On Wed, Jul 13, 2011 at 11:52:25AM +0400, Ilya Enkovich wrote:
> > However, it does not fix http://gcc.gnu.org/PR45671, which surprises me
> > as it was marked as a duplicate of this one. Any thoughts on why this
> > isn't sufficient to reassociate the linear chain of adds?
> >
> > Test case:
> >
> > int myfunction (int a, int b, int c, int d, int e, int f, int g, int h)
> > {
> > int ret;
> >
> > ret = a + b + c + d + e + f + g + h;
> > return ret;
> >
> > }
> >
> >
> >
>
> Reassociation does not work for signed integers because signed integer
> is not wrap-around type in C. You can change it by passing -fwrapv
> option but it will disable other useful optimization. Reassociation of
> signed integers without this option is not a trivial question because
> in that case you may introduce overflows and therefore undefined
> behavior.
Well, if it is clearly a win to reassociate, you can always reassociate
them by doing arithmetics in corresponding unsigned type and afterwards
converting back to the signed type.
Jakub
