Date: Sun, 9 Feb 2003 07:01:20 -0800 From: David Schultz <dschultz@uclink.berkeley.edu> To: Adrian Chadd <adrian@freebsd.org> Cc: Terry Lambert <tlambert2@mindspring.com>, Ray Kohler <ataraxia@cox.net>, freebsd-current@freebsd.org Subject: Re: Compiling with high optimization? Message-ID: <20030209150120.GA2263@HAL9000.homeunix.com> In-Reply-To: <20030209141006.GB33928@skywalker.creative.net.au> References: <20030208173756.GA56030@arkadia.nv.cox.net> <20030208232724.GA20435@HAL9000.homeunix.com> <3E459BF3.BB3FC381@mindspring.com> <20030209002542.GA20812@HAL9000.homeunix.com> <20030209141006.GB33928@skywalker.creative.net.au>
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Thus spake Adrian Chadd <adrian@freebsd.org>: > On Sat, Feb 08, 2003, David Schultz wrote: > > > Yes, the possibility of being bitten by compiler bugs is certainly > > higher with higher optimization levels. Alpha with -O2 seems to > > have been broken for years, and I have seen strange things happen > > on IA64 as well. But the i386 code generators have received much > > wider testing and debugging, so there is somewhat less danger there. > > Yet squid under i386 freebsd is .. well, finds -O bugs in gcc. > We gave up trying -O under FreeBSD a long time ago. :-) The last time someone told me, ``gcc -O is broken'', it turned out that they were doing some stack fiddling, and gcc's optimizations broke their faulty assumptions. On the other hand, I'm sure gcc -O does have bugs. Do you have an example snippet that gets miscompiled? > (note: I've seen better performance gains by telling gcc exactly what > CPU you have over -O65536 ..) Strangely, gcc in FreeBSD 5.0 actually generates *slower* code when compiling for more recent architectures than when compiling for a 386. I don't know whether that is a bug in gcc or whether gcc is using some fancy feature like SSE that the kernel handles poorly on context switches. I think there was some discussion on the lists about it earlier. To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-current" in the body of the message
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