On Wed, Feb 11, 2015 at 3:51 PM, Ian Romanick <i...@freedesktop.org> wrote: > On 02/11/2015 02:54 PM, Matt Turner wrote: >> We propagate negations to the right-most leaves of the multiplication >> expression trees: >> >> - mul(neg(x), neg(y)) -> mul(x, y) >> - mul(neg(x), y) -> neg(mul(x, y)) >> - mul(x, neg(y)) -> neg(mul(x, y)) >> >> total instructions in shared programs: 5943123 -> 5937229 (-0.10%) >> instructions in affected programs: 868221 -> 862327 (-0.68%) >> helped: 4518 >> HURT: 356 >> GAINED: 1 > > This seems very counter intuitive. Why does this help? It seems like > the negate modifier on the multiplication sources should be free... how > do we get cheaper than free? :)
Lots of shaders do things like var * 0.5 and var * -0.5. By making the second of those -(var * 0.5) we can do the multiplication once and use its result in both places, negated once. I also noticed we were generating a lot more LRPs after this. I guess the LRP peephole is able to recognize more stuff without some negates in the way. If you want, I can try to take a closer look at some affected shaders -- though it is kind of hard. Diff'ing GLSL IR is impossible, and even if you get past the compiler_temp@ variables being renumbered it's still really difficult to see what's going on. _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/mesa-dev
