Re: GCC loop optimizations

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GCC loop optimizations
\ linuxkaffee (28 Feb 2008)
. \ Zdenek Dvorak (29 Feb 2008)

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Subject:	Re: GCC loop optimizations
Group:	Gcc
From:	Zdenek Dvorak
Date:	29 Feb 2008

Hi,

> I'd like to know your experiences with the gcc loop optimizations.
>
> What loop optimizations (in your opinion) can be applied to a large
> number of programs and yield a (significant) improvement of the
> program run-time?

in general, I would say invariant motion, load/store motion, strength
reduction and loop unrolling. Most of the other loop optimizations tend
to apply only in special situations. Note however that the usefulness
also heavily depends on the type of the compiled code; e.g., wide class
of programs (simulations, optimization) spend most of the time doing
simple calculations on large arrays, and other optimizations (loop
interchange/reordering/..., prefetching) become important.

> I've tested the gcc compiler optimization "loop unswitching"
> (introduced in gcc 3.4) but going through a large number of different
> benchmarks I could find very few cases where the optimization could be
> applied. Do you share my experience?

yes, the situations where loop unswitching can be applied are not
very common.

Zdenek