[Mesa-dev] [PATCH 9/9] i965: Use NIR by default for Fragment shaders on SNB+

[Connor Abbott]

On Wed, Mar 18, 2015 at 1:56 AM, Matt Turner <mattst88 at gmail.com> wrote:
> Thanks. Looked through stats and at some of the regressions.
>
> Some of the areas I noticed we were doing worse:
>
> We generate two CMPs for discard_if; only one without NIR. I think you
> had an idea about solving this.
>
> SEL peephole interference -- we knew about this in general, but I
> found an interesting failure mode:
>
> NIR generates
>
> (+f0) if(8)     JIP: 4          UIP: 5
> else(8)         JIP: 3
> mov(8)          g124<1>F        -g124<8,8,1>F
> endif(8)        JIP: 6
>
> We should of course improve dead-control-flow-elimination to remove
> the else and set the if's predicate_inverse, but non-NIR generates
>
> (+f0) sel(8)    g124<1>F        g25<8,8,1>F     -g25<8,8,1>F
>
> which is probably better even than a predicated MOV since it's not a
> partial write and subject to optimization restrictions. The assembly
> matches the NIR exactly, so I'm assuming NIR is removing a
> self-assignment in the if-then block, preventing it from turning it
> into a conditional select.

I think this is because the sel peephole (along with all the other
optimizations) in NIR only works before we lower to source mods, where
this code probably looked something like:

ssa_1 = ...;
if ... {
} else {
    ssa_2 = fneg ssa_1;
}
ssa_3 = phi ssa_1, ssa_2;

So we can probably fix it just by adding fneg/ineg to
block_check_for_allowed_instrs().

>
> Also, since the fs's SEL peephole only recognizes MOVs to the same
> destination in order, it doesn't handle tropico-5/387 properly, which
> already improved from 110 to 79 instructions.
>
> I also noticed an interesting pattern in defense-grid/537 (and some
> others, I think). NIR generates
>
> mul(8)  g4<1>F    g2<8,8,1>F  g3<8,8,1>F
> mul(8)  g6<1>F    g5<8,8,1>F  g4<8,8,1>F
> mad(8)  g127<1>F  g6<4,4,1>F  g3<4,4,1>F  g2<4,4,1>F
>
> whereas without NIR we get:
>
> mul(8)  g5<1>F    g2<8,8,1>F  g4<8,8,1>F
> mad(8)  g127<1>F  g5<4,4,1>F  g3<4,4,1>F  g5<4,4,1>F
>
> Again, the assembly matches the NIR, so NIR is doing something bad.
>
> The changes to sun-temple/209 (and a lot of other sun temple shaders)
> look wrong. I'm not sure if the translation from NIR is doing
> something bad or what, but we should have 8 texture fetches and we
> only end up with 2.
>
>
> Some highlights --
>
> Some shaders are improved because NIR recognizes exp2(log(x) * y) as
> pow(x, y) when the GLSL optimizations couldn't, because its dead code
> elimination is much too stupid to allow tree grafting to put the IR in
> the form we expect. This accounts for a low of the high percentage
> gains.
>
> It cuts some Kerbal Space Program shaders in half (~1000 -> ~500
> instructions) and removes ~100 spills and ~100 fills from each. Not
> sure how it actually accomplished this. Too hard to read the assembly
> with all of the spills.
>
> Code generated by tropico-5/252 is really stupid. We MOVs both before
> and in an if statement that do the same thing. NIR cleans that up
> easily as a side-effect of SSA.
>
> In strike-suit-zero/156 we emit more MADs that we couldn't recognize
> from the GLSL IR, since again, its DCE is terrible and we can't
> combine trees.
>
> In the-binding-of-isaac-rebirth/18, it's able to CSE some duplicate
> rcp expressions in different basic blocks. I'm not sure why the
> backend isn't doing this.
>
>
> At this point I've looked through all of the shaders helped by >15% or more.
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