Re: Compiler performance tests on FreeBSD 10.0-CURRENT

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_at_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_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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