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Add two-level accumulation for SM90 FP8 FWD to mitigate long-context accuracy drops #104

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Merged
merged 5 commits into from
Nov 5, 2025
Merged

Add two-level accumulation for SM90 FP8 FWD to mitigate long-context accuracy drops #104

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1339384
fix sign-off
jmkuebler Oct 29, 2025
2ba74fa
simplify to always add to fp32 accumulator
jmkuebler Oct 30, 2025
b9e937f
add compile-time check to disable two-level accumulation
jmkuebler Oct 30, 2025
49d4c83
make BF16 path cleaner
jmkuebler Oct 31, 2025
96fe992
move to within utils. functionally correct
jmkuebler Nov 5, 2025
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2 changes: 2 additions & 0 deletions hopper/setup.py
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Original file line number Diff line number Diff line change
Expand Up @@ -48,6 +48,7 @@
DISABLE_SPLIT = os.getenv("FLASH_ATTENTION_DISABLE_SPLIT", "FALSE") == "TRUE"
DISABLE_PAGEDKV = os.getenv("FLASH_ATTENTION_DISABLE_PAGEDKV", "FALSE") == "TRUE"
DISABLE_APPENDKV = os.getenv("FLASH_ATTENTION_DISABLE_APPENDKV", "FALSE") == "TRUE"
DISABLE_FP8_TWO_LEVEL_ACCUMULATION = os.getenv("FLASH_ATTENTION_DISABLE_FP8_TWO_LEVEL_ACCUMULATION", "FALSE") == "TRUE"
DISABLE_LOCAL = os.getenv("FLASH_ATTENTION_DISABLE_LOCAL", "FALSE") == "TRUE"
DISABLE_SOFTCAP = os.getenv("FLASH_ATTENTION_DISABLE_SOFTCAP", "FALSE") == "TRUE"
DISABLE_PACKGQA = os.getenv("FLASH_ATTENTION_DISABLE_PACKGQA", "FALSE") == "TRUE"
Expand Down Expand Up @@ -476,6 +477,7 @@ def nvcc_threads_args():
+ (["-DFLASHATTENTION_DISABLE_PAGEDKV"] if DISABLE_PAGEDKV else [])
+ (["-DFLASHATTENTION_DISABLE_SPLIT"] if DISABLE_SPLIT else [])
+ (["-DFLASHATTENTION_DISABLE_APPENDKV"] if DISABLE_APPENDKV else [])
+ (["-DFLASHATTENTION_DISABLE_FP8_TWO_LEVEL_ACCUMULATION"] if DISABLE_FP8_TWO_LEVEL_ACCUMULATION else [])
+ (["-DFLASHATTENTION_DISABLE_LOCAL"] if DISABLE_LOCAL else [])
+ (["-DFLASHATTENTION_DISABLE_SOFTCAP"] if DISABLE_SOFTCAP else [])
+ (["-DFLASHATTENTION_DISABLE_PACKGQA"] if DISABLE_PACKGQA else [])
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24 changes: 24 additions & 0 deletions hopper/utils.h
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Original file line number Diff line number Diff line change
Expand Up @@ -235,6 +235,21 @@ auto mma_partition_fragment_AB(Mma const& mma, Tensor0 const& tensor0) {
template <bool zero_init=false, int wg_wait=0, bool SwapAB=false, int M_slice=-1,
typename Tensor0, typename Tensor1, typename Tensor2, typename TiledMma>
CUTLASS_DEVICE void gemm(TiledMma& tiled_mma, Tensor0 const& tCrA, Tensor1 const& tCrB, Tensor2& tCrC) {
#ifndef FLASHATTENTION_DISABLE_FP8_TWO_LEVEL_ACCUMULATION
static constexpr bool Is_FP8 = cute::is_same_v<typename Tensor0::value_type, cutlass::float_e4m3_t>
|| cute::is_same_v<typename Tensor0::value_type, cutlass::float_e5m2_t>;
static constexpr bool Use_Two_Level = Is_FP8 && !zero_init;

auto tCrC_original = cute::make_fragment_like(tCrC);
if constexpr (Use_Two_Level) {
// Copy original values to backup
#pragma unroll
for (int i = 0; i < cute::size(tCrC); ++i) {
tCrC_original(i) = tCrC(i);
}
cute::clear(tCrC);
}
#endif
if constexpr (M_slice >= 0) {
static constexpr int MMA_M = decltype(size<1>(tCrC))::value;
static_assert(M_slice < MMA_M);
Expand Down Expand Up @@ -301,6 +316,15 @@ CUTLASS_DEVICE void gemm(TiledMma& tiled_mma, Tensor0 const& tCrA, Tensor1 const
}
}
}

#ifndef FLASHATTENTION_DISABLE_FP8_TWO_LEVEL_ACCUMULATION
if constexpr (Use_Two_Level) {
#pragma unroll
for (int i = 0; i < cute::size(tCrC); ++i) {
tCrC(i) = tCrC_original(i) + tCrC(i); // Add temp results to original
}
}
#endif
}

////////////////////////////////////////////////////////////////////////////////////////////////////
Expand Down