Date: Thu, 21 Jul 2005 20:49:09 -0700 From: "David O'Brien" <obrien@freebsd.org> To: Olexandr Konovalenko <kono@kth.se> Cc: freebsd-amd64@freebsd.org Subject: Re: better optimization flags for opteron (amd64)// -ffast-math and -O3 are not friends Message-ID: <20050722034909.GD41313@dragon.NUXI.org> In-Reply-To: <49213.130.237.35.96.1121786769.squirrel@webmail.sys.kth.se> References: <49213.130.237.35.96.1121786769.squirrel@webmail.sys.kth.se>
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On Tue, Jul 19, 2005 at 05:26:09PM +0200, Olexandr Konovalenko wrote: > I found that on my system combination of -ffast-math AND -O3 is slower than > -O0 on AMD64 (interestingly it is true for Xeon EM64 as well), how can it be? > though just -O3 gives fastest double precision floating point computations. -O3 does a lot more than just FP optimizations. It also does a lot of inlining that can blow your caches. It is a falicay that -ON is better than -ON-1 level. Look in the GCC manual at http://gcc.gnu.org - you'll see all the optimizations that -O3 implies. Start with -O2 and add ones from the -O3 list to find the best performance for your benchmark on your machine. -- -- David (obrien@FreeBSD.org)
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