[Mesa-dev] [PATCH 1/3] glsl: Propagate negates through multiplication chains.

[Ben Widawsky]

On Wed, Feb 11, 2015 at 04:05:57PM -0800, Matt Turner wrote:
> On Wed, Feb 11, 2015 at 3:51 PM, Ian Romanick <idr at 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.
> 

Can you add this to the commit message? I had the exact same question, and this
was the exact right answer. Also does this same thing not apply for division?

> 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.

Have you already considered that this shortcircuits the next block which
according to the comment improves the ability to fold constants, or do we end up
through here again later? (I'm sure you're aware I know little about this code).

Anyway, with the explanation it sounds reasonable to me, and since it seemed
like nobody was actually opposed to it:
Reviewed-by: Ben Widawsky <ben at bwidawsk.net>