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swung0x48
c7e36986e7
The previous commit's fingerprint was pinned to one array's incidental
dimensions - workgroup exactly 32x16x1, element exactly vec2, length
exactly 32 - which is the auto-exposure reduction and nothing else. The
pack ships the same idiom twice:
- auto-exposure: 32x16 (512 invocations), shared vec2 prefixSumCache[32]
- RTW warp: 1024 invocations, shared float prefixSumCache[64]
so the warp kept writing 128 subgroups into 64 entries on an 8-lane
device and the retrace stayed bit-identically wrong (ssim 0.027902).
Key the fingerprint on the pack's idiom instead of one array's shape: a
workgroup array of 32-bit floats indexed by gl_SubgroupID, fed by a
subgroup scan, whose declared length is below ceil(invocations / native
width). Three properties keep that a targeted repair rather than a
general array resizer:
- the index must BE gl_SubgroupID (through OpCopyObject, a signedness
OpBitcast, or a spill whose every store is that id), so an index
masked or clamped into range is left alone;
- the >= 16-lane early-out is retained, so every module on the devices
the pack was written for passes through byte-identical;
- growth is certified against maxComputeSharedMemorySize using a
natural-alignment layout model, and declined outright when a
declaration cannot be sized, so a patched module can never fail
pipeline creation where the original would not have.
Verified against the shaders the CI trace actually contains: of the 14
compute modules in the fixture exactly these two change, the other
twelve are byte-identical, and all fourteen pass spirv-val. The
integration scenario grows a second case for the 1024-invocation shape;
both abort with heap corruption when the patch is disabled.