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Date: Wed, 24 May 2006 12:03:26 -0700
From: glen herrmannsfeldt <gah@ugcs.caltech.edu>
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Newsgroups: comp.lang.ada,comp.lang.fortran
Subject: Re: Ada vs Fortran for scientific applications
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Date: 2006-05-24T12:03:26-07:00
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Dick Hendrickson wrote:

(snip, someone wrote)

>> Is that through sequence association - i.e., I can assume (conceptually)
>> an EUIVALENCE of two reals, the real and imaginary part in that order, to
>> be in place for a complex variable?

(snip)

> However, essentially all of Fortran's rules have an "as if"
> clause somewhere in them.  If the compiler is sufficiently
> clever (and can see enough of the whole program) it can do
> anything with complex variables.  Keeping temporaries in
> registers and discarding them when they become dead is
> common practice and is standard conforming.

OK, but say for a certain program it is more efficient to store a 
complex array with all the real parts followed by all the imaginary 
parts, and the compiler figured this out.  The "as if" rule would work 
only if there was no other way for the program/programmer to discover 
the change.  If the array was in COMMON, EQUIVALENCEd, or passed to an 
external routine, this optimization most likely couldn't be done.
Without any legal way to view the storage association, it would seem 
possible to do it.

-- glen