[Mesa-dev] [PATCH] i965: Use NIR by default for vertex shaders on GEN8+

[Matt Turner]

On Fri, May 8, 2015 at 10:08 AM, Jason Ekstrand <jason at jlekstrand.net> wrote:
> On Fri, May 8, 2015 at 3:27 AM, Kenneth Graunke <kenneth at whitecape.org> wrote:
>> On Thursday, May 07, 2015 06:17:46 PM Matt Turner wrote:
>>> On Thu, May 7, 2015 at 4:50 PM, Jason Ekstrand <jason at jlekstrand.net> wrote:
>>> > GLSL IR vs. NIR shader-db results for SIMD8 vertex shaders on Broadwell:
>>> >
>>> >    total instructions in shared programs: 2724483 -> 2711790 (-0.47%)
>>> >    instructions in affected programs:     1860859 -> 1848166 (-0.68%)
>>> >    helped:                                4387
>>> >    HURT:                                  4758
>>> >    GAINED:                                1499
>>> >
>>> > The gained programs are ARB vertext programs that were previously going
>>> > through the vec4 backend.  Now that we have prog_to_nir, ARB vertex
>>> > programs can go through the scalar backend so they show up as "gained" in
>>> > the shader-db results.
>>>
>>> Again, I'm kind of confused and disappointed that we're just okay with
>>> hurting 4700 programs without more analysis. I guess I'll go do
>>> that...
>>
>> I took a stab at that tonight.  The good news is, the majority of the
>> hurt is pretty stupid.  Indirect uniform address calculations involve
>> a lot of integer multiplication by 4.
>>
>> For whatever reason, we're getting 4*x instead of x*4, which doesn't
>> support immediates.  So we get:
>>
>>     MOV tmp 4
>>     MUL dst tmp x
>>
>> Normally, constant propagation would commute the operands, giving us
>> "MUL dst x 4" like we want.  But it sees integer multiplication and
>> chickens out, due to the asymmetry on some platforms.
>
> Right.  I just sent out a patch that puts immediates in src1 for
> commutative ALU ops at the emit stage.  We probably still want to do
> something in constant propagation in case we can fold something, but
> it fixes the problem for now.  It also helps even more than the
> shifting patch.
>
> I don't know.  Maybe we just want to make constant propagation do the
> right thing on BDW+.  Matt?

Yes.