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Merged
merged 3 commits into from
Jun 28, 2025
Merged

Separate output and accumulator type for Flash Attention Prefill #443

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f64c7b7
Separate out type and accum type
muhammad-tanvir-1211 Jun 25, 2025
2a87b5f
Removed redundant wait from runners
muhammad-tanvir-1211 Jun 27, 2025
53c34ce
Merge branch 'sycl-develop' into flash_prefill_separate_out_type
aacostadiaz Jun 28, 2025
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17 changes: 12 additions & 5 deletions applications/flash_attention_v2/collective/xe_flash_attn_prefill_epilogue.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -53,15 +53,14 @@ template <class DispatchPolicy, class MMAOperation_, class TileShapeOutput_, cla
static_assert(cutlass::detail::dependent_false<DispatchPolicy>, "Could not find an epilogue specialization.");
};

template <class MMAOperation_, class TileShapeOutput_, class SubgroupLayout_, class ElementO_, class StrideO_, class ElementLSE_, class CopyOpO_>
class FlashPrefillEpilogue<epilogue::IntelXeXMX16, MMAOperation_, TileShapeOutput_, SubgroupLayout_, ElementO_, StrideO_, ElementLSE_, CopyOpO_> {
template <class MMAOperation_, class TileShapeOutput_, class SubgroupLayout_, class ElementCompute_, class ElementO_, class StrideO_, class ElementLSE_, class CopyOpO_>
class FlashPrefillEpilogue<epilogue::IntelXeXMX16, MMAOperation_, TileShapeOutput_, SubgroupLayout_, ElementCompute_, ElementO_, StrideO_, ElementLSE_, CopyOpO_> {
public:
//
// Type Aliases
//
using DispatchPolicy = epilogue::IntelXeXMX16;
using ElementO = ElementO_;
using ElementAccumulator = ElementO_;
using StrideO = StrideO_;
using ElementLSE = ElementLSE_;
using CopyOpO = CopyOpO_;
Expand All @@ -70,7 +69,8 @@ class FlashPrefillEpilogue<epilogue::IntelXeXMX16, MMAOperation_, TileShapeOutp
using TiledMmaOutput = typename TiledMMAHelper<MMA_Atom<MMAOperation_>, Layout<TileShapeOutput>, SubgroupLayout>::TiledMMA;
using GmemTiledCopyO = CopyOpO;
using ElementOutput = ElementO_;
using ElementCompute = ElementO_;
using ElementCompute = ElementCompute_;
using ElementAccumulator = ElementCompute_;
using SubgroupTileShape = decltype(cute::shape_div(TileShapeOutput{}, (SubgroupLayout{}.shape())));

static constexpr int SubgroupSize = DispatchPolicy::SubgroupSize;
Expand Down Expand Up @@ -196,7 +196,14 @@ class FlashPrefillEpilogue<epilogue::IntelXeXMX16, MMAOperation_, TileShapeOutp
auto thread_xe_store_o = params.xe_store_o.get_thread_slice(ThreadIdxX());
Tensor tOgO = thread_xe_store_o.partition_D(gO);

copy(params.xe_store_o, out_reg, tOgO);
Tensor final_out_reg = make_fragment_like<ElementOutput>(out_reg);
if constexpr (cute::is_any_of_v<ElementOutput, cute::float_e5m2_t, cute::float_e4m3_t> || cute::is_same_v<ElementOutput, ElementCompute>) {
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Could you add a comment here to explain this if condition?

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Added this comment, not sure why it doesn't show up here.

copy(out_reg, final_out_reg);
} else {
Tensor temp = convert_type<ElementOutput>(out_reg);
copy(temp, final_out_reg);
}
copy(params.xe_store_o, final_out_reg, tOgO);
}

// SequenceLengthShapeType = Shape<int, int>
Expand Down
2 changes: 1 addition & 1 deletion applications/flash_attention_v2/kernel/xe_flash_attn_prefill.hpp
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Expand Up @@ -370,7 +370,7 @@ class FMHAPrefill {
CUTLASS_PRAGMA_UNROLL
for (int row = 0; row < Vec; row++, row_idx++) { // 8
if (col_idx - full_tile_offset > row_idx - discard_seq_coord) {
tSr(row, m, n) = -INFINITY;
tSr(row, m, n) = ElementAccumulator{-INFINITY};
}
}
}
Expand Down
42 changes: 29 additions & 13 deletions benchmarks/flash_attention/flash_attention_prefill/benchmark_runner.hpp
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Expand Up @@ -189,18 +189,17 @@ template <class FMHAPrefillConfiguration> struct BenchmarkRunnerFMHA {
}
int kv_group_update=1;
for (int h = 0; h < num_heads_q; h++) {
cutlass::DeviceAllocation<ElementOutput> block_S;
cutlass::DeviceAllocation<ElementAccumulator> block_S;
block_S.reset(seq_len_qo * seq_len_kv);

cutlass::TensorRef ref_Q(block_Q[0].get() + offset_q, LayoutQ::packed({seq_len_qo, head_size_qk}));
cutlass::TensorRef ref_K(block_K[0].get() + offset_k, LayoutK::packed({head_size_qk, seq_len_kv}));
cutlass::TensorRef ref_V(block_V[0].get() + offset_v, LayoutV::packed({seq_len_kv, head_size_vo}));
cutlass::TensorRef ref_S(block_S.get(), LayoutQ::packed({seq_len_qo, seq_len_kv}));
cutlass::TensorRef ref_O(block_ref_O.get() + offset_o, LayoutO::packed({seq_len_qo, head_size_vo}));

cutlass::reference::device::GemmComplex({seq_len_qo, seq_len_kv, head_size_qk}, 1.f, ref_Q,
cutlass::reference::device::GemmComplex({seq_len_qo, seq_len_kv, head_size_qk}, ElementAccumulator{1.f}, ref_Q,
cutlass::ComplexTransform::kNone, ref_K, cutlass::ComplexTransform::kNone,
0.f, ref_S, ref_S, ElementAccumulator(0),
ElementAccumulator{0}, ref_S, ref_S, ElementAccumulator{0},
1, // batch_count
seq_len_qo * head_size_qk, // batch_stride_Q
seq_len_kv * head_size_qk, // batch_stride_K
Expand All @@ -210,8 +209,8 @@ template <class FMHAPrefillConfiguration> struct BenchmarkRunnerFMHA {

syclcompat::wait();

std::vector<ElementOutput> host_S(block_S.size());
syclcompat::memcpy<ElementOutput>(host_S.data(), block_S.get(), host_S.size());
std::vector<ElementAccumulator> host_S(block_S.size());
syclcompat::memcpy<ElementAccumulator>(host_S.data(), block_S.get(), host_S.size());
syclcompat::wait();
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Suggested change
syclcompat::wait();


// delete this memory as it is no longer needed
Expand All @@ -224,13 +223,13 @@ template <class FMHAPrefillConfiguration> struct BenchmarkRunnerFMHA {
for (int row = 0; row < seq_len_qo; row++) {
for (int col = 0; col < seq_len_kv; col++) {
if ((col - full_tile_offset) > (row - discard_seq_coord))
host_S[col + row * seq_len_kv] = -INFINITY;
host_S[col + row * seq_len_kv] = ElementAccumulator{-INFINITY};
}
}
}

// compute max element per row of S
std::vector<ElementOutput> max_vec(seq_len_qo, -INFINITY);
std::vector<ElementAccumulator> max_vec(seq_len_qo, ElementAccumulator{-INFINITY});
for (int row = 0; row < seq_len_qo; row++) {
int idx = row * seq_len_kv;
int max_idx = row;
Expand All @@ -246,12 +245,12 @@ template <class FMHAPrefillConfiguration> struct BenchmarkRunnerFMHA {
int idx = row * seq_len_kv;
int max_idx = row;
for (int col = 0; col < seq_len_kv; col++, idx++) {
host_S[idx] = expf((host_S[idx] - max_vec[max_idx]) / std::sqrt(static_cast<ElementOutput>((head_size_qk))));
host_S[idx] = expf((host_S[idx] - max_vec[max_idx]) / std::sqrt(static_cast<ElementAccumulator>((head_size_qk))));
}
}

// compute sum per row of S
std::vector<ElementOutput> sum_vec(seq_len_qo, ElementOutput{0});
std::vector<ElementAccumulator> sum_vec(seq_len_qo, ElementAccumulator{0});
for (int row = 0; row < seq_len_qo; row++) {
int idx = row * seq_len_kv;
int sum_idx = row;
Expand Down Expand Up @@ -283,9 +282,13 @@ template <class FMHAPrefillConfiguration> struct BenchmarkRunnerFMHA {

cutlass::TensorRef ref_P(block_P.get(), LayoutQ::packed({seq_len_qo, seq_len_kv}));

cutlass::reference::device::GemmComplex({seq_len_qo, head_size_vo, seq_len_kv}, 1.f, ref_P,
cutlass::DeviceAllocation<ElementAccumulator> block_acc;
block_acc.reset(seq_len_qo * head_size_vo);
cutlass::TensorRef ref_acc(block_acc.get(), LayoutO::packed({seq_len_qo, head_size_vo}));

cutlass::reference::device::GemmComplex({seq_len_qo, head_size_vo, seq_len_kv}, ElementAccumulator{1}, ref_P,
cutlass::ComplexTransform::kNone, ref_V, cutlass::ComplexTransform::kNone,
0.f, ref_O, ref_O, ElementAccumulator(0),
ElementAccumulator{0}, ref_acc, ref_acc, ElementAccumulator{0},
1, // batch_count
seq_len_qo * seq_len_kv, // batch_stride_P
seq_len_kv * head_size_vo, // batch_stride_V
Expand All @@ -297,6 +300,19 @@ template <class FMHAPrefillConfiguration> struct BenchmarkRunnerFMHA {
// delete this memory as it is no longer needed
block_P.reset();

std::vector<ElementAccumulator> vec_acc(block_acc.size());
syclcompat::memcpy<ElementAccumulator>(vec_acc.data(), block_acc.get(), vec_acc.size());
syclcompat::wait();
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Suggested change
syclcompat::wait();


// delete this memory as it is no longer needed
block_acc.reset();
std::vector<ElementOutput> vec_out(vec_acc.size());
for(int i = 0; i < vec_out.size(); i++) {
vec_out[i] = static_cast<ElementOutput>(vec_acc[i]);
}
syclcompat::memcpy<ElementOutput>(block_ref_O.get() + offset_o, vec_out.data(), vec_out.size());
syclcompat::wait();
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Suggested change
syclcompat::wait();


offset_q += seq_len_qo * head_size_qk;
if(kv_group_update % q_group_size==0) {
offset_k += seq_len_kv * head_size_qk;
Expand All @@ -311,7 +327,7 @@ template <class FMHAPrefillConfiguration> struct BenchmarkRunnerFMHA {

// Check if output from CUTLASS kernel and reference kernel are equal or not
bool passed = cutlass::reference::device::BlockCompareRelativelyEqual(block_ref_O.get(), block_O.get(),
block_O.size(), 0.5f, 0.5f);
block_O.size(), ElementOutput{0.5}, ElementOutput{0.5});

return passed;
}
Expand Down
2 changes: 1 addition & 1 deletion benchmarks/flash_attention/flash_attention_prefill/fmha_prefill_configuration.hpp
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Expand Up @@ -67,7 +67,7 @@ struct FMHAPrefillConfig {
using MMAOperation = typename MMAOP<GEMMDispatchPolicy, ElementInputType,ElementAccumulator>::Type;
using CollectiveEpilogue = cutlass::flash_attention::collective::FlashPrefillEpilogue<
EpilogueDispatchPolicy, MMAOperation, TileShapeOutput,
SubgroupLayout, ElementAccumulator,
SubgroupLayout, ElementAccumulator, ElementOutputType,
cutlass::gemm::TagToStrideC_t<LayoutO>, ElementOutput,
GmemTiledCopyO>;

Expand Down
40 changes: 28 additions & 12 deletions examples/sycl/06_bmg_flash_attention/bmg_flash_attn_prefill_runner.hpp
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Expand Up @@ -230,14 +230,13 @@ template <class FMHAPrefillKernel, bool isVarLen> struct ExampleRunner {
}
int kv_group_update=1;
for (int h = 0; h < num_heads_q; h++) {
cutlass::DeviceAllocation<ElementOutput> block_S;
cutlass::DeviceAllocation<ElementAccumulator> block_S;
block_S.reset(seq_len_qo * seq_len_kv);

cutlass::TensorRef ref_Q(block_Q_.get() + offset_q, LayoutQ::packed({seq_len_qo, head_size_qk}));
cutlass::TensorRef ref_K(block_K_.get() + offset_k, LayoutK::packed({head_size_qk, seq_len_kv}));
cutlass::TensorRef ref_V(block_V_.get() + offset_v, LayoutV::packed({seq_len_kv, head_size_vo}));
cutlass::TensorRef ref_S(block_S.get(), LayoutQ::packed({seq_len_qo, seq_len_kv}));
cutlass::TensorRef ref_O(block_ref_O.get() + offset_o, LayoutO::packed({seq_len_qo, head_size_vo}));

cutlass::reference::device::GemmComplex({seq_len_qo, seq_len_kv, head_size_qk}, 1.f, ref_Q,
cutlass::ComplexTransform::kNone, ref_K, cutlass::ComplexTransform::kNone,
Expand All @@ -251,8 +250,8 @@ template <class FMHAPrefillKernel, bool isVarLen> struct ExampleRunner {

syclcompat::wait();

std::vector<ElementOutput> host_S(block_S.size());
syclcompat::memcpy<ElementOutput>(host_S.data(), block_S.get(), host_S.size());
std::vector<ElementAccumulator> host_S(block_S.size());
syclcompat::memcpy<ElementAccumulator>(host_S.data(), block_S.get(), host_S.size());
syclcompat::wait();

// delete this memory as it is no longer needed
Expand All @@ -265,13 +264,13 @@ template <class FMHAPrefillKernel, bool isVarLen> struct ExampleRunner {
for (int row = 0; row < seq_len_qo; row++) {
for (int col = 0; col < seq_len_kv; col++) {
if ((col - full_tile_offset) > (row - discard_seq_coord))
host_S[col + row * seq_len_kv] = -INFINITY;
host_S[col + row * seq_len_kv] = ElementAccumulator{-INFINITY};
}
}
}

// compute max element per row of S
std::vector<ElementOutput> max_vec(seq_len_qo, -INFINITY);
std::vector<ElementAccumulator> max_vec(seq_len_qo, ElementAccumulator{-INFINITY});
for (int row = 0; row < seq_len_qo; row++) {
int idx = row * seq_len_kv;
int max_idx = row;
Expand All @@ -287,12 +286,12 @@ template <class FMHAPrefillKernel, bool isVarLen> struct ExampleRunner {
int idx = row * seq_len_kv;
int max_idx = row;
for (int col = 0; col < seq_len_kv; col++, idx++) {
host_S[idx] = expf((host_S[idx] - max_vec[max_idx]) / sqrt(static_cast<ElementOutput>((head_size_qk))));
host_S[idx] = expf((host_S[idx] - max_vec[max_idx]) / sqrt(static_cast<ElementAccumulator>((head_size_qk))));
}
}

// compute sum per row of S
std::vector<ElementOutput> sum_vec(seq_len_qo, ElementOutput{0});
std::vector<ElementAccumulator> sum_vec(seq_len_qo, ElementAccumulator{0});
for (int row = 0; row < seq_len_qo; row++) {
int idx = row * seq_len_kv;
int sum_idx = row;
Expand Down Expand Up @@ -324,9 +323,13 @@ template <class FMHAPrefillKernel, bool isVarLen> struct ExampleRunner {

cutlass::TensorRef ref_P(block_P.get(), LayoutQ::packed({seq_len_qo, seq_len_kv}));

cutlass::reference::device::GemmComplex({seq_len_qo, head_size_vo, seq_len_kv}, 1.f, ref_P,
cutlass::DeviceAllocation<ElementAccumulator> block_acc;
block_acc.reset(seq_len_qo * head_size_vo);
cutlass::TensorRef ref_acc(block_acc.get(), LayoutO::packed({seq_len_qo, head_size_vo}));

cutlass::reference::device::GemmComplex({seq_len_qo, head_size_vo, seq_len_kv}, ElementAccumulator{1}, ref_P,
cutlass::ComplexTransform::kNone, ref_V, cutlass::ComplexTransform::kNone,
0.f, ref_O, ref_O, ElementAccumulator(0),
ElementAccumulator{0}, ref_acc, ref_acc, ElementAccumulator{0},
1, // batch_count
seq_len_qo * seq_len_kv, // batch_stride_P
seq_len_kv * head_size_vo, // batch_stride_V
Expand All @@ -338,6 +341,19 @@ template <class FMHAPrefillKernel, bool isVarLen> struct ExampleRunner {
// delete this memory as it is no longer needed
block_P.reset();

std::vector<ElementAccumulator> vec_acc(block_acc.size());
syclcompat::memcpy<ElementAccumulator>(vec_acc.data(), block_acc.get(), vec_acc.size());
syclcompat::wait();
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Suggested change
syclcompat::wait();


// delete this memory as it is no longer needed
block_acc.reset();
std::vector<ElementOutput> vec_out(vec_acc.size());
for(int i = 0; i < vec_out.size(); i++) {
vec_out[i] = static_cast<ElementOutput>(vec_acc[i]);
}
syclcompat::memcpy<ElementOutput>(block_ref_O.get() + offset_o, vec_out.data(), vec_out.size());
syclcompat::wait();
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Suggested change
syclcompat::wait();


offset_q += seq_len_qo * head_size_qk;
if(kv_group_update % q_group_size==0) {
offset_k += seq_len_kv * head_size_qk;
Expand All @@ -352,7 +368,7 @@ template <class FMHAPrefillKernel, bool isVarLen> struct ExampleRunner {

// Check if output from CUTLASS kernel and reference kernel are equal or not
bool passed = cutlass::reference::device::BlockCompareRelativelyEqual(block_ref_O.get(), block_O.get(),
block_O.size(), 0.5f, 0.5f);
block_O.size(), ElementOutput{0.5}, ElementOutput{0.5});

return passed;
}
Expand Down Expand Up @@ -619,7 +635,7 @@ template <bool Causal,
using EpilogueDispatchPolicy = cutlass::epilogue::IntelXeXMX16;
using GEMMDispatchPolicy = cutlass::gemm::MainloopIntelXeXMX16<PipelineStages>;
using CollectiveEpilogue = cutlass::flash_attention::collective::FlashPrefillEpilogue<
EpilogueDispatchPolicy, MMAOperation, TileShapeOutput, SubgroupLayout, ElementAccumulator, cutlass::gemm::TagToStrideC_t<LayoutO>, ElementOutput,
EpilogueDispatchPolicy, MMAOperation, TileShapeOutput, SubgroupLayout, ElementComputeEpilogue, ElementOutput, cutlass::gemm::TagToStrideC_t<LayoutO>, ElementOutput,
GmemTiledCopyStore>;
using CollectiveSoftmaxEpilogue = cutlass::flash_attention::collective::FlashPrefillSoftmaxEpilogue<Causal, EpilogueDispatchPolicy, ElementAccumulator>;

Expand Down
1 change: 1 addition & 0 deletions include/cute/arch/copy_xe_U8.hpp
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Expand Up @@ -682,6 +682,7 @@ struct XE_2D_U8x4x16_ST_N {
};

struct XE_2D_U8x8x16_ST_N {
using BlockShape = Shape<_8, _16>;
template <class T>
CUTE_HOST_DEVICE static void copy(void *baseoffset, int width, int height,
int pitch, intel::coord_t coord,
Expand Down
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