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[Impeller] Take subgroup size into account when prefix summing #40509
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A fun note to add: I was scratching my head for a bit debugging this problem because if I simplified the quad shader even a little, it jumped back up to a subgroup size of 32 on the intel mac and everything looked good. I finally thought to check the size specifically when I noticed that the |
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auto label is removed for flutter/engine, pr: 40509, due to - The status or check suite Mac Host clang-tidy has failed. Please fix the issues identified (or deflake) before re-applying this label.
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Fixes flutter/flutter#122828
On Intel hardware, the subgroup size is coming out as 16 for the quad shader in particular. There are, however, 26 quads to compute. The test works fine on M1 hardware where the subgroup size stays at 32 for this shader, but fails on Intel.
This change makes both the test shaders safe against smaller threadgroup sizes.