This tries to calculate an underestimate (lower bound) for the register
pressure at various SIMD widths, by counting live values in the NIR
shader. This fundamentally won't be accurate, but it can give us an
idea of whether it's even worth trying a certain SIMD-width compile.
Doing this at the NIR level means we:
- Can use SSA structure rather than fuzzy liveness intervals
- Can avoid the backend scheduler aggressively trying to hide latency,
presenting an overinflated view of the register pressure
- Have divergence information on-hand, making it easier to "scale up"
- Can skip cloning and optimizing NIR for compute shader SIMD widths
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/36750>
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