Fix Variable Sequence Length Support for Flash Attention Decode #362
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t4c1
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May 8, 2025
examples/sycl/06_pvc_flash_attention/pvc_flash_decode_runner.hpp
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test/unit/flash_attention/flash_attention_decode/flash_decode_testbed_3x.hpp
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applications/flash_attention_v2/collective/xe_flash_attn_decode_epilogue.hpp
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muhammad-tanvir-1211
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mehdi-goli
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May 9, 2025
| if constexpr (!is_var_len) { | ||
| return params; | ||
| } else { | ||
| auto [batch, num_heads_q, num_heads_kv, seq_len_qo, seq_len_kv, seq_len_kv_cache, head_size_qk, head_size_vo] = problem_shape; | ||
| auto [batch, num_heads_q, num_heads_kv, seq_len_qo, seq_len_kv, seq_len_kv_cache, head_size_qk, head_size_vo] = logical_problem_shape; |
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| auto [batch, num_heads_q, num_heads_kv, seq_len_qo, seq_len_kv, seq_len_kv_cache, head_size_qk, head_size_vo] = logical_problem_shape; | |
| auto [num_heads_q, num_heads_kv, seq_len_qo, seq_len_kv, seq_len_kv_cache, head_size_qk, head_size_vo] = select<1, 2, 3, 4, 5, 6, 7>logical_problem_shape; |
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The logical problem shape and problem shape holding a lot of duplicate inputs unnecessarily which takes up register space. The logical problem shape only need to hold shape<int, int, int> for seq_len_qo, seq_len_kv, seq_len_kv_cache the remaining is already provided in the problem shape
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This PR fixes the variable sequence length support for Flash Attention Decode. It also fixes the causal masking on device code and matches the verification with the one for prefill along with the flops and gbps calculation.