Re: Compiler performance tests on FreeBSD 10.0-CURRENT

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_at_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. :)
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