Instead of emitting a single one at the top, and making reference to it,
emit the virtual instruction as needed and let CSE do its job.
Since load_subgroup_invocation now can appear not at the start of the
shader, use UNDEF in all cases to ensure that the liveness of the
destination doesn't extend to the first partial write done here (it was
being used only for SIMD > 8 before).
Note this option was considered in the past
6132992cdb but at the time dismissed. The
difference now is that the lowering of the virtual instruction happens
earlier than the scheduling.
The motivation for this change is to allow passes other than the NIR
conversion to use this value. The alternative of storing a `brw_reg` in
the shader (instead of NIR state) gets complicated by passes like
compact_vgrfs, that move VGRFs around (and update the instructions).
This and maybe other passes would have to care about the brw_reg.
Fossil-db numbers, TGL
```
*** Shaders only in 'after' results are ignored:
steam-native/shadow_of_the_tomb_raider/c683ea5067ee157d/fs.32/0, steam-native/shadow_of_the_tomb_raider/f4df450c3cef40b4/fs.32/0, steam-native/shadow_of_the_tomb_raider/94b708fb8e3d9597/fs.32/0, steam-native/shadow_of_the_tomb_raider/19d44c328edabd30/fs.32/0, steam-native/shadow_of_the_tomb_raider/8a7dcbd5a74a19bf/fs.32/0, and 366 more
from 4 apps: steam-dxvk/alan_wake, steam-dxvk/batman_arkham_city_goty, steam-dxvk/batman_arkham_origins, steam-native/shadow_of_the_tomb_raider
*** Shaders only in 'before' results are ignored:
steam-dxvk/octopath_traveler/aaa3d10acb726906/fs.32/0, steam-dxvk/batman_arkham_origins/e6872ae23569c35f/fs.32/0, steam-dxvk/octopath_traveler/fd33a99fa5c271a8/fs.32/0, steam-dxvk/octopath_traveler/9a077cdc16f24520/fs.32/0, steam-dxvk/batman_arkham_city_goty/fac7b438ad52f622/fs.32/0, and 12 more
from 4 apps: steam-dxvk/batman_arkham_city_goty, steam-dxvk/batman_arkham_origins, steam-dxvk/octopath_traveler, steam-native/shadow_of_the_tomb_raider
Totals:
Instrs: 149752381 -> 149751337 (-0.00%); split: -0.00%, +0.00%
Cycle count: 11553609349 -> 11549970294 (-0.03%); split: -0.06%, +0.03%
Spill count: 42763 -> 42764 (+0.00%); split: -0.01%, +0.01%
Fill count: 75650 -> 75651 (+0.00%); split: -0.00%, +0.01%
Max live registers: 31725096 -> 31671792 (-0.17%)
Max dispatch width: 5546008 -> 5551672 (+0.10%); split: +0.11%, -0.00%
Totals from 52574 (8.34% of 630441) affected shaders:
Instrs: 9535159 -> 9534115 (-0.01%); split: -0.03%, +0.02%
Cycle count: 1006627109 -> 1002988054 (-0.36%); split: -0.65%, +0.29%
Spill count: 11588 -> 11589 (+0.01%); split: -0.03%, +0.03%
Fill count: 21057 -> 21058 (+0.00%); split: -0.01%, +0.02%
Max live registers: 1992493 -> 1939189 (-2.68%)
Max dispatch width: 559696 -> 565360 (+1.01%); split: +1.06%, -0.05%
```
and DG2
```
*** Shaders only in 'after' results are ignored:
steam-native/shadow_of_the_tomb_raider/1f95a9d3db21df85/fs.32/0, steam-native/shadow_of_the_tomb_raider/56b87c4a46613a2a/fs.32/0, steam-native/shadow_of_the_tomb_raider/a74b4137f85dbbd3/fs.32/0, steam-native/shadow_of_the_tomb_raider/e07e38d3f48e8402/fs.32/0, steam-native/shadow_of_the_tomb_raider/206336789c48996c/fs.32/0, and 268 more
from 4 apps: steam-dxvk/alan_wake, steam-dxvk/batman_arkham_city_goty, steam-dxvk/batman_arkham_origins, steam-native/shadow_of_the_tomb_raider
*** Shaders only in 'before' results are ignored:
steam-native/shadow_of_the_tomb_raider/0420d7c3a2ea99ec/fs.32/0, steam-native/shadow_of_the_tomb_raider/2ff39f8bf7d24abb/fs.32/0, steam-native/shadow_of_the_tomb_raider/92d7be2824bd9659/fs.32/0, steam-native/shadow_of_the_tomb_raider/f09ca6d2ecf18015/fs.32/0, steam-native/shadow_of_the_tomb_raider/490f8ffd59e52949/fs.32/0, and 205 more
from 3 apps: steam-dxvk/batman_arkham_city_goty, steam-dxvk/batman_arkham_origins, steam-native/shadow_of_the_tomb_raider
Totals:
Instrs: 151597619 -> 151599914 (+0.00%); split: -0.00%, +0.00%
Subgroup size: 7699776 -> 7699784 (+0.00%)
Cycle count: 12738501989 -> 12739841170 (+0.01%); split: -0.01%, +0.02%
Spill count: 61283 -> 61274 (-0.01%)
Fill count: 119886 -> 119849 (-0.03%)
Max live registers: 31810432 -> 31758920 (-0.16%)
Max dispatch width: 5540128 -> 5541136 (+0.02%); split: +0.08%, -0.06%
Totals from 49286 (7.81% of 631231) affected shaders:
Instrs: 8607753 -> 8610048 (+0.03%); split: -0.01%, +0.04%
Subgroup size: 857752 -> 857760 (+0.00%)
Cycle count: 305939495 -> 307278676 (+0.44%); split: -0.28%, +0.72%
Spill count: 6339 -> 6330 (-0.14%)
Fill count: 12571 -> 12534 (-0.29%)
Max live registers: 1788346 -> 1736834 (-2.88%)
Max dispatch width: 510920 -> 511928 (+0.20%); split: +0.85%, -0.66%
```
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/30489>
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