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

[Jason Ekstrand]

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?

Right...  constants...  NIR doesn't know what a negative constant is,
at least not at the moment.  Sorry for the noise.

> 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.
>
>> 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 irony being that this patch with 19 and 20 are mostly helpful with
> NIR. :)
>
>> 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.
>
> I think patch 19 & 20 cover all the cases for CSE.
>
>> So I'd probably rather not take this patch.
>
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