biondizzle 4564758466 Stage B Bug 4b debugging: P/A alias proven working, V layout issue for (128,64) PV ...

Key findings:
- P/A alias WORKS: PV reads non-zero P from TMEM at offset 32 (proven by no-softmax test)
- V mode bug: V=(128,64) only loads 64 K-values, PV needs 128. Output = sum(S[:,:64]) = 0.67 cosine
- FMHA-style V reconstruction (hd,n,1) stride (1,hd) gives NaN for (128,64) PV
- K-major V (64,128) contiguous gives NaN for (128,64) PV
- Square (128,128) PV works with ALL V approaches (cosine 0.999999)
- Non-square PV consistently broken regardless of V layout

Test files:
- test_128_128_fmha_v.py: (128,128) with FMHA V - PASS
- test_pv64_fmha_v.py: (128,64) with FMHA V - NaN
- test_pv64_kmajor_v.py: (128,64) with K-major V - NaN
- test_pv64_with_softmax.py: (128,64) with original V - 0.67
- test_pv64_no_softmax.py: proves P/A alias works
- test_fmha_v3.py: full pipeline with QK C-fragment composition store

2026-05-21 15:20:14 +00:00

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