Re: GCC Benchmarks (coybench), AMD64 and i686, 14 August 2004

[Daniel Berlin <dberlin at dberlin dot org>]

On Aug 17, 2004, at 8:13 PM, Joe Buck wrote:

> On Tue, Aug 17, 2004 at 07:53:40PM -0400, Robert Dewar wrote:
> > Richard Kenner wrote:
> >
> > >     The trig identity that 2*sin(x)*cos(x) is sin(2*x) is more iffy; 
> > > we'd
> > >     waste time trying to apply such transformations to every tree, 
> > > and
> > >     it's pretty much exactly what caused such controversy when 
> > > everyone
> > >     did it to Whetstone.
> > >
> > > Although I agree with you in general about tuning to benchmarks, I 
> > > disagree
> > > with the above.  When you consider some of the even more obscure 
> > > cases in
> > > both tree and RTL folding, that doesn't seem so peculiar anymore.
> >
> > It seems completely wrong to me. I guess people who use -ffast-math
> > tolerate these kind of non-optimizations (an optimization to me is
> > a transformation that is meaning preserving, not a substitution of
> > some other computation that gives a different result!)
>
> Are we sure that ICC does 2*sin(x)*cos(x) -> sin(2*x)?  It seems to me
> that just being able to produce x < PI2 ? 2*sin(x)*cos(x) : 0.0 gives
> most of the available speedup.  Such a transformation should be 
> achievable
> and preserves the result.
Neither is what ICC does, actually.
At least according to it's debug dumps.