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

[Matt Turner]

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.

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.