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Message from discussion Global array operations: a performance hit?
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glen herrmannsfeldt  
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 More options Jun 18 2008, 6:00 pm
Newsgroups: comp.lang.fortran
From: glen herrmannsfeldt <g...@ugcs.caltech.edu>
Date: Wed, 18 Jun 2008 14:00:39 -0800
Local: Wed, Jun 18 2008 6:00 pm
Subject: Re: Global array operations: a performance hit?
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Dennis Wassel wrote:

(snip)

> Is (compiler) optimisation of array operations that much of a
> nontrivial task or just something that nobody has come around to
> doing, yet? Judging from their mailing list, gfortran seems to have
> many construction sites and a number of them apparently with higher
> priorities.

If you do exactly the same thing in both cases, the compiler
should do pretty well, except in cases that modify the source array.
There are cases where you know that no array elements are used
after they are modified (using DO loops), but the compiler doesn't
(using array expressions).

Using vector subscripts as an example, say you have arrays X and Y,
both dimensioned N, where X has a permutation of the numbers from
1 to N.

       DO I=1,N
          Y(I)=Y(X(I))
       ENDDO

reorders Y based on the permutation, where you know that
the values in X are unique.

       Y=Y(X)

does it using vector subscripts, but the compiler does not
know that the values are unique, so a temporary array is
needed.  If you read in X from a file, there is no way
that the compiler could possibly know the values are unique.
If you generated X directly, it is unlikely that even the
best optimizer would figure it out.

-- glen


 
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