Nodes are allocated in order to registers so initially sorting
was used to ensure that nodes with smaller life ranges would
be assigned first and therefore be more likely to get
accumulators.
However, since d81a6e5f1d now we don't rely on order to make
decisions about accumulators and instead we make policy decisions
based on actual liveness, so sorting is no longer strictly
relevant to this decision.
Furthermore, we are not re-sorting nodes after each spill either,
since that would probably require that we rebuild the interference
graph after each spill (the graph identifies nodes by their index).
Shader-db results show a significant improvement in instruction
counts, due to more optimal accumulator assignments. The reason for
this is that we use a round-robin policy for choosing the next
accumulator to assign. The idea behind this is preventing nearby
temps to be assigned to the same accumulator so that QPU scheduling
is more flexible, but if we sort our nodes, we are basically not
assigning temps in program order any more and the round-robin policy
becomes less effective:
total instructions in shared programs: 13000420 -> 12663189 (-2.59%)
instructions in affected programs: 11791267 -> 11454036 (-2.86%)
helped: 62890
HURT: 19987
total threads in shared programs: 415874 -> 415870 (<.01%)
threads in affected programs: 20 -> 16 (-20.00%)
helped: 2
HURT: 4
total uniforms in shared programs: 3711652 -> 3711624 (<.01%)
uniforms in affected programs: 43430 -> 43402 (-0.06%)
helped: 134
HURT: 173
total max-temps in shared programs: 2144876 -> 2138822 (-0.28%)
max-temps in affected programs: 123334 -> 117280 (-4.91%)
helped: 4112
HURT: 1195
total spills in shared programs: 3870 -> 3860 (-0.26%)
spills in affected programs: 1013 -> 1003 (-0.99%)
helped: 14
HURT: 12
total fills in shared programs: 5560 -> 5573 (0.23%)
fills in affected programs: 1765 -> 1778 (0.74%)
helped: 14
HURT: 17
Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15168>
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