[Mesa-dev] [PATCH 3/4] nir: allow nir_intrinsic_load_ubo in opt_peephole_select

[Eric Anholt]

Timothy Arceri <tarceri at itsqueeze.com> writes:

> This makes this opt behave more like the GLSL IR opt
> lower_if_to_cond_assign(). With this we can disable that GLSL IR
> opt on drivers with a NIR backend without causing spill
> regressions.
>
> shader-db results for radeonsi (RX580):
>
> Totals from affected shaders:
> SGPRS: 12200 -> 13072 (7.15 %)
> VGPRS: 13496 -> 11840 (-12.27 %)
> Spilled SGPRs: 285 -> 290 (1.75 %)
> Spilled VGPRs: 115 -> 0 (-100.00 %)
> Private memory VGPRs: 0 -> 0 (0.00 %)
> Scratch size: 116 -> 0 (-100.00 %) dwords per thread
> Code Size: 781304 -> 770168 (-1.43 %) bytes
> LDS: 0 -> 0 (0.00 %) blocks
> Max Waves: 1558 -> 1586 (1.80 %)
> Wait states: 0 -> 0 (0.00 %)

Moving UBO loads out of conditionals seems questionable to me, and
I don't think shader-db would represent the effect of this kind of code
transformation well.  I'd love to see some actual performance numbers on
affected applications.

That said, lower_if_to_cond_assign() was already doing this, and I'm
assuming these shader-db stats are just from
with-glsl-optimization-disabled to with-nir-patch.  So, while we should
probably investigate tuning this, since it lets us dump some GLSL
optimization:

Acked-by: Eric Anholt <eric at anholt.net>
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