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

[Robert Dewar <dewar at gnat dot com>]

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!)