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Re: Gcc 3.1 performance regressions with respect to 2.95.3
- From: law at redhat dot com
- To: Jan Hubicka <jh at suse dot cz>
- Cc: Peter Schmid <schmid at snake dot iap dot physik dot tu-darmstadt dot de>, Jason Merrill <jason at redhat dot com>, gcc at gcc dot gnu dot org, libstdc++ at gcc dot gnu dot org
- Date: Mon, 18 Mar 2002 10:19:31 -0700
- Subject: Re: Gcc 3.1 performance regressions with respect to 2.95.3
- Reply-to: law at redhat dot com
In message <20020317180245.GB10722@atrey.karlin.mff.cuni.cz>, Jan Hubicka write
s:
> > It seems that the option -frerun-cse-after-loop causes a 10 percent
> > slowdown. But I am not sure if that was the option you asked me to
> > turn off. On the other hand even when -fno-rerun-cse-after-loop is
> > added to the -O2 optimization level the performance is still slower
>
> I now realise that there is no way to turn off loop optimizer, that ineed
> is mistake. I will send patch for this together with crossjumping.
> This is another pass that may well be responsible for peroformance loss
> and misscompilations so it should be possible to disable it.
crossjumping is not always a performance win -- if you look at the old
crossjumping code, it had heuristics to guess when crossjumping would
not be profitable. Perhaps you should review the code and add some heuristics
to your new version.
jeff