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Path: euryale.cc.adfa.oz.au!newshost.carno.net.au!harbinger.cc.monash.edu.au!munnari.OZ.AU!news.ecn.uoknor.edu!news.wildstar.net!cancer.vividnet.com!hunter.premier.net!news.mathworks.com!enews.sgi.com!super.zippo.com!zdc!szdc!szdc-e!news From: John Dyson <dyson@freebsd.org> Newsgroups: comp.unix.bsd.freebsd.misc Subject: Re: gcc optimizations for kernel Date: Sat, 14 Sep 1996 11:03:21 -0500 Organization: John S. Dyson's home machine Lines: 28 Message-ID: <323AD749.41C67EA6@freebsd.org> References: <51d0eg$9uf@usenet4.interramp.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit X-Mailer: Mozilla 3.0Gold (X11; U; FreeBSD 2.2-CURRENT i386) dcmyers@access.digex.net wrote: > > Following up on a comment made in the "Benchmarking different UNIX systems" > thread, I'd like to know what the best gcc optimizations are for a FreeBSD > kernel running on a Pentium. For example, I noticed that the default > Makefile uses plain ol' -O, as opposed to -O2 or -O3. Is it safe to > turn on the higher optimization levels? And what sort of performance > increase has been measured? I'd happily trade a few tens of kilobytes > of kernel size for better performance... > Specifically, even though we (the FreeBSD team) do not currently support it (and perhaps never will), you can (usually) compile and run your kernel at -O2 -fomit-frame-pointer and get away with it. On the older, released code you might have to tack on a -fno-strength-reduce because of a gcc bug. On the plus side, you will find measurable kernel performance improvement, but the kernel will be bigger, and there will be more cache-misses. Also, you will not be able to use kernel stack tracebacks. I also once-in-a-while test the kernel with the pgcc. Sometimes it works, and sometimes it doesn't :-). When I get some time, I'll start giving the pgcc guys some feedback. John