On Wed, Jan 13, 2010 at 10:53 AM, Craig Schlenter <craig.schlen...@gmail.com
> wrote:
> On Wed, Jan 13, 2010 at 8:34 PM, Antoine Labour <pi...@google.com> wrote:
> >
> > On Wed, Jan 13, 2010 at 10:06 AM, Stephen White <
> senorbla...@chromium.org>
> > wrote:
> >>
> >> On Wed, Jan 13, 2010 at 12:19 PM, Dan Kegel <d...@kegel.com> wrote:
> >>>
> >>> On Wed, Jan 13, 2010 at 9:12 AM, Stephen White <
> senorbla...@chromium.org>
> >>> wrote:
> >>> > 2) Most of the supposed performance advantage of strict aliasing
> rules
> >>> > is
> >>> > probably taken care of by memory disambiguation in modern (ie., Core2
> >>> > and
> >>> > later) CPUs.
> >>>
> >>> I kind of doubt that. Disallowing aliasing lets the compiler do
> >>> a number of high-level optimizations that the chip could never do on
> its
> >>> own.
> >>> See e.g.
> >>>
> >>>
> http://cellperformance.beyond3d.com/articles/2006/06/understanding-strict-aliasing.html
> >>
> >> (As a side note, I find this example somewhat contrived: if this were
> >> performance-critical code, the programmer would pull the dereference out
> of
> >> the loop anyway, not rely on the compiler to do it.)
> >> That said, the "data-driven" answer would be to run perf benchmarks with
> >> and without strict aliasing, and see the effect. It might even help us
> find
> >> some hotspots which we could improve under MSVC.
> >>>
> >>> I'm for -fstrict-aliasing except for third_party.
> >>
> >> Whichever is decided, I think making it explicit would make life easier
> >> for the people on different compiler versions.
> >> Stephen
> >
> > I have a bias towards -fstrict-aliasing, but whichever the choice, I
> think
> > we should have the same default for all the compiler versions (i.e.
> forcing
> > it either way).
> > For Chrome OS, we will use gcc 4.4 so we'll get more exposure in-house to
> > that particular compiler.
>
> I'm one try-server run away from possibly turning -fno-strict-aliasing on
> for
> all linux/bsd gcc: http://codereview.chromium.org/519034
>
> From a "process" standpoint, given that there is some disagreement here
> is someone going to come find me with a clue bat if I commit this?
>
> Get your votes in now! :)
>
I say go for it, it's easy enough to revert if we discover problems.
Antoine
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