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1f15a0f8b2
The caller already loops over the sources. This means that the caller
must loop over the sources in reverse because constant propagation
prefers to propagate into the last sources first.
The shader-db and fossil-db changes (below) are all due to SEL
instructions. Changing the order sources are visited changes whether a
SEL with two immediate sources is
(+f0.0) sel g12 IMM_A IMM_B
or
(-f0.0) sel g12 IMM_B IMM_A
The ordering of the sources affects the order the constant combining
encounters the values, and the determines which value is "combined"
and which value remains an immediate.
This affects the results by luck. If there are two instructions:
(+f0.0) sel g12 IMM_A IMM_B
(+f0.0) sel g13 IMM_A IMM_C
Picking IMM_A is advantageous over picking IMM_B and IMM_C. Since the
selection algorithm in constant combining is greedy, this case
requires the algorithm see the values in just the right order for the
right thing to happen.
v2: Rebase on many, many changes. Move instruction source fixup
reordering out or try_constant_propagate.
v3: Rebase on !7698.
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25091>
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