189 lines
9.1 KiB
C++
189 lines
9.1 KiB
C++
/******************************************************************************
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* Copyright (c) 2024, Tri Dao.
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******************************************************************************/
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#pragma once
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#include <cmath>
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#include <cute/tensor.hpp>
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#include <cutlass/numeric_types.h>
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#include "philox.cuh"
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#include "utils.h"
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namespace flash {
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using namespace cute;
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////////////////////////////////////////////////////////////////////////////////////////////////////
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template<bool zero_init=true, typename Engine0, typename Layout0, typename Engine1, typename Layout1, typename Operator>
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__device__ __forceinline__ void thread_reduce_(Tensor<Engine0, Layout0> const &tensor, Tensor<Engine1, Layout1> &summary, Operator &op) {
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static_assert(Layout0::rank == 2, "Only support 2D Tensor");
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static_assert(Layout1::rank == 1, "Only support 1D Tensor");
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CUTE_STATIC_ASSERT_V(size<0>(summary) == size<0>(tensor));
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#pragma unroll
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for (int mi = 0; mi < size<0>(tensor); mi++) {
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summary(mi) = zero_init ? tensor(mi, 0) : op(summary(mi), tensor(mi, 0));
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#pragma unroll
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for (int ni = 1; ni < size<1>(tensor); ni++) {
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summary(mi) = op(summary(mi), tensor(mi, ni));
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}
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}
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}
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template<typename Engine0, typename Layout0, typename Engine1, typename Layout1, typename Operator>
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__device__ __forceinline__ void quad_allreduce_(Tensor<Engine0, Layout0> &dst, Tensor<Engine1, Layout1> &src, Operator &op) {
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CUTE_STATIC_ASSERT_V(size(dst) == size(src));
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#pragma unroll
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for (int i = 0; i < size(dst); i++){
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dst(i) = Allreduce<4>::run(src(i), op);
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}
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}
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template<bool zero_init=true, typename Engine0, typename Layout0, typename Engine1, typename Layout1, typename Operator>
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__device__ __forceinline__ void reduce_(Tensor<Engine0, Layout0> const& tensor, Tensor<Engine1, Layout1> &summary, Operator &op) {
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thread_reduce_<zero_init>(tensor, summary, op);
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quad_allreduce_(summary, summary, op);
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}
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template<bool zero_init=true, typename Engine0, typename Layout0, typename Engine1, typename Layout1>
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__device__ __forceinline__ void reduce_max(Tensor<Engine0, Layout0> const& tensor, Tensor<Engine1, Layout1> &max){
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MaxOp<float> max_op;
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reduce_<zero_init>(tensor, max, max_op);
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}
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template<bool zero_init=true, typename Engine0, typename Layout0, typename Engine1, typename Layout1>
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__device__ __forceinline__ void reduce_sum(Tensor<Engine0, Layout0> const& tensor, Tensor<Engine1, Layout1> &sum){
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SumOp<float> sum_op;
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thread_reduce_<zero_init>(tensor, sum, sum_op);
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}
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// Apply the exp to all the elements.
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template <bool Scale_max=true, typename Engine0, typename Layout0, typename Engine1, typename Layout1>
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__forceinline__ __device__ void scale_apply_exp2(Tensor<Engine0, Layout0> &tensor, Tensor<Engine1, Layout1> const &max, const float scale) {
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static_assert(Layout0::rank == 2, "Only support 2D Tensor");
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static_assert(Layout1::rank == 1, "Only support 1D Tensor");
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CUTE_STATIC_ASSERT_V(size<0>(max) == size<0>(tensor));
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#pragma unroll
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for (int mi = 0; mi < size<0>(tensor); ++mi) {
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// If max is -inf, then all elements must have been -inf (possibly due to masking).
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// We don't want (-inf - (-inf)) since that would give NaN.
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// If we don't have float around M_LOG2E the multiplication is done in fp64.
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const float max_scaled = max(mi) == -INFINITY ? 0.f : max(mi) * (Scale_max ? scale : float(M_LOG2E));
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#pragma unroll
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for (int ni = 0; ni < size<1>(tensor); ++ni) {
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// Instead of computing exp(x - max), we compute exp2(x * log_2(e) -
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// max * log_2(e)) This allows the compiler to use the ffma
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// instruction instead of fadd and fmul separately.
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// The following macro will disable the use of fma.
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// See: https://github.com/pytorch/pytorch/issues/121558 for more details
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// This macro is set in PyTorch and not FlashAttention
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#ifdef UNFUSE_FMA
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tensor(mi, ni) = exp2f(__fmul_rn(tensor(mi, ni), scale) - max_scaled);
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#else
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tensor(mi, ni) = exp2f(tensor(mi, ni) * scale - max_scaled);
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#endif
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}
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}
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}
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// Apply the exp to all the elements.
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template <bool zero_init=true, typename Engine0, typename Layout0, typename Engine1, typename Layout1>
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__forceinline__ __device__ void max_scale_exp2_sum(Tensor<Engine0, Layout0> &tensor, Tensor<Engine1, Layout1> &max, Tensor<Engine1, Layout1> &sum, const float scale) {
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static_assert(Layout0::rank == 2, "Only support 2D Tensor");
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static_assert(Layout1::rank == 1, "Only support 1D Tensor");
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CUTE_STATIC_ASSERT_V(size<0>(max) == size<0>(tensor));
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#pragma unroll
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for (int mi = 0; mi < size<0>(tensor); ++mi) {
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MaxOp<float> max_op;
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max(mi) = zero_init ? tensor(mi, 0) : max_op(max(mi), tensor(mi, 0));
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#pragma unroll
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for (int ni = 1; ni < size<1>(tensor); ni++) {
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max(mi) = max_op(max(mi), tensor(mi, ni));
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}
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max(mi) = Allreduce<4>::run(max(mi), max_op);
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// If max is -inf, then all elements must have been -inf (possibly due to masking).
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// We don't want (-inf - (-inf)) since that would give NaN.
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const float max_scaled = max(mi) == -INFINITY ? 0.f : max(mi) * scale;
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sum(mi) = 0;
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#pragma unroll
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for (int ni = 0; ni < size<1>(tensor); ++ni) {
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// Instead of computing exp(x - max), we compute exp2(x * log_2(e) -
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// max * log_2(e)) This allows the compiler to use the ffma
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// instruction instead of fadd and fmul separately.
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tensor(mi, ni) = exp2f(tensor(mi, ni) * scale - max_scaled);
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sum(mi) += tensor(mi, ni);
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}
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SumOp<float> sum_op;
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sum(mi) = Allreduce<4>::run(sum(mi), sum_op);
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}
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}
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////////////////////////////////////////////////////////////////////////////////////////////////////
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template <int kNRows>
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struct Softmax {
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using TensorT = decltype(make_tensor<float>(Shape<Int<kNRows>>{}));
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TensorT row_max, row_sum;
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__forceinline__ __device__ Softmax() {};
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template<bool Is_first, bool Check_inf=false, typename Tensor0, typename Tensor1>
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__forceinline__ __device__ void softmax_rescale_o(Tensor0 &acc_s, Tensor1 &acc_o, float softmax_scale_log2) {
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// Reshape acc_s from (MMA=4, MMA_M, MMA_N) to (nrow=(2, MMA_M), ncol=(2, MMA_N))
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Tensor scores = make_tensor(acc_s.data(), flash::convert_layout_acc_rowcol(acc_s.layout()));
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static_assert(decltype(size<0>(scores))::value == kNRows);
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if (Is_first) {
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flash::template reduce_max</*zero_init=*/true>(scores, row_max);
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flash::scale_apply_exp2(scores, row_max, softmax_scale_log2);
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flash::reduce_sum</*zero_init=*/true>(scores, row_sum);
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} else {
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Tensor scores_max_prev = make_fragment_like(row_max);
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cute::copy(row_max, scores_max_prev);
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flash::template reduce_max</*zero_init=*/false>(scores, row_max);
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// Reshape acc_o from (MMA=4, MMA_M, MMA_K) to (nrow=(2, MMA_M), ncol=(2, MMA_K))
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Tensor acc_o_rowcol = make_tensor(acc_o.data(), flash::convert_layout_acc_rowcol(acc_o.layout()));
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static_assert(decltype(size<0>(acc_o_rowcol))::value == kNRows);
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#pragma unroll
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for (int mi = 0; mi < size(row_max); ++mi) {
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float scores_max_cur = !Check_inf
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? row_max(mi)
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: (row_max(mi) == -INFINITY ? 0.0f : row_max(mi));
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float scores_scale = exp2f((scores_max_prev(mi) - scores_max_cur) * softmax_scale_log2);
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row_sum(mi) *= scores_scale;
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#pragma unroll
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for (int ni = 0; ni < size<1>(acc_o_rowcol); ++ni) { acc_o_rowcol(mi, ni) *= scores_scale; }
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}
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flash::scale_apply_exp2(scores, row_max, softmax_scale_log2);
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// We don't do the reduce across threads here since we don't need to use the row_sum.
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// We do that reduce at the end when we need to normalize the softmax.
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flash::reduce_sum</*zero_init=*/false>(scores, row_sum);
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}
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};
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template<bool Is_dropout=false, bool Split=false, typename Tensor0>
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__forceinline__ __device__ TensorT normalize_softmax_lse(Tensor0 &acc_o, float softmax_scale, float rp_dropout=1.0) {
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SumOp<float> sum_op;
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quad_allreduce_(row_sum, row_sum, sum_op);
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TensorT lse = make_fragment_like(row_sum);
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Tensor acc_o_rowcol = make_tensor(acc_o.data(), flash::convert_layout_acc_rowcol(acc_o.layout()));
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static_assert(decltype(size<0>(acc_o_rowcol))::value == kNRows);
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#pragma unroll
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for (int mi = 0; mi < size<0>(acc_o_rowcol); ++mi) {
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float sum = row_sum(mi);
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float inv_sum = (sum == 0.f || sum != sum) ? 1.f : 1.f / sum;
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lse(mi) = (sum == 0.f || sum != sum) ? (Split ? -INFINITY : INFINITY) : row_max(mi) * softmax_scale + __logf(sum);
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float scale = !Is_dropout ? inv_sum : inv_sum * rp_dropout;
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#pragma unroll
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for (int ni = 0; ni < size<1>(acc_o_rowcol); ++ni) { acc_o_rowcol(mi, ni) *= scale; }
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}
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return lse;
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};
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};
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} // namespace flash
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