Once we have exhausted compile strategies at 4 threads and we start
enabling lower thread counts, there is no point in starting compiles
with 4 threads for them, we know these will fail, so let's start at
2 in these cases.
This also has another nice implication: if the driver compiles at 4
threads and fails to register allocate, we were allowing it to try
with 2 threads, but this would only retry the register allocation
process and would not really recompile the shader with 2 threads. This
is not optimal, because at 2 threads we have more TMU fifo space for
each thread and we can do more TMU pipelining, so we were missing that
opportunity.
This improves performance in Sponza by ~1.5% and also seems to help
UE4 slightly.
Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10647>
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