We usually emit flags right before consuming them but this is
suboptimal from the point of view of register pressure: if an
instruction is only used to generate flags then waiting to emit
it right before reading the flags extends the liveness of the
sources used to generate the flags for no gain. This pass will
check for such instructions and try to move them as early as
possible.
Shader-db results below show this is effective to reduce register
pressure, allowing a few shaders to increase thread counts and/or
reduce spilling:
total instructions in shared programs: 11057173 -> 11057076 (<.01%)
instructions in affected programs: 1955543 -> 1955446 (<.01%)
helped: 4214
HURT: 3905
Inconclusive result (value mean confidence interval includes 0).
total threads in shared programs: 425096 -> 425170 (0.02%)
threads in affected programs: 74 -> 148 (100.00%)
helped: 37
HURT: 0
Threads are helped.
total uniforms in shared programs: 3846275 -> 3845674 (-0.02%)
uniforms in affected programs: 23574 -> 22973 (-2.55%)
helped: 217
HURT: 30
Uniforms are helped.
total max-temps in shared programs: 2222910 -> 2220488 (-0.11%)
max-temps in affected programs: 61904 -> 59482 (-3.91%)
helped: 2145
HURT: 113
Max-temps are helped.
total spills in shared programs: 4294 -> 4280 (-0.33%)
spills in affected programs: 148 -> 134 (-9.46%)
helped: 8
HURT: 0
total fills in shared programs: 6497 -> 6468 (-0.45%)
fills in affected programs: 291 -> 262 (-9.97%)
helped: 8
HURT: 0
total sfu-stalls in shared programs: 14344 -> 14611 (1.86%)
sfu-stalls in affected programs: 1308 -> 1575 (20.41%)
helped: 217
HURT: 335
Inconclusive result (%-change mean confidence interval includes 0).
total inst-and-stalls in shared programs: 11071517 -> 11071687 (<.01%)
inst-and-stalls in affected programs: 1946767 -> 1946937 (<.01%)
helped: 4191
HURT: 3909
Inconclusive result (value mean confidence interval includes 0).
total nops in shared programs: 270628 -> 269829 (-0.30%)
nops in affected programs: 22032 -> 21233 (-3.63%)
helped: 1213
HURT: 571
Inconclusive result (%-change mean confidence interval includes 0).
Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/30511>
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