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intel/fs: reduce liveness of variables in lowering passes

Lionel Landwerlin requested to merge llandwerlin/mesa:review/intel-fs-undef-lowering.2 into main

When lowering a single instruction with a destination VGRF to 2 or more, the VGRF is now considered partially written by each generated instruction and that increases its liveness especially in loops. Thus potentially increasing the number of spills/fills due to register allocation.

Putting an UNDEF instruction in front of the lowered instructions allows the IR to limit the liveness of the VGRF, reducing register pressure.

This has a pretty dramatic effect on spills/fills for RT shaders. Here the stats on Q2RTX shaders on DG2 (wiping out any spills/fills due to register allocation) :

Instructions in all programs: 78269 -> 72254 (-7.7%)
SENDs in all programs: 3177 -> 3177 (+0.0%)
Loops in all programs: 40 -> 40 (+0.0%)
Cycles in all programs: 2389042 -> 2001762 (-16.2%)
Spills in all programs: 320 -> 91 (-71.6%)
Fills in all programs: 486 -> 133 (-72.6%)

Shader-db results on TGL :

total instructions in shared programs: 21206365 -> 21206602 (<.01%)
instructions in affected programs: 76629 -> 76866 (0.31%)
helped: 18
HURT: 20
helped stats (abs) min: 1 max: 60 x̄: 18.89 x̃: 12
helped stats (rel) min: 0.21% max: 3.61% x̄: 1.02% x̃: 0.77%
HURT stats (abs)   min: 1 max: 79 x̄: 28.85 x̃: 18
HURT stats (rel)   min: 0.04% max: 2.81% x̄: 1.13% x̃: 0.79%
95% mean confidence interval for instructions value: -4.82 17.30
95% mean confidence interval for instructions %-change: -0.34% 0.57%
Inconclusive result (value mean confidence interval includes 0).

total loops in shared programs: 5774 -> 5774 (0.00%)
loops in affected programs: 0 -> 0
helped: 0
HURT: 0

total cycles in shared programs: 800180557 -> 800194411 (<.01%)
cycles in affected programs: 6208395 -> 6222249 (0.22%)
helped: 22
HURT: 17
helped stats (abs) min: 2 max: 8794 x̄: 1438.18 x̃: 782
helped stats (rel) min: 0.05% max: 2.28% x̄: 0.63% x̃: 0.44%
HURT stats (abs)   min: 2 max: 19178 x̄: 2676.12 x̃: 1358
HURT stats (rel)   min: 0.04% max: 23.49% x̄: 2.25% x̃: 0.71%
95% mean confidence interval for cycles value: -952.19 1662.65
95% mean confidence interval for cycles %-change: -0.64% 1.90%
Inconclusive result (value mean confidence interval includes 0).

total spills in shared programs: 4078 -> 4066 (-0.29%)
spills in affected programs: 40 -> 28 (-30.00%)
helped: 2
HURT: 0

total fills in shared programs: 2856 -> 2832 (-0.84%)
fills in affected programs: 127 -> 103 (-18.90%)
helped: 2
HURT: 0

total sends in shared programs: 1002091 -> 1002091 (0.00%)
sends in affected programs: 0 -> 0
helped: 0
HURT: 0

LOST:   0
GAINED: 0

CI (green) :

Edited by Lionel Landwerlin

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