is concerned) between a constant and a loop invariant :-) ?
>
> Kind regards,
>
> --
> Toon Moene - e-mail: toon@moene.indiv.nluug.nl - phone: +31 346 214290
> Saturnushof 14, 3738 XG Maartensdijk, The Netherlands
> A maintainer of GNU Fortran 95: http://gcc.gnu.org/fortran/