kernels/kernels/wu_arch_cases/case20_flash_bwd_fused/README.md

case20_flash_bwd_fused

FlashAttention backward-style fused pipeline smoke test.

The tensor warp performs one score MMA, then waits for the scalar warp to run softmax plus dsoftmax on the TMEM C row. The scalar warp writes the dS row back to TMEM A using signed fp16 values. The tensor warp then performs four more MMA steps, for five MMA operations total in this case.

This case verifies:

• tensor warp MMA sequencing around a scalar TMEM handoff;
• scalar-only FEXP.S use for stable softmax;
• dsoftmax shape dS = P * (dP - sum(P * dP));
• signed scalar TMEM stores feeding later tensor MMA operations.

The input score row is uniform, so P = 1/32. The synthetic upstream gradient uses two buckets, producing exact dS values -1/32 for row entries 0..15 and +1/32 for row entries 16..31.