Staging K/V through shared memory looks like an obvious win on GCN: without it
each rowgroup re-reads the whole K/V block from global memory, and with row_split
4 that pulls one 16KB block through a 16KB vector L1 four times.
Measured, it loses: -6.7% pp2048 at depth 16k and -7.4% at 32k on Polaris at head
size 128. The kvsh stride of D/4+1 dwords is 4 mod 32, which costs an 8-way LDS
bank conflict on wave64 - the +1 padding is tuned for warp32 - and the extra
shared memory eats occupancy this shader is already short of.
Comment only, no behaviour change. Leaving a note so the next person does not
spend a GPU on it.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
LumpiastyandClaude Opus 5
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