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feat: _flash_attn_forward add out args #2648

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Merged
valarLip merged 2 commits into from
Apr 14, 2026
Merged

feat: _flash_attn_forward add out args #2648

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1a112eb
feat: _flash_attn_forward add out args
RuibinCheung Apr 2, 2026
1a3c7c6
Merge branch 'main' into feat/fa_fwd_add_out_args
RuibinCheung Apr 12, 2026
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11 changes: 6 additions & 5 deletions aiter/ops/mha.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1263,6 +1263,7 @@ def _flash_attn_forward(
cu_seqlens_q: Optional[torch.Tensor] = None,
cu_seqlens_kv: Optional[torch.Tensor] = None,
sink_ptr: Optional[Tensor] = None,
out: Optional[torch.Tensor] = None,
) -> Tuple[torch.Tensor, torch.Tensor, torch.Tensor, torch.Tensor]:
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batch_size, seqlen_q, nhead_q, hdim_q = q.shape
Expand Down Expand Up @@ -1323,7 +1324,7 @@ def _validate_cu(name: str, x: Optional[torch.Tensor]):
_validate_cu("cu_seqlens_kv", cu_seqlens_kv)

if can_impl_fmha_v3_fwd() and seqlen_q > 128: # Prefer CK for decode cases
out, softmax_lse, S_dmask, rng_state = fmha_v3_fwd(
out_, softmax_lse, S_dmask, rng_state = fmha_v3_fwd(
q,
k,
v,
Expand All @@ -1335,7 +1336,7 @@ def _validate_cu(name: str, x: Optional[torch.Tensor]):
return_lse,
return_softmax,
how_v3_bf16_cvt,
None,
out,
bias,
alibi_slopes,
q_descale,
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Expand All @@ -1344,7 +1345,7 @@ def _validate_cu(name: str, x: Optional[torch.Tensor]):
None,
)
else:
out, softmax_lse, S_dmask, rng_state = mha_fwd(
out_, softmax_lse, S_dmask, rng_state = mha_fwd(
q,
k,
v,
Expand All @@ -1358,7 +1359,7 @@ def _validate_cu(name: str, x: Optional[torch.Tensor]):
return_softmax,
cu_seqlens_q,
cu_seqlens_kv,
None,
out,
bias,
alibi_slopes,
q_descale,
Expand All @@ -1368,7 +1369,7 @@ def _validate_cu(name: str, x: Optional[torch.Tensor]):
None,
# custom_build_args={"md_name": md_name, "blob_gen_cmd": blob_gen_cmd},
)
return out, softmax_lse, S_dmask, rng_state
return out_, softmax_lse, S_dmask, rng_state


# @torch_compile_guard(mutates_args=[])
Expand Down
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