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/*! \file
    \brief This defines a "fragment" iterator for visiting the fragments of an accumulator tile
      that participate in one warp-level store operation.

      Typically, the accumulator tile is the largest single block of register-backed storage 
      within the kernel. Storing it to memory is best accomplished by partitioning it into
      smaller tiles and storing these sequentially.

      Round trips through shared memory during the Epilogue phase require partitioning, as
      shared memory capacity is typically insufficient for a threadblock's total accumulator
      size.
*/

#pragma once

#include "cutlass/array.h"
#include "cutlass/layout/matrix.h"

#include "cutlass/epilogue/warp/simt_policy.h"

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

namespace cutlass {
namespace epilogue {
namespace warp {

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

/// Fragment iterator for SIMT accumulator arrangements
template <
  typename WarpShape,             ///< shape of warp-level GEMM (concept: MatrixShape)
  typename Operator,              ///< matrix multiply operation (concept: arch::Mma)
  typename Layout,                ///< target shared memory layout
  typename MmaSimtPolicy          ///< policy defining lane arrangement (concept: MmaSimtPolicy)
>
class FragmentIteratorSimt;

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

/// Partial specialization for row-major shared memory
template <
  typename WarpShape_,     ///< shape of the warp-level GEMM tile
  typename Operator_ ,     ///< matrix multiply operator (concept: arch::Mma)
  typename MmaSimtPolicy_  ///< policy defining lane arrangement (concept: MmaSimtPolicy)
>
class FragmentIteratorSimt<WarpShape_, Operator_, layout::RowMajor, MmaSimtPolicy_> {
public:

  using WarpShape = WarpShape_;
  using Operator = Operator_;
  using Layout = layout::RowMajor;

  /// Policy for warp-level epilogue components
  using Policy = SimtPolicy<WarpShape, Operator, Layout, MmaSimtPolicy_>;

  /// This is the fragment size produced by one access of the iterator.
  using Fragment = Array<
    typename Operator::ElementC, 
    Policy::kElementsPerIteration>;

  /// This is the complete warp-level accumulator tile.
  using AccumulatorTile = Array<
    typename Operator::ElementC, 
    Policy::kAccumulatorElementCount>;

  using OutputAccumulatorTile = AccumulatorTile;

  /// Number of times this iterator can be incremented
  static int const kIterations = Policy::kIterations;

private:

  /// Internal access type
  using AccessType = Array<typename Operator::ElementC, Policy::kElementsPerAccess>;

private:

  //
  // Data members
  //

  /// Accumulator tile
  AccessType const *accumulators_;

  /// Internal index
  int index_;

public:

  /// Constructs an iterator
  CUTLASS_HOST_DEVICE
  FragmentIteratorSimt(AccumulatorTile const &accum): 
    accumulators_(reinterpret_cast<AccessType const *>(&accum)), 
    index_(0) {

  }

  /// Increments
  CUTLASS_HOST_DEVICE
  FragmentIteratorSimt &operator++() {
    ++index_;
    return *this;
  }

  /// Decrements
  CUTLASS_HOST_DEVICE
  FragmentIteratorSimt &operator--() {
    --index_;
    return *this;
  }

  /// Loads a fragment from the referenced part of the accumulator tile
  CUTLASS_HOST_DEVICE
  void load(Fragment &frag, int index_offset = 0) const {

    AccessType *frag_ptr = reinterpret_cast<AccessType *>(&frag);

    CUTLASS_PRAGMA_UNROLL
    for (int n = 0; n < Policy::kAccessesPerIteration; ++n) {

      int accumulator_access_offset = index_ * Policy::kAccessesPerIteration + n;

      frag_ptr[n] = accumulators_[accumulator_access_offset];
    }
  }
};

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

} // namespace warp
} // namespace epilogue
} // namespace cutlass

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