On Thu, May 10, 2018 at 10:33:39AM +0200, Marc Glisse wrote:
> (not a review)
>
> On Thu, 10 May 2018, Hans-Peter Nilsson wrote:
>
> >Replacing a division feeding a division helps only when the
> >second division is the only user, and "fusing" the divisions is
>
> Well, that's not quite true.
> int x, y;
> void f(int n){
> int c = 3 << 20;
> x = n / c;
> y = x / c;
> }
[ fixed the typo ]
> Here we can optimize the last division to y = 0. After your patch, we
> likely need VRP to do that simplification. There are probably more
> complicated transformations this disables.
Without the replacement we have two dependent divisions; with the
replacement we have two independent divisions, and that is much faster
on some (many?) systems (that can do two fixed-point divisions in parallel),
even if things do not simplify further.
Segher
