nir: introduce discard_is_demote compiler option
This new option indicates that the driver emits the same
code for nir_intrinsic_discard and nir_intrinsic_demote.
Otherwise, it is assumed that discard is implemented as
terminate.
spirv_to_nir uses this option in order to directly emit
nir_demote in case of OpKill.
RADV GFX11:
Totals from 3965 (4.99% of 79439) affected shaders:
MaxWaves: 119418 -> 119424 (+0.01%); split: +0.03%, -0.03%
Instrs: 1608753 -> 1620830 (+0.75%); split: -0.18%, +0.93%
CodeSize: 8759152 -> 8785152 (+0.30%); split: -0.18%, +0.48%
VGPRs: 152292 -> 149232 (-2.01%); split: -2.37%, +0.36%
Latency: 9162314 -> 10033923 (+9.51%); split: -0.46%, +9.97%
InvThroughput: 1491656 -> 1493408 (+0.12%); split: -0.10%, +0.22%
VClause: 21424 -> 21452 (+0.13%); split: -0.31%, +0.44%
SClause: 53598 -> 55871 (+4.24%); split: -2.15%, +6.39%
Copies: 90553 -> 90462 (-0.10%); split: -2.91%, +2.81%
Branches: 16283 -> 16311 (+0.17%)
PreSGPRs: 113993 -> 113254 (-0.65%); split: -1.84%, +1.19%
PreVGPRs: 110951 -> 108914 (-1.84%); split: -2.08%, +0.24%
VALU: 963192 -> 963167 (-0.00%); split: -0.01%, +0.01%
SALU: 87926 -> 90795 (+3.26%); split: -2.92%, +6.18%
VMEM: 25937 -> 25936 (-0.00%)
SMEM: 110012 -> 109799 (-0.19%); split: -0.20%, +0.01%
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Faith Ekstrand <faith.ekstrand@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27617>
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