Date: Wed, 5 Sep 2012 13:44:00 -0400 From: Justin Hibbits <chmeeedalf@gmail.com> To: Roman Divacky <rdivacky@freebsd.org> Cc: Garrett Cooper <yanegomi@gmail.com>, Dimitry Andric <dimitry@andric.com>, freebsd-current@freebsd.org, David Chisnall <theraven@freebsd.org>, Steve Kargl <sgk@troutmask.apl.washington.edu> Subject: Re: Compiler performance tests on FreeBSD 10.0-CURRENT Message-ID: <CAHSQbTAx8Js%2BbgTXnt6=OeOGnns%2BH-k8A2zbugFyizRsafyCOA@mail.gmail.com> In-Reply-To: <20120905173704.GA31250@freebsd.org> References: <5046670C.6050500@andric.com> <20120904214344.GA17723@troutmask.apl.washington.edu> <504679CB.90204@andric.com> <20120904221413.GA19395@troutmask.apl.washington.edu> <CAGH67wTQavfh9ExsjypnCjw4yrV2RpdUUjxAD2kaZy-PiDocHA@mail.gmail.com> <50471BEE.6030708@andric.com> <96BD00DE-865C-4690-A2F1-E5B7C5D221C0@FreeBSD.org> <50472FC7.8010500@andric.com> <CAHSQbTDy7WS0_N7wc68Vi3tzijObDNLchG7jVMWy1uejxYFd8Q@mail.gmail.com> <20120905173704.GA31250@freebsd.org>
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Actually, Nathan does say it's gcc's fault in a comment on that bug. However, I do all my clang work compiling it with gcc4.2.1, so run into this constantly when I forget to add the flag. - Justin On Wed, Sep 5, 2012 at 1:37 PM, Roman Divacky <rdivacky@freebsd.org> wrote: > What makes you think it's a bug in llvm code and not a plain gcc > miscompile? > Other people seem to compile llvm on PPC64 with gcc and -fstrict-aliasing > just fine. They just dont happen to use gcc4.2.1. Ie. gcc47 is reported > to not have this problem. I personally can confirm that fbsd+gcc48 is ok to > > On Wed, Sep 05, 2012 at 09:11:22AM -0400, Justin Hibbits wrote: > > On Wed, Sep 5, 2012 at 6:56 AM, Dimitry Andric <dimitry@andric.com> > wrote: > > > > > On 2012-09-05 11:36, David Chisnall wrote: > > > > > >> On 5 Sep 2012, at 10:31, Dimitry Andric wrote: > > >> > > >>> TThe > > >>> > > >>> -fno-strict-aliasing is not really my choice, but it was > introduced > > >>> in the past by Nathan Whitehorn, who apparently saw problems > without > > >>> it. It will hopefully disappear in the future. > > >>> > > >> Clang currently defaults to no strict aliasing on FreeBSD. > > >> > > > > > > Yes, but upstream has never used -fno-strict-aliasing, just plain -O2. > > > I run regular separate builds of pristine upstream clang on FreeBSD, > and > > > I haven't seen any failures due aliasing problems in all the regression > > > tests. That doesn't guarantee there are no problems, of course... > > > > > > Aliasing problems are seen much more frequently on PowerPC than any other > > platform for Clang. I found this a while back when doing some Clang > > testing, and I still see problems with upstream unless I explicitly set > > -fno-strict-aliasing. Nathan had mentioned wanting to get upstream to > use > > -fno-strict-aliasing by default on all platforms, but I don't think that > > ever made it beyond his suggesting. > > > > I filed this bug to track it: http://llvm.org/bugs/show_bug.cgi?id=11955 > > > > > > In my experience, most C programmers misunderstand the aliasing rules of > C > > >> and even people on the C++ standards committee often get them wrong > for > > >> C++, so trading a 1-10% performance increase for a significant > chance of > > >> generating non-working code seems like a poor gain. If people are > certain > > >> that they do understand the rules, then they can add > -fstrict-aliasing to > > >> their own CFLAGS. > > >> > > > > > > I'm actually quite interested in the performance difference; I think I > > > will run a few tests. :) > > _______________________________________________ > > freebsd-current@freebsd.org mailing list > > http://lists.freebsd.org/mailman/listinfo/freebsd-current > > To unsubscribe, send any mail to " > freebsd-current-unsubscribe@freebsd.org" >
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