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FlashAttention FP8: Added support for asymmetric head dimensions, variable sequence lengths (varlen), and KV splitting - #1097
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coderfeli
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akii96
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Sep 10, 2026
Conflict in kernels/attention/flash_attn_interface.py. Upstream ROCm#1097 (fp8 asymmetric head dims / varlen / split-K) hoisted the fp8 route out of the dense `else:` branch, which de-nested the dualwave-vs-generic routing by one level. This branch had edited that same block at the old nesting depth, so the whole region conflicted. Resolved by keeping upstream's restructured control flow and re-applying the two things this branch adds inside it: - causal_lpt=causal_lpt is not False on the dense dualwave build - the XCD-swizzle rationale comment Verified: black + ruff clean; all 20 causal_lpt sites intact; numerics match SDPA on gfx950 (max abs err <= 0.011) across dense/causal/generic routes and for causal_lpt in {None, True, False}.
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Motivation
FlashAttention FP8: Added support for asymmetric head dimensions, variable sequence lengths (varlen), and KV splitting
Perf: MI355X, B=1, H=16, Hkv=1, causal, Skv = 2*Sq