A glc buffer_load_dwordx2 is one 64-bit access on the GPU, so it never
sees half of a buffer_atomic_swap_x2. It was emitted as two dword loads,
which can: the first can read the flag from before the swap and the
second the value from after it.
GTA V gives its BVH nodes their record indices with a single-pass
decoupled look-back prefix sum. Each workgroup publishes a {flag, value}
pair for its block with buffer_atomic_swap_x2 and reads the previous 64
blocks' pairs with glc dwordx2 loads. A torn read counts earlier blocks
twice, two nodes then share records, the BVH is corrupt and the ray
tracing refit hangs. It only happens in scans of more than 64 blocks (the
largest BVHs) and depends on timing.
In a program that has the u64 storage-buffer view (it uses 64-bit buffer
atomics), a coherent raw two-dword buffer load that is 8-byte aligned and
in bounds is now read with one device-scope OpAtomicLoad on that view and
split with OpUConvert and a shift (not OpBitcast to a uvec2, which
crashes NVIDIA's shader compiler). All other loads keep the per-dword
path.
tests/execution/LookBackScan runs the game's scan kernel: the look-back
from given blocks, and 50 whole 125-block scans checked against a CPU
prefix sum. Without this change the first whole scan fails.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
55447e1c77