TensorRT-LLMs/cpp/tensorrt_llm/kernels/flashMLA/softmax.h

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 * SOFTWARE.
 *
 * Copyright (c) 2022-2024, NVIDIA CORPORATION.  All rights reserved.
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 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
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 * reference: https://github.com/deepseek-ai/FlashMLA
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// Adapted from https://github.com/Dao-AILab/flash-attention/blob/main/csrc/flash_attn/src/softmax.h

#pragma once

#include <cmath>

#include <cute/tensor.hpp>
#include <cutlass/numeric_types.h>

#include "utils.h"

namespace flash
{

using namespace cute;

////////////////////////////////////////////////////////////////////////////////////////////////////

template <bool zero_init = true, typename Engine0, typename Layout0, typename Engine1, typename Layout1,
    typename Operator>
__device__ __forceinline__ void thread_reduce_(
    Tensor<Engine0, Layout0> const& tensor, Tensor<Engine1, Layout1>& summary, Operator& op)
{
    static_assert(Layout0::rank == 2, "Only support 2D Tensor");
    static_assert(Layout1::rank == 1, "Only support 1D Tensor");
    CUTE_STATIC_ASSERT_V(size<0>(summary) == size<0>(tensor));
#pragma unroll
    for (int mi = 0; mi < size<0>(tensor); mi++)
    {
        summary(mi) = zero_init ? tensor(mi, 0) : op(summary(mi), tensor(mi, 0));
#pragma unroll
        for (int ni = 1; ni < size<1>(tensor); ni++)
        {
            summary(mi) = op(summary(mi), tensor(mi, ni));
        }
    }
}

template <typename Engine0, typename Layout0, typename Engine1, typename Layout1, typename Operator>
__device__ __forceinline__ void quad_allreduce_(
    Tensor<Engine0, Layout0>& dst, Tensor<Engine1, Layout1>& src, Operator& op)
{
    CUTE_STATIC_ASSERT_V(size(dst) == size(src));
#pragma unroll
    for (int i = 0; i < size(dst); i++)
    {
        dst(i) = Allreduce<4>::run(src(i), op);
    }
}

template <bool zero_init = true, typename Engine0, typename Layout0, typename Engine1, typename Layout1,
    typename Operator>
__device__ __forceinline__ void reduce_(
    Tensor<Engine0, Layout0> const& tensor, Tensor<Engine1, Layout1>& summary, Operator& op)
{
    thread_reduce_<zero_init>(tensor, summary, op);
    quad_allreduce_(summary, summary, op);
}

template <bool zero_init = true, typename Engine0, typename Layout0, typename Engine1, typename Layout1>
__device__ __forceinline__ void reduce_max(Tensor<Engine0, Layout0> const& tensor, Tensor<Engine1, Layout1>& max)
{
    MaxOp<float> max_op;
    reduce_<zero_init>(tensor, max, max_op);
}

template <bool zero_init = true, typename Engine0, typename Layout0, typename Engine1, typename Layout1>
__device__ __forceinline__ void reduce_sum(Tensor<Engine0, Layout0> const& tensor, Tensor<Engine1, Layout1>& sum)
{
    SumOp<float> sum_op;
    thread_reduce_<zero_init>(tensor, sum, sum_op);
}

// Apply the exp to all the elements.
template <bool Scale_max = true, typename Engine0, typename Layout0, typename Engine1, typename Layout1>
__forceinline__ __device__ auto scale_apply_exp2(
    Tensor<Engine0, Layout0>& tensor, Tensor<Engine1, Layout1> const& max, float const scale)
{
    static_assert(Layout0::rank == 2, "Only support 2D Tensor");
    static_assert(Layout1::rank == 1, "Only support 1D Tensor");
    CUTE_STATIC_ASSERT_V(size<0>(max) == size<0>(tensor));
#pragma unroll
    for (int mi = 0; mi < size<0>(tensor); ++mi)
    {
        // If max is -inf, then all elements must have been -inf (possibly due to masking).
        // We don't want (-inf - (-inf)) since that would give NaN.
        // If we don't have float around M_LOG2E the multiplication is done in fp64.
        float const max_scaled = max(mi) == -INFINITY ? 0.f : max(mi) * (Scale_max ? scale : float(M_LOG2E));
#pragma unroll
        for (int ni = 0; ni < size<1>(tensor); ++ni)
        {
// Instead of computing exp(x - max), we compute exp2(x * log_2(e) -
// max * log_2(e)) This allows the compiler to use the ffma
// instruction instead of fadd and fmul separately.
// The following macro will disable the use of fma.
// See: https://github.com/pytorch/pytorch/issues/121558 for more details
// This macro is set in PyTorch and not FlashAttention
#ifdef UNFUSE_FMA
            tensor(mi, ni) = exp2f(__fmul_rn(tensor(mi, ni), scale) - max_scaled);
#else
            tensor(mi, ni) = exp2f(tensor(mi, ni) * scale - max_scaled);
#endif
        }
    }
    return tensor;
}

// Apply the exp to all the elements.
template <bool zero_init = true, typename Engine0, typename Layout0, typename Engine1, typename Layout1>
__forceinline__ __device__ void max_scale_exp2_sum(
    Tensor<Engine0, Layout0>& tensor, Tensor<Engine1, Layout1>& max, Tensor<Engine1, Layout1>& sum, float const scale)
{
    static_assert(Layout0::rank == 2, "Only support 2D Tensor");
    static_assert(Layout1::rank == 1, "Only support 1D Tensor");
    CUTE_STATIC_ASSERT_V(size<0>(max) == size<0>(tensor));
#pragma unroll
    for (int mi = 0; mi < size<0>(tensor); ++mi)
    {
        MaxOp<float> max_op;
        max(mi) = zero_init ? tensor(mi, 0) : max_op(max(mi), tensor(mi, 0));
#pragma unroll
        for (int ni = 1; ni < size<1>(tensor); ni++)
        {
            max(mi) = max_op(max(mi), tensor(mi, ni));
        }
        max(mi) = Allreduce<4>::run(max(mi), max_op);
        // If max is -inf, then all elements must have been -inf (possibly due to masking).
        // We don't want (-inf - (-inf)) since that would give NaN.
        float const max_scaled = max(mi) == -INFINITY ? 0.f : max(mi) * scale;
        sum(mi) = 0;
#pragma unroll
        for (int ni = 0; ni < size<1>(tensor); ++ni)
        {
            // Instead of computing exp(x - max), we compute exp2(x * log_2(e) -
            // max * log_2(e)) This allows the compiler to use the ffma
            // instruction instead of fadd and fmul separately.
            tensor(mi, ni) = exp2f(tensor(mi, ni) * scale - max_scaled);
            sum(mi) += tensor(mi, ni);
        }
        SumOp<float> sum_op;
        sum(mi) = Allreduce<4>::run(sum(mi), sum_op);
    }
}

template <typename Tensor0, typename Tensor1>
__forceinline__ __device__ void rescale_o(Tensor0& acc_o, Tensor1& scale_o)
{
    // Reshape acc_s from ((2, 2, V), MMA_M, MMA_N) to (nrow=(2, MMA_M), ncol=(2, V, MMA_N))
    Tensor acc_o_rowcol = make_tensor(acc_o.data(), flash::convert_layout_acc_rowcol(acc_o.layout()));
#pragma unroll
    for (int mi = 0; mi < size(scale_o); ++mi)
    {
#pragma unroll
        for (int ni = 0; ni < size<1>(acc_o_rowcol); ++ni)
        {
            acc_o_rowcol(mi, ni) *= scale_o(mi);
        }
    }
}

////////////////////////////////////////////////////////////////////////////////////////////////////

template <int kNRows>
struct Softmax
{

    using TensorT = decltype(make_tensor<float>(Shape<Int<kNRows>>{}));
    TensorT row_max, row_sum;

    __forceinline__ __device__ Softmax(){};

    template <bool Is_first, bool Check_inf = false, typename Tensor0>
    __forceinline__ __device__ TensorT softmax(Tensor0& acc_s, float softmax_scale_log2)
    {
        // Reshape acc_s from ((2, 2, V), MMA_M, MMA_N) to (nrow=(2, MMA_M), ncol=(2, V, MMA_N))
        Tensor scores = make_tensor(acc_s.data(), flash::convert_layout_acc_rowcol(acc_s.layout()));
        static_assert(decltype(size<0>(scores))::value == kNRows);
        TensorT scale_o;
        clear(scale_o);
        if (Is_first)
        {
            flash::template reduce_max</*zero_init=*/true>(scores, row_max);
            flash::scale_apply_exp2(scores, row_max, softmax_scale_log2);
            flash::reduce_sum</*zero_init=*/true>(scores, row_sum);
        }
        else
        {
            Tensor scores_max_prev = make_fragment_like(row_max);
            cute::copy(row_max, scores_max_prev);
            flash::template reduce_max</*zero_init=*/false>(scores, row_max);
// Reshape acc_o from (MMA=4, MMA_M, MMA_K) to (nrow=(2, MMA_M), ncol=(2, MMA_K))
#pragma unroll
            for (int mi = 0; mi < size(row_max); ++mi)
            {
                float scores_max_cur = !Check_inf ? row_max(mi) : (row_max(mi) == -INFINITY ? 0.0f : row_max(mi));
                float scores_scale = exp2f((scores_max_prev(mi) - scores_max_cur) * softmax_scale_log2);
                scale_o(mi) = scores_scale;
                row_sum(mi) *= scores_scale;
            }
            flash::scale_apply_exp2(scores, row_max, softmax_scale_log2);
            // We don't do the reduce across threads here since we don't need to use the row_sum.
            // We do that reduce at the end when we need to normalize the softmax.
            flash::reduce_sum</*zero_init=*/false>(scores, row_sum);
        }
        return scale_o;
    };

    template <bool Is_dropout = false, bool Split = false, typename Tensor0>
    __forceinline__ __device__ TensorT normalize_softmax_lse(
        Tensor0& acc_o, float softmax_scale, float descale_v, float rp_dropout = 1.0)
    {
        SumOp<float> sum_op;
        quad_allreduce_(row_sum, row_sum, sum_op);
        TensorT lse = make_fragment_like(row_sum);
        // Reshape acc_s from ((2, 2, V), MMA_M, MMA_N) to (nrow=(2, MMA_M), ncol=(2, V, MMA_N))
        Tensor acc_o_rowcol = make_tensor(acc_o.data(), flash::convert_layout_acc_rowcol(acc_o.layout()));
        static_assert(decltype(size<0>(acc_o_rowcol))::value == kNRows);
#pragma unroll
        for (int mi = 0; mi < size<0>(acc_o_rowcol); ++mi)
        {
            float sum = row_sum(mi);
            float inv_sum = (sum == 0.f || sum != sum) ? 1.f : descale_v / sum;
            lse(mi) = (sum == 0.f || sum != sum) ? (Split ? -INFINITY : INFINITY)
                                                 : row_max(mi) * softmax_scale + __logf(sum);
            float scale = !Is_dropout ? inv_sum : inv_sum * rp_dropout;
#pragma unroll
            for (int ni = 0; ni < size<1>(acc_o_rowcol); ++ni)
            {
                acc_o_rowcol(mi, ni) *= scale;
            }
        }
        return lse;
    };
};

} // namespace flash