Skip to content

agx: promote constants to uniforms

Alyssa Rosenzweig requested to merge alyssa/mesa:agx/promote-unifs into main 
Add an optimization pass to promote constants loaded in the shader to dedicated
uniform registers preloaded before the shader. This is beneficial for two
reasons:

* fewer mov_imm instructions
* less GPR pressure (uniforms have dedicated registers)

The latter can significantly improve occupancy since we don't remat constants
for occupancy. We do remat to avoid spilling so it won't affect spilling,
although it can still be a win by reducing remat when a shader would otherwise
spill.

The problem is that we have limited uniform registers so can't promote
everything that we would want to. We model this as a 0-1 knapsack problem and
use the well-known heuristic to prioritize frequently used constants. This is
not optimal but works quite well in practice.

This gives a nice fps win in some complex shaders, including:

* Dolphin ubers from 10.25fps to 10.85fps at 4K in MMG.
* "Wall and chimney" shadertoy from 24.8fps to 29.5fps at 1188x658.

shader-db results are excellent as well.

total instructions in shared programs: 2088290 -> 2039709 (-2.33%)
instructions in affected programs: 1478061 -> 1429480 (-3.29%)
helped: 8246
HURT: 85
Instructions are helped.

total bytes in shared programs: 14321004 -> 14111800 (-1.46%)
bytes in affected programs: 10108742 -> 9899538 (-2.07%)
helped: 7999
HURT: 1416
Bytes are helped.

total regs in shared programs: 602415 -> 590371 (-2.00%)
regs in affected programs: 92177 -> 80133 (-13.07%)
helped: 1887
HURT: 209
Regs are helped.

total uniforms in shared programs: 1457531 -> 1533232 (5.19%)
uniforms in affected programs: 835522 -> 911223 (9.06%)
helped: 0
HURT: 11042
Uniforms are HURT.

total threads in shared programs: 20325824 -> 20329216 (0.02%)
threads in affected programs: 29632 -> 33024 (11.45%)
helped: 41
HURT: 0
Threads are helped.

Merge request reports