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Support H-max to 256 in JAX/SPDA for Blackwell. #33504

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Support H-max to 256 in JAX/SPDA for Blackwell. #33504

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Support H-max to 256 in JAX/SPDA for Blackwell.
mingxu1067 Nov 24, 2025
3feb474
Apply check_compute_capability
mingxu1067 Nov 24, 2025
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3 changes: 2 additions & 1 deletion jax/_src/cudnn/fused_attention_stablehlo.py
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Expand Up @@ -380,7 +380,8 @@ def check_is_flash_attention(
# bf16/fp16 attention conditions
# Check the head dim.
is_on_hopper = is_cuda_compute_capability_equal("9.0")
H_max = 256 if is_on_hopper else 128
is_on_blackwell = is_cuda_compute_capability_equal("10.0")
H_max = 256 if (is_on_hopper or is_on_blackwell) else 128
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@gemini-code-assist gemini-code-assist bot Nov 24, 2025

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medium

To make this check more future-proof for upcoming GPU architectures, it would be better to check for a minimum compute capability rather than listing specific architectures. Assuming all architectures from Hopper (9.0) onwards will support H_max = 256, you can simplify this logic using check_compute_capability. This also aligns with how other capability checks are performed in this file (e.g., for is_packed and is_mla).

Suggested change
is_on_hopper = is_cuda_compute_capability_equal("9.0")
H_max = 256 if is_on_hopper else 128
is_on_blackwell = is_cuda_compute_capability_equal("10.0")
H_max = 256 if (is_on_hopper or is_on_blackwell) else 128
is_hopper_or_later = check_compute_capability("9.0")
H_max = 256 if is_hopper_or_later else 128

# check if multi-head latent attention is needed
is_mla = qH != vH
if not (qH <= H_max and qH % 8 == 0):
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