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Message from discussion Global array operations: a performance hit?
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Dennis Wassel  
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 More options Jun 19 2008, 4:16 am
Newsgroups: comp.lang.fortran
From: Dennis Wassel <dennis.was...@googlemail.com>
Date: Thu, 19 Jun 2008 01:16:34 -0700 (PDT)
Local: Thurs, Jun 19 2008 4:16 am
Subject: Re: Global array operations: a performance hit?
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On 19 Jun., 00:00, glen herrmannsfeldt <g...@ugcs.caltech.edu> wrote:

> 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

*Errrrrm*
Am I getting something wrong? This won't work, consider for instance X
= (2 1).
Except for the special case X(I) >= I \forall I, this will produce
garbage (and for permutations, that special case is only satisfied by
the identity).
Point is, I'm fairly sure you need temporaries here as well.
> 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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