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[cpu][performance] CPU Paged Attention NEON BFMMLA BF16 Implementation #32263

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Merged
bigPYJ1151 merged 2 commits into from
Feb 6, 2026
Merged

[cpu][performance] CPU Paged Attention NEON BFMMLA BF16 Implementation #32263

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eb17631
[cpu][performance] CPU Paged Attention NEON BFMMLA BF16 Implementation
gassan-arm Jan 4, 2026
af07337
Merge branch 'main' into neon-bfmmla-04-01-upstremable
bigPYJ1151 Feb 6, 2026
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3 changes: 2 additions & 1 deletion csrc/cpu/cpu_attn_impl.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1107,7 +1107,8 @@ class AttentionMainLoop {
if (sliding_window_left != -1) {
pos = std::max(pos, curr_token_pos - sliding_window_left);
}
return pos;
// Clamp to tile end to avoid OOB when window starts past the tile
return std::min(pos, kv_tile_end_pos);
}();

int32_t right_kv_pos = [&]() {
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19 changes: 17 additions & 2 deletions csrc/cpu/cpu_attn_neon.hpp
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Expand Up @@ -4,6 +4,9 @@
#include "cpu_attn_impl.hpp"
#include <arm_neon.h>
#include <type_traits>
#ifdef ARM_BF16_SUPPORT
#include "cpu_attn_neon_bfmmla.hpp"
#endif
namespace cpu_attention {

namespace {
Expand Down Expand Up @@ -57,7 +60,7 @@ FORCE_INLINE void load_row8_B_as_f32<c10::BFloat16>(const c10::BFloat16* p,
#endif
}

// Mx8, with 1 <= M <= 8 , K streamed, unroll-by-4 with NEON FMLAs
// Mx8, with 1 <= M <= 8 , K streamed, unroll-by-4 with ASIMD FMLAs
// #Loads = (K // 4) * (M + 4 * sizeof(kv_cache_t) / 2)
// #FMLAs = (K // 4) * (4 * 2 * M)
// We have (4 * 2 * M) FMLAs for (M + 4 * sizeof(kv_cache_t) / 2) loads
Expand Down Expand Up @@ -381,6 +384,18 @@ class AttentionImpl<ISA::NEON, scalar_t, head_dim> {
}
}
};

#ifdef ARM_BF16_SUPPORT
// For BF16 on Arm, reuse the BFMMLA kernels with 32-token alignment.
template <int64_t head_dim>
class AttentionImpl<ISA::NEON, c10::BFloat16, head_dim>
: public AttentionImplNEONBFMMLA<BLOCK_SIZE_ALIGNMENT, ISA::NEON,
head_dim> {};
#endif
} // namespace cpu_attention

#endif // #ifndef CPU_ATTN_NEON_HPP
#undef BLOCK_SIZE_ALIGNMENT
#undef HEAD_SIZE_ALIGNMENT
#undef MAX_Q_HEAD_NUM_PER_ITER

#endif // #ifndef CPU_ATTN_ASIMD_HPP
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