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Re: GCC mini-summit - compiling for a particular architecture
- From: Zdenek Dvorak <rakdver at atrey dot karlin dot mff dot cuni dot cz>
- To: Robert Dewar <dewar at adacore dot com>
- Cc: Steve Ellcey <sje at cup dot hp dot com>, aaw at google dot com, kenneth dot hoste at elis dot ugent dot be, gcc at gcc dot gnu dot org
- Date: Sat, 21 Apr 2007 02:19:28 +0200
- Subject: Re: GCC mini-summit - compiling for a particular architecture
- References: <46294CB9.3030301@adacore.com>
Hello,
> Steve Ellcey wrote:
>
> >This seems unfortunate. I was hoping I might be able to turn on loop
> >unrolling for IA64 at -O2 to improve performance. I have only started
> >looking into this idea but it seems to help performance quite a bit,
> >though it is also increasing size quite a bit too so it may need some
> >modification of the unrolling parameters to make it practical.
>
> To me it is obvious that optimizations are target dependent. For
> instance loop unrolling is really a totally different optimization
> on the ia64 as a result of the rotating registers.
that we do not use. Nevertheless, there are still compelling reasons
for why unrolling is more useful on ia64 then on other architectures
(importance of scheduling, insensitivity to code size growth).
Another option would be to consider enabling (e.g.) -funroll-loops
-fprefetch-loop-arrays by default on -O3. I think it is fairly rare
for these flags to cause performance regressions (although of course
more measurements to support this claim would be necessary).
Zdenek