[Mesa-dev] [PATCH 4/5] i965/fs: Allow saturate propagation to propagate negations into MADs.

[Ilia Mirkin]

On Mon, Feb 22, 2016 at 7:58 PM, Matt Turner <mattst88 at gmail.com> wrote:
> On Mon, Feb 22, 2016 at 4:39 PM, Ilia Mirkin <imirkin at alum.mit.edu> wrote:
>> On Mon, Feb 22, 2016 at 7:13 PM, Matt Turner <mattst88 at gmail.com> wrote:
>>
>> Unless there's something unexpected going on, let's say
>>
>> src0 = 0, src1 = 0, src2 = -0.5
>>
>>> Allows us to transform
>>>
>>>    mad      res  src0   src1   src2
>>
>> res = -0.5
>
> I think you're assuming that MAD is src0 * src1 + src2, but it's
> actually src0 + src1 * src2.
>
> Does that clear things up?

Considerably. Ignore my comments :)

>
> Generally, the thinking is to negate a MUL you negate one of its
> arguments, and to negate an ADD you negate both of its arguments, so
> to negate a MAD you negate one of the multiplicands and the additive
> argument.