scatter_epilogue.hpp

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/*! \file
  \brief Functor performing elementwise operations used by epilogues.
*/

#pragma once

#include "cutlass/cutlass.h"
#include "cutlass/gemm/dispatch_policy.hpp"
#include "cutlass/epilogue/collective/detail.hpp"

#include "cute/tensor.hpp"
#include "cute/numeric/numeric_types.hpp"

#include "gather_tensor.hpp"

namespace cutlass::epilogue::collective {

/// Applies an element wise operation to all elements within the fragment
/// and scatter-writes them out to destination storage.
/// GatherC and ScatterD are types of user-defined functions that apply the
/// transoformation of the strided coordinate (e.g. through an index array).
template <
  class StrideC_,
  class StrideD_,
  class ThreadEpilogueOp_,
  class EpilogueSchedule_,
  class GatherC_,
  class ScatterD_
>
class EpilogueGatherScatter {
public:
  //
  // Type Aliases
  //
  using EpilogueSchedule = EpilogueSchedule_;
  
  // derived types of output thread level operator
  using ThreadEpilogueOp = ThreadEpilogueOp_;
  using ElementOutput = typename ThreadEpilogueOp::ElementOutput;
  using ElementAccumulator = typename ThreadEpilogueOp::ElementAccumulator;
  using ElementCompute = typename ThreadEpilogueOp::ElementCompute;
  using ElementScalar = ElementCompute;
  using ElementC = typename ThreadEpilogueOp::ElementC;
  using StrideC = StrideC_;
  using ElementD = typename ThreadEpilogueOp::ElementD;
  using StrideD = StrideD_;

  // Every epilogue needs these two GmemTiledCopy{C,D} aliases.
  // If you don't know what they should be, just use void.
  using GmemTiledCopyC = void;
  using GmemTiledCopyD = void;

  using GatherC = GatherC_;
  using ScatterD = ScatterD_;

  static const int kOutputAlignment = ThreadEpilogueOp::kCount;
  using AlignmentType = typename cute::uint_bit<sizeof_bits<ElementOutput>::value * kOutputAlignment>::type;

  static_assert(cute::rank(StrideC{}) == 3, "StrideCD must be rank-3: [M, N, L]");
  static_assert(cute::rank(StrideD{}) == 3, "StrideCD must be rank-3: [M, N, L]");

  struct SharedStorage { };

  // Host side epilogue arguments
  struct Arguments {
    typename ThreadEpilogueOp::Params thread_params{};
    ElementC const* ptr_C = nullptr;
    StrideC dC{};
    ElementD* ptr_D = nullptr;
    StrideD dD{};
    GatherC gather_C{};
    ScatterD scatter_D{};
  };

  // Device side epilogue params
  using Params = Arguments;

  //
  // Methods
  //

  template <class ProblemShape>
  static constexpr Params
  to_underlying_arguments(
      [[maybe_unused]] ProblemShape const& _,
      Arguments const& args,
      [[maybe_unused]] void* workspace) {
    return args;
  }

  template<class ProblemShape>
  CUTLASS_HOST_DEVICE static bool
  can_implement(
      [[maybe_unused]] ProblemShape const& problem_shape,
      [[maybe_unused]] Arguments const& args) {
    return true;
  }

  CUTLASS_HOST_DEVICE
  EpilogueGatherScatter(Params const& params_) : params(params_) { }

  template<
    class ProblemShapeMNKL,
    class BlockShapeMNK,
    class BlockCoordMNKL,
    class FrgEngine, class FrgLayout,
    class TiledMma,
    class ResidueMNK
  >
  CUTLASS_DEVICE void
  operator()(
      ProblemShapeMNKL problem_shape_mnkl,
      BlockShapeMNK blk_shape_MNK,
      BlockCoordMNKL blk_coord_mnkl,
      cute::Tensor<FrgEngine, FrgLayout> const& accumulators,
      TiledMma tiled_mma,
      ResidueMNK residue_mnk,
      int thread_idx,
      char* smem_buf)
  {
    using namespace cute;
    using X = Underscore;

    static_assert(cute::rank(ProblemShapeMNKL{}) == 4, "ProblemShapeMNKL must be rank 4");
    static_assert(is_static<BlockShapeMNK>::value, "ThreadBlock tile shape must be static");
    static_assert(cute::rank(BlockShapeMNK{}) == 3, "BlockShapeMNK must be rank 3");
    static_assert(cute::rank(BlockCoordMNKL{}) == 4, "BlockCoordMNKL must be rank 3");

    (void) smem_buf;
    ThreadEpilogueOp epilogue_op{params.thread_params};

    // Separate out problem shape for convenience
    auto M = get<0>(problem_shape_mnkl);
    auto N = get<1>(problem_shape_mnkl);
    auto L = get<3>(problem_shape_mnkl);

    auto stride_c = detail::get_epilogue_stride<EpilogueSchedule>(params.dC);
    auto stride_d = detail::get_epilogue_stride<EpilogueSchedule>(params.dD);

    // Represent the full output tensor
    Tensor mC_mnl = make_gather_tensor(make_gmem_ptr(params.ptr_C), make_shape(M,N,L), stride_c, params.gather_C);  // (m,n,l)
    Tensor mD_mnl = make_gather_tensor(make_gmem_ptr(params.ptr_D), make_shape(M,N,L), stride_d, params.scatter_D); // (m,n,l)

    Tensor gC_mnl = local_tile(mC_mnl, blk_shape_MNK, make_coord(_,_,_), Step<_1,_1, X>{});    // (BLK_M,BLK_N,m,n,l)
    Tensor gD_mnl = local_tile(mD_mnl, blk_shape_MNK, make_coord(_,_,_), Step<_1,_1, X>{});    // (BLK_M,BLK_N,m,n,l)

    // Slice to get the tile this CTA is responsible for
    auto [m_coord, n_coord, k_coord, l_coord] = blk_coord_mnkl;
    Tensor gC = gC_mnl(_,_,m_coord,n_coord,l_coord);                                                 // (BLK_M,BLK_N)
    Tensor gD = gD_mnl(_,_,m_coord,n_coord,l_coord);                                                 // (BLK_M,BLK_N)

    // Partition source and destination tiles to match the accumulator partitioning
    auto thr_mma = tiled_mma.get_thread_slice(thread_idx);
    Tensor tCgD = thr_mma.partition_C(gD);                                       // (VEC,THR_M,THR_N)
    Tensor tCgC = thr_mma.partition_C(gC);                                       // (VEC,THR_M,THR_N)

    static_assert(is_static<FrgLayout>::value, "Accumulator layout must be static");
    CUTE_STATIC_ASSERT_V(size(tCgC) == size(tCgD),
        "Source and destination must have the same number of elements.");
    CUTE_STATIC_ASSERT_V(size(tCgD) == size(accumulators),
        "Accumulator count must have the same destination element count.");

    // Make an identity coordinate tensor for predicating our output MN tile
    auto cD = make_identity_tensor(make_shape(unwrap(shape<0>(gD)), unwrap(shape<1>(gD))));
    Tensor tCcD = thr_mma.partition_C(cD);

    // source is needed
    if (epilogue_op.is_source_needed()) {
      CUTLASS_PRAGMA_UNROLL
      for (int i = 0; i < size(accumulators); ++i) {
        if (elem_less(tCcD(i), make_coord(get<0>(residue_mnk), get<1>(residue_mnk)))) {
          tCgD(i) = epilogue_op(accumulators(i), tCgC(i));
        }
      }
    }
    // source is not needed, avoid load
    else {
      CUTLASS_PRAGMA_UNROLL
      for (int i = 0; i < size(accumulators); ++i) {
        if (elem_less(tCcD(i), make_coord(get<0>(residue_mnk), get<1>(residue_mnk)))) {
          tCgD(i) = epilogue_op(accumulators(i));
        }
      }
    }
  }

private:
  Params params;
};

} // namespace cutlass::epilogue::collective