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[Bug target/63503] [AArch64] A57 executes fused multiply-add poorly in some situations
- From: "wdijkstr at arm dot com" <gcc-bugzilla at gcc dot gnu dot org>
- To: gcc-bugs at gcc dot gnu dot org
- Date: Tue, 28 Oct 2014 22:54:05 +0000
- Subject: [Bug target/63503] [AArch64] A57 executes fused multiply-add poorly in some situations
- Auto-submitted: auto-generated
- References: <bug-63503-4 at http dot gcc dot gnu dot org/bugzilla/>
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63503
--- Comment #24 from Wilco <wdijkstr at arm dot com> ---
(In reply to Evandro from comment #23)
> (In reply to Wilco from comment #22)
> > Unrolling alone isn't good enough in sum reductions. As I mentioned before,
> > GCC doesn't enable any of the useful loop optimizations by default. So add
> > -fvariable-expansion-in-unroller to get a good speedup with unrolling. Again
> > these are all generic GCC issues.
>
> Adding -fvariable-expansion-in-unroller when using -funroll-loops results in
> practically the same code being emitted.
Correct, all it does is cut the dependency chain of the accumulates. But that's
enough to get the speedup.