epilogue_workspace.h

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#pragma once

#include "cutlass/cutlass.h"
#include "cutlass/numeric_types.h"
#include "cutlass/array.h"


namespace cutlass {
namespace epilogue {


template <
  typename Shape_,      
  int WarpCount,        
  typename FragmentC_   
>
class EpilogueWorkspace {
public:

  using Shape = Shape_;
  using FragmentC = FragmentC_;
  using ElementC = typename FragmentC::value_type;

  static int const kWarpCount = WarpCount;

  static int const kAccessSizeInBits = 128;

  static int const kWarpSize = 32;

  static int const kElementsPerAccess = 
    kAccessSizeInBits / sizeof_bits<ElementC>::value;

  static int const kIterations = FragmentC::kElements / kElementsPerAccess;

  static_assert(
    !(FragmentC::kElements % kElementsPerAccess), 
    "The number of accumulators must be divisible by the access size.");

  static int const kWarpAccesses = kIterations * kWarpSize;

  static int const kThreadblockAccesses = kWarpAccesses * kWarpCount;

  struct Params {

    ElementC *ptr_C;

    int stride_n;

    int stride_k;

    //
    // Methods
    //

    CUTLASS_HOST_DEVICE
    Params(
      ElementC *ptr_C,   
      int stride_n_,      
      int stride_k_       
    ):
      ptr_C(ptr_C), stride_n(stride_n_ / kElementsPerAccess), stride_k(stride_k_ / kElementsPerAccess) {

    }
  };

  struct SharedStorage {
    // Intentionally empty
  };

private:

  struct alignas((kAccessSizeInBits / 8)) AccessType {
    Array<ElementC, kElementsPerAccess> storage;
  };

  AccessType *pointer_;

  int stride_n_;

  int stride_k_;

public:

  CUTLASS_DEVICE
  EpilogueWorkspace(
    Params const &params,     
    SharedStorage &,          
    int warp_idx,             
    int lane_idx              

  ):
    pointer_(reinterpret_cast<AccessType *>(params.ptr_C)),
    stride_n_(params.stride_n), 
    stride_k_(params.stride_k) {

    // Add per-thread offset
    pointer_ += lane_idx + warp_idx * kWarpAccesses;
  }

  CUTLASS_DEVICE
  void operator()(
    cutlass::gemm::GemmCoord problem_size,       
    cutlass::gemm::GemmCoord tb_tile_coord,      
    FragmentC const &accum) {     
    
    // Compute offset for entire threadblock (note, per-thread offset has been folded in already)
    AccessType *pointer = pointer_ + 
      tb_tile_coord.m() * kThreadblockAccesses + 
      tb_tile_coord.n() * stride_n_ +
      tb_tile_coord.k() * stride_k_;

    // Cast to vectorized view of accumulator fragments
    AccessType const * src_pointer = reinterpret_cast<AccessType const *>(&accum);

    // Write out accumulators at full speed
    CUTLASS_PRAGMA_UNROLL
    for (int i = 0; i < kIterations; ++i) {
      pointer[i * kWarpSize] = src_pointer[i];
    }
  }
};


} // namespace epilogue
} // namespace cutlass