[Bug c/46186] Clang creates code running 1600 times faster than gcc's

[joseph at codesourcery dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46186

--- Comment #19 from joseph at codesourcery dot com <joseph at codesourcery dot com> 2010-10-26 20:29:56 UTC ---
On Tue, 26 Oct 2010, dominiq at lps dot ens.fr wrote:

> --- Comment #13 from Dominique d'Humieres <dominiq at lps dot ens.fr> 2010-10-26 16:36:05 UTC ---
> > This multiplication transformation is incorrect if the loop wraps  
> > (unsigned always wraps; never overflows).
> 
> I think this is wrong: wrapping is nothing but a modulo 2^n operation (n=64
> here) which "works" for additions and multiplications, so if there is wrapping,
> the result is sum=(b*(b-1)-a*(a-1))/2 modulo 2^n, i.e. correctly wrapped.

It's a bit more complicated than that, in that you can't just compute 
(b*(b-1)-a*(a-1)) mod 2^n, then divide by 2, as that will lose the top 
bit.  (I haven't checked exactly what the generated code is doing here.)