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

[Ian Romanick]

On 02/11/2015 04:05 PM, 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.
> 
> 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.

Naw, that's fine.  Enabling additional CSE is a perfectly valid
explanation... though it makes me wonder if CSE should be smarter.  For
all the programs helped, about 8% are hurt.  *shrug*