apex/csrc/multi_tensor_apply.cuh

#include <ATen/ATen.h>
#include <ATen/AccumulateType.h>
#include <ATen/cuda/CUDAContext.h>
#include <ATen/cuda/Exceptions.h>

#include <assert.h>

// #include <iostream>

// This header is the one-stop shop for all your multi-tensor apply needs.


// TODO:  Kernel arg size limit may be <4KB for some other cards (ie Jetson)
constexpr int depth_to_max_tensors[5] = {110, 64, 48, 36, 30};
constexpr int depth_to_max_blocks[5] = {320, 320, 320, 320, 320};

template<int n> struct TensorListMetadata
{
  void* addresses[n][depth_to_max_tensors[n-1]];
  int sizes[depth_to_max_tensors[n-1]];
  unsigned char block_to_tensor[depth_to_max_blocks[n-1]];
  int block_to_chunk[depth_to_max_blocks[n-1]]; // I fear this needs to be a full int.
  int start_tensor_this_launch;
};


template<typename T, typename U, typename... ArgTypes>
__global__ void multi_tensor_apply_kernel(
    int chunk_size,
    volatile int* noop_flag,
    T tl,
    U callable,
    ArgTypes... args)
{
  // Hand the chunk information to the user-supplied functor to process however it likes.
  callable(chunk_size, noop_flag, tl, args...); 
}

template<int depth, typename T, typename... ArgTypes>
void multi_tensor_apply(
  int block_size,
  int chunk_size,
  const at::Tensor& noop_flag,
  const std::vector<std::vector<at::Tensor>>& tensor_lists,
  T callable,
  ArgTypes... args)
{
  AT_CHECK(tensor_lists.size() == depth, "tensor_lists.size() != depth");
  int len0 = tensor_lists[0].size();
  AT_CHECK(len0 > 0, "tensor_lists[0].size() is not > 0");

  for(int l = 0; l < tensor_lists.size(); l++) // No range-based for because I need indices
  {
    AT_CHECK(tensor_lists[l].size() == len0, "Size mismatch among tensor lists");
    for(int t = 0; t < tensor_lists[l].size(); t++)
    {
      // TODO:  Print which tensor fails.
      AT_CHECK(tensor_lists[l][t].is_contiguous(), "A tensor was not contiguous.");
      AT_CHECK(tensor_lists[l][t].is_cuda(), "A tensor was not cuda.");
      AT_CHECK(tensor_lists[l][t].numel() == tensor_lists[0][t].numel(), "Size mismatch");
    }
  }

  int ntensors = tensor_lists[0].size();

  TensorListMetadata<depth> tl;

  auto stream = at::cuda::getCurrentCUDAStream();
  
  tl.start_tensor_this_launch = 0;
  int loc_block_info = 0;
  int loc_tensor_info = 0;
  for(int t = 0; t < ntensors; t++)
  {
    tl.sizes[loc_tensor_info] = tensor_lists[0][t].numel();
    for(int d = 0; d < depth; d++)
      tl.addresses[d][loc_tensor_info] = tensor_lists[d][t].data_ptr();
    loc_tensor_info++;

    int chunks_this_tensor = (tensor_lists[0][t].numel() + chunk_size - 1)/chunk_size;

    for(int chunk = 0; chunk < chunks_this_tensor; chunk++)
    {
      // std::cout << chunks_this_tensor << std::endl;
      tl.block_to_tensor[loc_block_info] = loc_tensor_info - 1;
      tl.block_to_chunk[loc_block_info] = chunk;
      loc_block_info++;
  
      bool tensors_full = (loc_tensor_info == depth_to_max_tensors[depth-1] &&
                           chunk == chunks_this_tensor - 1);
      bool blocks_full = (loc_block_info == depth_to_max_blocks[depth-1]);
      bool last_chunk = (t == ntensors - 1 && chunk == chunks_this_tensor - 1);
      if(tensors_full || blocks_full || last_chunk)
      {
        // using accscalar_t = acc_type<scalar_t, true>;
        multi_tensor_apply_kernel<<<loc_block_info, block_size, 0, stream>>>(
          chunk_size,
          noop_flag.data<int>(),
          tl,
          callable,
          args...);

        AT_CUDA_CHECK(cudaGetLastError());

        // Reset.  The control flow possibilities here make my brain hurt.
        loc_block_info = 0;
        if(chunk == chunks_this_tensor - 1)
        {
          // std::cout << "Hit case 1 " << cond1 << " " << cond2 << " " << cond3 << std::endl;
          loc_tensor_info = 0; 
          tl.start_tensor_this_launch = t + 1;
        }
        else
        {
          // std::cout << "Hit case 2 " << cond1 << " " << cond2 << " " << cond3 << std::endl;
          tl.sizes[0] = tl.sizes[loc_tensor_info-1];
          for(int d = 0; d < depth; d++)
            tl.addresses[d][0] = tl.addresses[d][loc_tensor_info-1];
          loc_tensor_info = 1;
          tl.start_tensor_this_launch = t;
        }
      }
    }
  }
}