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

[Kenneth Graunke]

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.

I think we can extend that - on Broadwell it should work fine, and
might work fine for 16-bit immediates on Gen7 and Cherryview, too.

Alternatively, I wrote a nir_opt_algebraic_late optimization that turns
4*x into x << 2, which works around the problem, and is also apparently
much better for R600.

Statistics on the shift patch are:

    total instructions in shared programs: 7432587 -> 7388982 (-0.59%)
    instructions in affected programs:     1360411 -> 1316806 (-3.21%)
    helped:                                5772
    HURT:                                  0

Statistics for GLSL IR vs. NIR+(4*x => x << 2):

    total instructions in shared programs: 7232451 -> 7175983 (-0.78%)
    instructions in affected programs:     1586917 -> 1530449 (-3.56%)
    helped:                                5780
    HURT:                                  1654

which is much better.

Looking at a couple of the shaders that are still worse off...it looks
like a ton of Source shaders used to do MUL/ADD with an attribute and
two immediates, and now are doing MOV/MOV/MAD.
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