Scheduling complete loop unrolling early and unconditional?

Richard Guenther rguenth@tat.physik.uni-tuebingen.de
Wed Dec 22 18:25:00 GMT 2004


Dorit Naishlos wrote:

> just a comment -
> for better or for worse, scheduling complete-unrolling early would take
> away opportunities from autovectorization (at least as long as we're
> looking for vectorization opportunities only across loop iterations and not
> in straight-line code). Indeed if a loop gets completely unrolled then in
> many cases it may be  too short to gain much if anything from
> vectorization, but ideally the vectorizer would decide what's worth while
> to vectorize and what not before such opportunities are eliminated (of
> course we do not have such a cost model yet).

Yes, of course.  But there is also a huge amount of manually unrolled
code around like autogenerated stuff, but I also do manual unrolling
sometimes.  So the best thing would be if the autovectorizer recognizes
such straight-line "loop" code and other optimizations like SRA
recognize constant-small-times running loops.

Richard.



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