[Mesa-dev] [PATCH 17/23] glsl: Distribute ir_unop_neg over ir_binop_mul of a constant

[Matt Turner]

On Fri, Mar 20, 2015 at 4:14 PM, Ian Romanick <idr at freedesktop.org> wrote:
> On 03/20/2015 02:51 PM, Matt Turner wrote:
>> On Fri, Mar 20, 2015 at 1:58 PM, Ian Romanick <idr at freedesktop.org> wrote:
>>> From: Ian Romanick <ian.d.romanick at intel.com>
>>>
>>> An expression like -(x * 8) becomes (x * -8).
>>>
>>> Much of the hurt caused by this change appears to be from CSE not being
>>> able to see (x * -8) and (x * 8) as common.  Previously -(x * 8) and (x
>>> * 8) would have had a common subexpression.  Later patches address this
>>> at the GLSL IR level, but additional work will be needed in NIR and the
>>> i965 backend.
>>
>> I've got patches that do the opposite of this: pull the negate out of
>> the multiplication.
>
> Why is pulling the negation off a constant useful?

Because shaders do x * -4.0 and x * 4.0 and if we just calculate x *
4.0 once we can negate its use for free.

> This patch only pushes the negation onto a constant, and constant
> folding will eliminate that.  There's even an assertion in the code to
> be sure. :)
>
> You have links (or a branch) to those patches?  I'd like to compare the
> results with these patches.

Pushed to

   git://people.freedesktop.org/~mattst88/mesa mul-neg