CHR (Control Height Reduction) merges multiple biased branches into a
single speculative check, cloning the region into hot/cold paths. On
GPU targets, the merged branch may be divergent (evaluated per-thread),
splitting the wavefront: some threads take the hot path, others the
cold path.
A convergent call like ds_bpermute (a cross-lane operation on AMDGPU)
requires a specific set of threads to be active — when thread X reads
from thread Y, thread Y must be active and participating in the same
call. After CHR cloning, thread Y may have gone to the cold path while
thread X is on the hot path, so the hot-path ds_bpermute reads a stale
register value from thread Y instead of the intended value.
This caused a miscompilation in rocPRIM's lookback scan: CHR duplicated
a region containing ds_bpermute, and the hot-path copy executed with a
different set of active threads, reading incorrect cross-lane data and
causing a memory access fault.
The fix skips any region containing convergent or noduplicate calls,
following the same pattern as SimplifyCFG's block-duplication guard.
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