Date: Wed, 5 Sep 2012 09:11:22 -0400 From: Justin Hibbits <chmeeedalf@gmail.com> To: Dimitry Andric <dimitry@andric.com> Cc: Garrett Cooper <yanegomi@gmail.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: <CAHSQbTDy7WS0_N7wc68Vi3tzijObDNLchG7jVMWy1uejxYFd8Q@mail.gmail.com> In-Reply-To: <50472FC7.8010500@andric.com> 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>
next in thread | previous in thread | raw e-mail | index | archive | help
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. :)
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?CAHSQbTDy7WS0_N7wc68Vi3tzijObDNLchG7jVMWy1uejxYFd8Q>