[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 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.

It was a three patch series and one of the patches was never reviewed.
I would have just pushed them, but they hurt NIR and I haven't figured
out how to fix that yet.

The third patch recognizes neg(mul(...)) in the GLSL -> i965
translators and set a source modifier to avoid an additional
instruction from the case where we can't eliminate the negates
entirely.

It's not just recognizing -(x * y) and (x * -y) that's problematic,
it's (-x * y) as well. My patches handle that case too.

So I'd probably rather not take this patch.