#include <dlfcn.h>
#include <ATen/ceil_div.h>
#include <c10/cuda/CUDAGuard.h>

#include <torch/csrc/distributed/c10d/symm_mem/env.hpp>
#include <torch/csrc/distributed/c10d/symm_mem/nvshmem_extension.cuh>
#include <torch/csrc/distributed/c10d/symm_mem/nvshmem_team_manager.hpp>
#include <torch/csrc/distributed/c10d/symm_mem/CUDASymmetricMemory-inl.h>
#include <torch/csrc/distributed/c10d/symm_mem/CUDASymmetricMemoryUtils.hpp>
#include <torch/csrc/distributed/c10d/symm_mem/SymmetricMemory.hpp>

#include <ATen/ceil_div.h>
// Use torch's cub wrapper instead of CUDA's <cub/cub.cuh>, see #55292
#include <ATen/cuda/cub.cuh>

// NVSHMEM minimum SM arch
#define _NVSHMEM_MIN_SM_ARCH 700

// If CUDA_ARCH is less than sm_70, or on sm_110, skip NVSHMEM device APIs
#define _NVSHMEM_DEVICELIB_SUPPORTED 1
#if defined(__CUDA_ARCH__)
#  if (__CUDA_ARCH__ < _NVSHMEM_MIN_SM_ARCH) || (__CUDA_ARCH__ == 1100)
#    undef _NVSHMEM_DEVICELIB_SUPPORTED
#  endif
#endif

// Some NVSHMEM device APIs do not compile on older SM archs
#ifndef _NVSHMEM_DEVICELIB_SUPPORTED
// Only include host APIs. See nvshmem.h for details.
#  define NVSHMEM_HOSTLIB_ONLY
#endif  // Must be done before nvshmem.h is included

#include <nvshmem.h>
#include <nvshmemx.h>

namespace c10d::nvshmem_extension {

#define THREADS_PER_BLOCK 512
#define WARP_SIZE 32

extern "C" void nvshmem_init() __attribute__((weak));

// Check if NVSHMEM is available
bool is_nvshmem_available() {
  // Runtime check
  static std::mutex mutex;
  static int is_available = -2;
  std::lock_guard<std::mutex> lock(mutex);

  // Checked if the symbol is statically linked
  if(is_available == -2 && nvshmem_init) {
    is_available = 1;
  }

  if (is_available == -2) {
    void* handle{};
    // Open the shared library, RTLD_LAZY defers symbol resolution until needed
    handle = dlopen("libnvshmem_host.so.3", RTLD_LAZY);
    if (!handle) {
      std::cerr << dlerror() << "\n";
      is_available = 0;
    } else {
      is_available = 1;
      // Close the shared library
      dlclose(handle);
    }
  }
  return is_available == 1;
}

// Initializes the device state in CUmodule so that it’s able to perform NVSHMEM
// operations.
void nvshmemx_cumodule_init(uintptr_t module) {
  auto cumodule = reinterpret_cast<CUmodule>(module);
  NVSHMEM_CHECK(
    ::nvshmemx_cumodule_init(cumodule),
    "nvshmemx_cumodule_init failed");
}

at::Tensor nvshmem_broadcast(at::Tensor& input, const int64_t root, const std::string& group_name) {
  auto input_hdl = c10d::symmetric_memory::rendezvous(input, group_name);
  int rank = input_hdl->get_rank();
  void* buffer_ptr = input.mutable_data_ptr();
  auto buffer_size = input.numel() * input.element_size();
  auto& team_manager = TeamManager::get(input.device());
  auto team = team_manager.get_team(group_name, input_hdl->get_rank_to_global_rank());
  int team_size = nvshmem_team_n_pes(team);
  TORCH_CHECK(root < team_size, "root must be smaller than group size");

  auto stream = at::cuda::getCurrentCUDAStream();
  nvshmemx_broadcastmem_on_stream(team, buffer_ptr, buffer_ptr, buffer_size, root, stream);
  return input;
}

void nvshmem_put(at::Tensor& tensor, const int64_t peer) {
  // TODO: support non-contiguous tensors
  TORCH_CHECK(tensor.is_contiguous(),
      "put op currently supports contiguous tensors only");
  // TODO: rendezvous should remember the group name
  auto hdl = c10d::symmetric_memory::rendezvous(tensor, "0");
  auto rank = hdl->get_rank();
  void* buffer_ptr = hdl->get_buffer_ptrs()[rank];
  auto buffer_size = tensor.numel() * tensor.element_size();
  TORCH_CHECK(peer < hdl->get_world_size(), "peer must be smaller than world size");

  c10::cuda::CUDAGuard guard(tensor.device());
  auto stream = at::cuda::getCurrentCUDAStream();
  nvshmemx_putmem_on_stream(buffer_ptr, tensor.data_ptr(), buffer_size, peer, stream);
}

void nvshmem_wait_for_signal(at::Tensor& sigpad, int64_t signal, int64_t peer) {
  c10::cuda::CUDAGuard guard(sigpad.device());
  auto stream = at::cuda::getCurrentCUDAStream();
  nvshmemx_signal_wait_until_on_stream(static_cast<uint64_t*>(sigpad.data_ptr()), NVSHMEM_CMP_EQ, signal, stream);
}

void nvshmem_put_with_signal(at::Tensor& tensor, at::Tensor& sigpad, int64_t signal, int64_t peer) {
  auto buffer_size = tensor.numel() * tensor.element_size();

  c10::cuda::CUDAGuard guard(tensor.device());
  auto stream = at::cuda::getCurrentCUDAStream();
  nvshmemx_putmem_signal_on_stream(
    tensor.mutable_data_ptr(),
    tensor.mutable_data_ptr(),
    buffer_size,
    static_cast<uint64_t*>(sigpad.mutable_data_ptr()),
    signal,
    NVSHMEM_SIGNAL_SET,
    peer,
    stream);
}

void nvshmem_get(at::Tensor& tensor, const int64_t peer) {
  // TODO: support non-contiguous tensors
  TORCH_CHECK(tensor.is_contiguous(),
      "get op currently supports contiguous tensors only");
  // TODO: rendezvous should remember the group name
  auto hdl = c10d::symmetric_memory::rendezvous(tensor, "0");
  auto rank = hdl->get_rank();
  void* buffer_ptr = hdl->get_buffer_ptrs()[rank];
  auto buffer_size = tensor.numel() * tensor.element_size();
  TORCH_CHECK(peer < hdl->get_world_size(), "peer must be smaller than world size");

  c10::cuda::CUDAGuard guard(tensor.device());
  auto stream = at::cuda::getCurrentCUDAStream();
  nvshmemx_getmem_on_stream(tensor.mutable_data_ptr(), buffer_ptr, buffer_size, peer, stream);
}

at::Tensor nvshmem_all_to_all(
    at::Tensor& input,
    at::Tensor& out,
    std::string group_name) {
  auto input_hdl = c10d::symmetric_memory::rendezvous(input, group_name);
  auto out_hdl = c10d::symmetric_memory::rendezvous(out, group_name);
  int rank = input_hdl->get_rank();
  int world_size = input_hdl->get_world_size();
  auto& team_manager = TeamManager::get(input.device());
  auto team = team_manager.get_team(group_name, input_hdl->get_rank_to_global_rank());

  void* input_ptr = input.data_ptr();
  void* output_ptr = out.mutable_data_ptr();
  TORCH_CHECK(input.is_contiguous() && out.is_contiguous());
  TORCH_CHECK_EQ(input.numel(), out.numel());
  TORCH_CHECK_EQ(input.dtype(), out.dtype());
  TORCH_CHECK_EQ(input.numel() % world_size, 0);
  auto buffer_size = input.numel() * input.element_size();
  size_t bytes_per_rank = buffer_size / world_size;

  auto stream = at::cuda::getCurrentCUDAStream(input.device().index());
  nvshmemx_alltoallmem_on_stream(team, output_ptr, input_ptr, bytes_per_rank, stream);
  return out;
}

// This is an exclusive prefix sum function that calculates read (or write) offsets for each peer.
__device__ int64_t prefixSum(int64_t *odata, int64_t *idata, int n) {
  // Specialize BlockScan for a 1D block of threads, of type int64_t.
  // - `BLOCK_SCAN_WARP_SCANS` is a low-latency scan algorithm (instead of high
  // throughput which we don't need here).
  // - `at_cuda_detail::cub` is torch's cub wrapper, see #55292.
  using BlockScanT = at_cuda_detail::cub::BlockScan<int64_t, THREADS_PER_BLOCK, at_cuda_detail::cub::BLOCK_SCAN_WARP_SCANS>;
  // Allocate shared memory for BlockScan
  __shared__ typename BlockScanT::TempStorage temp_storage;

  // TODO: currently it is assumed that the number of PE's is smaller than
  // `THREADS_PER_BLOCK`
  CUDA_KERNEL_ASSERT(n <= THREADS_PER_BLOCK);

  // Obtain input item for each thread
  int tid = threadIdx.x;
  int64_t thread_data = (tid < n) ? idata[tid] : 0;

  // Collectively compute the block-wide exclusive prefix sum
  int64_t block_aggregate;
  BlockScanT(temp_storage).ExclusiveSum(thread_data, thread_data, block_aggregate);

  // Store the result
  odata[tid] = thread_data;
  return block_aggregate;
}

// This kernel is used to exchange output splits and source offsets between peers.
// `in_out_splits` is of size (3, npes) and contains:
// - input splits (IN)
// - output splits (OUT) and
// - source offsets (OUT).
__global__ void exchangeSplitAndOffset(int64_t* input_splits, int64_t* out_splits_offsets, nvshmem_team_t team) {
#ifndef _NVSHMEM_DEVICELIB_SUPPORTED
  CUDA_KERNEL_ASSERT_MSG(false, "SM arch unsupported for NVSHMEM");
#else
  CUDA_KERNEL_ASSERT(team != NVSHMEM_TEAM_INVALID);
  int mype = nvshmem_team_my_pe(team);
  int npes = nvshmem_team_n_pes(team);
  auto output_splits = out_splits_offsets;
  auto source_offsets = out_splits_offsets + npes;
  int tid = threadIdx.x;

  CUDA_KERNEL_ASSERT(npes <= THREADS_PER_BLOCK);
  __shared__ int64_t peer_offsets[THREADS_PER_BLOCK];

  // Scan input splits to get the source offsets
  prefixSum(peer_offsets, input_splits, npes);
  __syncthreads();;

  // Use 1 block to do the exchange
  if (tid < npes) {
    // tid is peer index within team, but put calls require global rank
    int peer_global = nvshmem_team_translate_pe(team, tid, NVSHMEM_TEAM_WORLD);
    nvshmem_int64_p(source_offsets + mype, peer_offsets[tid], peer_global);
    nvshmem_int64_p(output_splits + mype, input_splits[tid], peer_global);
  }
  // This barrier ensures that all remote PEs see the updated values
  nvshmemx_barrier_block(team);
#endif
}

// This kernel is used to do the actual data exchange.
// `in_out_splits` has the same definition as in `exchangeSplitAndOffset`.
// `stride` is the stride at dim 0, unit in byte.
__global__ void allToAllV(void *send_data, void *recv_data, int64_t* out_splits_offsets, size_t stride, nvshmem_team_t team) {
#ifndef _NVSHMEM_DEVICELIB_SUPPORTED
  CUDA_KERNEL_ASSERT_MSG(false, "SM arch unsupported for NVSHMEM");
#else
  CUDA_KERNEL_ASSERT(team != NVSHMEM_TEAM_INVALID);
  int mype = nvshmem_team_my_pe(team);
  int npes = nvshmem_team_n_pes(team);
  auto output_splits = out_splits_offsets;
  auto source_offsets = out_splits_offsets + npes;
  int bid = blockIdx.x;
  int tid = threadIdx.x;
  int blocks_per_peer = max(gridDim.x / npes, 1);

  // Calculate the output offsets
  CUDA_KERNEL_ASSERT(npes <= THREADS_PER_BLOCK);
  __shared__ int64_t peer_offsets[THREADS_PER_BLOCK];
  prefixSum(peer_offsets, output_splits, npes);
  __syncthreads();

  // Target a different peer based on bid
  for (int i = bid / blocks_per_peer; i < npes; i += gridDim.x / blocks_per_peer) {
    int peer = (mype + i) % npes;
    auto peer_global = nvshmem_team_translate_pe(team, peer, NVSHMEM_TEAM_WORLD);
    // Total amount from `peer`
    auto peer_size = output_splits[peer] * stride;
    // Amount to get from `peer` in this block
    auto block_size = peer_size / blocks_per_peer;
    // Being lazy here, we should handle the residual if the division is not exact
    CUDA_KERNEL_ASSERT(block_size * blocks_per_peer == peer_size);
    // This block's offset in the data from `peer`
    auto block_offset = block_size * (bid % blocks_per_peer);
    auto source_offset = source_offsets[peer] * stride + block_offset;
    auto write_offset = peer_offsets[peer] * stride + block_offset;
    nvshmemx_getmem_nbi_block(
      (char*)recv_data + write_offset,
      (char*)send_data + source_offset,
      block_size,
      peer_global);
  }
  // Write out the output offsets (to the scratchpad line)
  if (bid == 0 && tid < npes) {
    source_offsets[tid] = peer_offsets[tid];
  }
  // Make sure getmem_nbi calls finish
  nvshmem_quiet();
#endif
}

static int get_a2a_nblocks(size_t size, int world_size, bool intra_node) {
  // Check user setting first
  int num_blocks = c10d::symmetric_memory::getenv_nblocks();
  if (num_blocks > 0) {  // set by user
    return num_blocks;
  }
  // 16B per thread, 8 loops
  constexpr size_t chunk_size = 16 * THREADS_PER_BLOCK * 8;
  num_blocks = at::ceil_div(size, chunk_size);
  // Allow kernel to target even number of blocks per peer
  num_blocks = at::round_up(num_blocks, world_size);
  const int max_blocks = intra_node ? 64 : 16;
  return std::min(num_blocks, max_blocks);
}

void all_to_all_vdev(
    at::Tensor& input,
    at::Tensor& out,
    at::Tensor& in_splits,
    at::Tensor& out_splits_offsets,
    std::string group_name) {
  /* Perform AllToAllv operation using NVSHMEM, with split information provided on device.
   * Arguments:
   *  - `input` is the input tensor
   *  - `out` is the output tensor
   *  - `in_splits` is a 1D tensor of size (npes), containing the input splits
   *  - `out_splits_offsets` is a 2D tensor of size (2, npes). The rows are (in order):
        output splits and output offsets.
  */
  auto input_hdl = c10d::symmetric_memory::rendezvous(input, group_name);
  auto out_hdl = c10d::symmetric_memory::rendezvous(out, group_name);
  auto in_splits_hdl = c10d::symmetric_memory::rendezvous(in_splits, group_name);
  auto out_splits_offsets_hdl = c10d::symmetric_memory::rendezvous(out_splits_offsets, group_name);
  int rank = input_hdl->get_rank();
  int world_size = input_hdl->get_world_size();

  void* input_ptr = input.data_ptr();
  void* output_ptr = out.mutable_data_ptr();
  int64_t* in_splits_ptr = (int64_t*)(in_splits.const_data_ptr());
  int64_t* out_splits_offsets_ptr = (int64_t*)(out_splits_offsets.mutable_data_ptr());

  TORCH_CHECK_EQ(input.device(), out.device());
  auto device = input.device();
  c10::cuda::CUDAGuard guard(device);
  auto& team_manager = TeamManager::get(device);
  auto team = team_manager.get_team(group_name, input_hdl->get_rank_to_global_rank());
  auto stream = at::cuda::getCurrentCUDAStream(device.index());

  // Exchange output splits and source offsets
  // Use collective launch because kernel involves nvshmem barrier
  void* args0[] = {
      &in_splits_ptr,
      &out_splits_offsets_ptr,
      &team};
  nvshmemx_collective_launch(
      (const void*)exchangeSplitAndOffset,
      dim3(1),
      dim3(THREADS_PER_BLOCK),
      args0,
      0,
      stream);

  // CTA Tuning
  auto input_size = input.numel() * input.element_size();
  int num_blocks = get_a2a_nblocks(
    input_size,
    input_hdl->get_world_size(),
    input_hdl->world_within_direct_access());

  // Stride at dim 0 (assuming input is contiguous, TODO)
  size_t stride_bytes = input.stride(0) * input.element_size();

  // All to all data exchange
  void* args1[] = {
      &input_ptr,
      &output_ptr,
      &out_splits_offsets_ptr,
      &stride_bytes,
      &team};
  nvshmemx_collective_launch(
      (const void*)allToAllV,
      dim3(num_blocks),
      dim3(THREADS_PER_BLOCK),
      args1,
      0,
      stream);
}

// Start of `all_to_all_vdev_2d`

// `exchangeSplitAndOffset_2d` is used to exchange output splits and source
// offsets between peers.

/* Arguments:
 * `in_splits_offsets`: input splits and offsets (optinoal), of size (2, nsplits), or (1, nsplits) if no offsets are provided.
 * `out_splits_offsets`: output splits and offsets, of size (2, nsplits).
 * `mype`: the rank of the current PE.
 * `npes`: the number of PEs.
 * `ne`: the number of experts.
 * `input_dim0`: the size of dim 0 of the input tensor.
 * `rank_is_row_in` is a boolean flag indicating whether the input has ranks as row or experts as row.
*/

/* Template parameters:
 * `HAS_IN_OFFSETS` is a boolean flag indicating whether `in_splits_offsets` has offsets (2nd row) or not.
*/

template <bool HAS_IN_OFFSETS>
__global__ void exchangeSplitAndOffset_2d(int64_t* in_splits_offsets, int64_t* out_splits_offsets, nvshmem_team_t team, int ne, size_t input_dim0, bool rank_is_row_in) {
#ifndef _NVSHMEM_DEVICELIB_SUPPORTED
  CUDA_KERNEL_ASSERT_MSG(false, "SM arch unsupported for NVSHMEM");
#else
  CUDA_KERNEL_ASSERT(team != NVSHMEM_TEAM_INVALID);
  int mype = nvshmem_team_my_pe(team);
  int npes = nvshmem_team_n_pes(team);
  int nsplits = npes * ne;
  auto input_splits = in_splits_offsets;
  auto output_splits = out_splits_offsets;
  // Borrowing the space below as a temporary exchange pad.
  auto source_offsets = out_splits_offsets + nsplits;
  int tid = threadIdx.x;

  int64_t* input_offsets = nullptr;
  if (HAS_IN_OFFSETS) {
    // input offset are provided, so we can use them directly
    input_offsets = in_splits_offsets + nsplits;
  } else {
    // input offset are not provided, so we need to calculate them.
    // Scan input splits to get the source offsets
    __shared__ int64_t peer_offsets[THREADS_PER_BLOCK];
    auto sum_of_splits = prefixSum(peer_offsets, input_splits, nsplits);
    __syncthreads();;
    CUDA_KERNEL_ASSERT(sum_of_splits <= input_dim0 && "sum of splits is larger than input dim\n");
    // Redirect the input splits to the calculated result
    input_offsets = peer_offsets;
  }

  // Use 1 block to do the exchange
  if (tid < nsplits) {
    int peer, e, dst_offset;
    if (rank_is_row_in) {
      peer = tid / ne;
      e = tid % ne;
      dst_offset = e * npes + mype;
    } else {  // expert is row in input
      peer = tid % npes;
      e = tid / npes;
      dst_offset = mype * ne + e;
    }
    // This does a transpose from rank-major order to expert-major order
    // (or vice versa).
    auto split_val = input_splits[tid];
    CUDA_KERNEL_ASSERT(split_val >= 0 && "split value is negative\n");
    auto peer_global = nvshmem_team_translate_pe(team, peer, NVSHMEM_TEAM_WORLD);
    nvshmem_int64_p(source_offsets + dst_offset, input_offsets[tid], peer_global);
    nvshmem_int64_p(output_splits + dst_offset, split_val, peer_global);
  }
  // This barrier ensures that all remote PEs see the updated values
  nvshmemx_barrier_block(team);
#endif
}

// This is an warp-scope, exclusive prefix sum. When called by a block of
// threads, each warp will perform an independent prefix sum, concurrently.
// Returns the sum of all elements in the warp.
// `NUM_WARPS` is the number of warps participating the concurrent prefix sum.
template <int NUM_WARPS>
__device__ int64_t prefixSum_warp(int64_t *odata, int64_t *idata, int n) {
  CUDA_KERNEL_ASSERT(n <= WARP_SIZE);

  // Specialize WarpScan for type int
  using WarpScan = at_cuda_detail::cub::WarpScan<int64_t>;
  // Allocate WarpScan shared memory for N warps
  __shared__ typename WarpScan::TempStorage temp_storage[NUM_WARPS];

  int warp_id = threadIdx.x / WARP_SIZE;
  if (warp_id >= NUM_WARPS) {
    return 0;
  }

  // Obtain input item for each thread
  int tid = threadIdx.x % WARP_SIZE;
  int64_t thread_data = (tid < n) ? idata[tid] : 0;

  // Total sum of all elements in the warp
  int64_t warp_aggregate;
  // Compute the warp-wide exclusive prefix sum
  WarpScan(temp_storage[warp_id]).ExclusiveSum(thread_data, thread_data, warp_aggregate);

  // Store the result
  odata[tid] = thread_data;
  return warp_aggregate;
}

// This is for abstracting a thread-group-scope, exclusive prefix sum.
// Since we use warp-scope prefix sum, the thread group size is limited to warp size.
#define A2AV_TILE_SIZE WARP_SIZE

// This kernel is used to do the actual data exchange.
// `in_out_splits` has the same definition as in `exchangeSplitAndOffset`.
// `stride` is the stride at dim 0, unit in byte.
// For meaning of `mype` and `npes`, see the docstring of `all_to_all_vdev_2d`.
// `major_align` is the alignment at dim 0, unit in element. If 0, no alignment is needed.

// `rank_is_row_out` is a boolean flag indicating whether the output has ranks as rows or experts as rows.
// In dispatch case, rank_is_row_out = false, major_size = ne, minor_size = npes.
// In combine case, rank_is_row_out = true, major_size = npes, minor_size = ne.

__global__ void allToAllV_2d(void *send_data, void *recv_data, int64_t* in_splits, int64_t* out_splits_offsets, size_t stride, int minor_size, int major_size, int64_t major_align, bool rank_is_row_out, nvshmem_team_t team) {
#ifndef _NVSHMEM_DEVICELIB_SUPPORTED
  CUDA_KERNEL_ASSERT_MSG(false, "SM arch unsupported for NVSHMEM");
#else
  int nsplits = minor_size * major_size;
  auto output_splits = out_splits_offsets;
  auto source_offsets = out_splits_offsets + nsplits;
  int bid = blockIdx.x;
  int tid = threadIdx.x;

  // Split the thread block into tiles
  constexpr int NUM_TILES = THREADS_PER_BLOCK / A2AV_TILE_SIZE;
  int tileId = tid / A2AV_TILE_SIZE;
  int laneId = tid % A2AV_TILE_SIZE;
  // Each tile calculates its own prefix sum
  __shared__ int64_t tile_prefix_sums[NUM_TILES][A2AV_TILE_SIZE];
  // A tile takes care of minor_size worth of splits
  int nsplits_per_tile = min(minor_size, nsplits - tileId * minor_size);
  // TODO: currently it is assumed that the number of PE's is smaller than
  // `A2AV_TILE_SIZE` bc the warp-scope prefix sum can only handle up to
  // WARP_SIZE elements
  CUDA_KERNEL_ASSERT(minor_size <= A2AV_TILE_SIZE && "minor_size is too large\n");
  // Similarly, the number of experts per rank is also assumed to be smaller
  // than `NUM_TILES`
  CUDA_KERNEL_ASSERT(major_size <= NUM_TILES && "major_size is too large\n");

  // Total length of each tile
  __shared__ int64_t len_per_tile[NUM_TILES];
  // When `nsplits` is small, not every tile gets data to sum. They can skip
  // this local prefix sum.
  if (nsplits_per_tile > 0) {
    // Each tile calculates its own prefix sum, return value is the sum of all elements in the tile.
    int64_t my_tile_len = prefixSum_warp<NUM_TILES>(tile_prefix_sums[tileId], output_splits + tileId * minor_size, nsplits_per_tile);
    // Last thread in each tile does the up aligning.
    if (laneId == A2AV_TILE_SIZE - 1) {
      if (major_align != 0) {  // Needs alignment
        auto aligned_len = (my_tile_len + major_align - 1) / major_align * major_align;
        // In case `aligned_len` is 0, we set it to `major_align` to avoid an
        // empty bin, bc cutlass currently does not support it. See
        // https://github.com/pytorch/pytorch/issues/152668.
        len_per_tile[tileId] = max(aligned_len, major_align);
      } else {  // 0 means alignment not needed
        len_per_tile[tileId] = my_tile_len;
      }
    }
  }
  __syncthreads();

  // Starting offset of each tile
  __shared__ int64_t start_offset_per_tile[NUM_TILES];
  // Prefix sum again to get the tiles' start offsets.
  // `NUM_TILES` is typically not greater than 32, because 32 tiles * 32 threads
  // = 1024 threads, and this kernel is launched within 1024 threads. Thus, we
  // can use warp-scope prefix sum.
  static_assert(NUM_TILES <= WARP_SIZE);
  // Only 1 warp is needed
  prefixSum_warp<1>(start_offset_per_tile, len_per_tile, NUM_TILES);
  __syncthreads();

  // Add tile offset to every element in the tile
  tile_prefix_sums[tileId][laneId] += start_offset_per_tile[tileId];
  __syncthreads();

  // Target a different e based on bid
  for (int eid = bid; eid < nsplits; eid += gridDim.x) {
    int row = eid / minor_size;
    int col = eid % minor_size;
    // Amount from `peer` for `e`
    auto peer_size = output_splits[eid] * stride;
    auto source_offset = source_offsets[eid] * stride;
    auto e_offset = tile_prefix_sums[row][col];
    auto write_offset = e_offset * stride;
    auto peer_global = nvshmem_team_translate_pe(team, rank_is_row_out ? row : col, NVSHMEM_TEAM_WORLD);
    nvshmemx_getmem_nbi_block(
      (char*)recv_data + write_offset,
      (char*)send_data + source_offset,
      peer_size,
      peer_global);  // peer's global index
  }
  // Write out the output offsets (to the scratchpad line)
  if (bid == 0 && tid < nsplits) {
    source_offsets[tid] = tile_prefix_sums[tid / minor_size][tid % minor_size];
  }
  // Make sure getmem_nbi calls finish
  nvshmem_quiet();
#endif
}

void all_to_all_vdev_2d(
    at::Tensor& input,
    at::Tensor& out,
    at::Tensor& in_splits,
    at::Tensor& out_splits_offsets,
    std::string group_name,
    std::optional<int64_t> major_align) {
  /* Perform a 2D AllToAllv shuffle operation using NVSHMEM, with split information provided on device.
   * Arguments:
   *  - `input` is the input tensor
   *  - `out` is the output tensor
   *  - `in_out_splits` is a 2D tensor of size (3, `world_size` * `ne`). In the
        scenario of Mixture-of-Experts models, `ne` is the number of experts per
        rank. The rows of `in_out_splits` are (in order):
        input splits (IN)
        output splits (OUT) and
        output offsets (OUT).
   *  - `group_name` is the name of the group to use for the collective operation.
   *  - `major_align` is the alignment of the "major dimension" of the output
        sequence. See below for details.

   *  A 2D AllToAllv shuffle is illustrated below:
        (world_size = 2, ne = 2, total number of experts = 4)
        Source: |       Rank 0      |       Rank 1      |
                | c0 | c1 | c2 | c3 | d0 | d1 | d2 | d3 |

        Dest  : |       Rank 0      |       Rank 1      |
                | c0 | d0 | c1 | d1 | c2 | d2 | c3 | d3 |
        where each `c_i` / `d_i` are slices of the `input` tensor, targeting
        expert `i`, with length indicated by input splits (in
        `in_out_splits[0]`).  That is, the 2D AllToAllv shuffle achieves a
        transpose from rank-major order at input to expert-major order at
        output.

   *  If `major_align` is not 1, the output offsets of c1, c2, c3 will be
      up-aligned to this value. For example, if c0 has length 5 and d0 has
      length 7 (making a total of 12), and if the `major_align` is set to 16,
      the output offset of c1 will be 16. Similar for c2 and c3. This value has
      no effect on the offset of the minor dimension, i.e.  d0, d1, d2 and d3.
      Note: since cutlass does not support empty bins, we set the aligned length
      to `major_align` if it is 0. See
      https://github.com/pytorch/pytorch/issues/152668.
  */
  auto input_hdl = c10d::symmetric_memory::rendezvous(input, group_name);
  auto out_hdl = c10d::symmetric_memory::rendezvous(out, group_name);
  auto in_splits_hdl = c10d::symmetric_memory::rendezvous(in_splits, group_name);
  auto out_splits_offsets_hdl = c10d::symmetric_memory::rendezvous(out_splits_offsets, group_name);
  int rank = input_hdl->get_rank();
  int world_size = input_hdl->get_world_size();
  // TODO: world_size is currently limited by the number of elements in a WarpScan.
  TORCH_CHECK(world_size <= A2AV_TILE_SIZE, "world_size must be smaller than A2AV_TILE_SIZE", A2AV_TILE_SIZE);

  // If `major_align` is not provided, use 1 as the default value.
  int64_t major_align_val = major_align.value_or(1);
  TORCH_CHECK(major_align_val > 0, "major_align must be positive");

  void* input_ptr = input.data_ptr();
  void* output_ptr = out.mutable_data_ptr();
  int64_t* in_splits_ptr = (int64_t*)(in_splits.data_ptr());
  int64_t* out_splits_offsets_ptr = (int64_t*)(out_splits_offsets.mutable_data_ptr());

  // Shape checks
  TORCH_CHECK(in_splits.is_contiguous()
      && out_splits_offsets.is_contiguous()
      && input.is_contiguous()
      && out.is_contiguous(),
      "input, out, in_splits and out_splits_offsets must be contiguous");
  auto in_split_shape = in_splits.sizes();
  auto out_split_shape = out_splits_offsets.sizes();
  TORCH_CHECK(out_split_shape.size() == 2
      && out_split_shape[0] == 2
      && out_split_shape[1] == in_split_shape[0]
      && in_split_shape[0] % world_size == 0,
      "out_splits_offsets must be 2D with 2 rows, "
      "each row must be a multiple of world_size");

  // Consistency checks
  TORCH_CHECK(input.dtype() == out.dtype()
      && input.stride(0) == out.stride(0),
      "input and out must have the same dtype and same stride at dim 0");
  TORCH_CHECK(in_splits.scalar_type() == at::kLong
      && out_splits_offsets.scalar_type() == at::kLong,
      "splits and offsets must be int64");

  // Number of experts per rank
  int ne = in_split_shape[0] / world_size;
  constexpr int NUM_TILES = THREADS_PER_BLOCK / A2AV_TILE_SIZE;
  TORCH_CHECK(ne <= NUM_TILES, "Number of experts must be smaller than NUM_TILES", NUM_TILES);

  // Set device context for getting the stream and launching kernels below
  auto device = input.device();
  TORCH_CHECK(device.type() == at::DeviceType::CUDA &&
      out.device() == device &&
      in_splits.device() == device &&
      out_splits_offsets.device() == device,
      "all tensor arguments must be on the same CUDA device");
  c10::cuda::CUDAGuard guard(device);
  auto stream = at::cuda::getCurrentCUDAStream();
  auto& team_manager = TeamManager::get(device);
  auto team = team_manager.get_team(group_name, input_hdl->get_rank_to_global_rank());

  // Exchange output splits and source offsets
  auto input_dim0 = input.size(0);
  bool rank_is_row_in = true;
  // Use collective launch because kernel involves nvshmem barrier
  void* args0[] = {
      &in_splits_ptr,
      &out_splits_offsets_ptr,
      &team,
      &ne,
      &input_dim0,
      &rank_is_row_in};
  nvshmemx_collective_launch(
      (const void*)exchangeSplitAndOffset_2d<false>,  // false: input offsets not provided
      dim3(1),
      dim3(THREADS_PER_BLOCK),
      args0,
      0,
      stream);

  // CTA Tuning
  // Naive for now, use 1 block per expert.
  // Total number of blocks is limited to 64 (intra-node) or 8 (inter-node).
  int num_blocks = std::min(world_size * ne, world_size > 8 ? 8 : 64);

  // Stride at dim 0
  size_t stride_bytes = input.stride(0) * input.element_size();
  bool rank_is_row_out = !rank_is_row_in;

  // All to all data exchange
  void* args1[] = {
      &input_ptr,
      &output_ptr,
      &in_splits_ptr,
      &out_splits_offsets_ptr,
      &stride_bytes,
      &world_size,
      &ne,
      &major_align_val,
      &rank_is_row_out,
      &team};
  nvshmemx_collective_launch(
      (const void*)allToAllV_2d,
      dim3(num_blocks),
      dim3(THREADS_PER_BLOCK),
      args1,
      0,
      stream);
}

void all_to_all_vdev_2d_offset(
    at::Tensor& input,
    at::Tensor& out,
    at::Tensor& in_splits_offsets,
    at::Tensor& out_splits_offsets,
    std::string group_name) {
  /* Perform a 2D AllToAllv shuffle operation, with input split and offset
   * information provided on device. The input offsets are not required to be
   * exact prefix sum of the input splits, i.e. paddings are allowed between the
   * split chunks. The paddings, however, will not be transferred to peer
   * ranks.

   * In Mixture of Experts models, this operation can be used to combine tokens
   * processed by experts on parallel ranks. This operation can be viewed as an
   * "reverse" operation to the `all_to_all_vdev_2d` operation (which shuffles
   * tokens to experts).

   * Arguments:
   *  - `input` is the input tensor
   *  - `out` is the output tensor
   *  - `in_splits_offsets` is a 2D tensor of size (2, `ne` * `world_size`). In the
        scenario of Mixture-of-Experts models, `ne` is the number of experts per
        rank. The rows of `in_splits_offsets` are (in order):
        input splits (IN) and
        input offsets (IN)
   *  - `out_splits_offsets` is a 2D tensor of size (2, `world_size` * `ne`). The
        rows are (in order):
        output splits (OUT) and
        output offsets (OUT).
   *  - `group_name` is the name of the group to use for the collective operation.
  */
  auto input_hdl = c10d::symmetric_memory::rendezvous(input, group_name);
  auto out_hdl = c10d::symmetric_memory::rendezvous(out, group_name);
  auto out_splits_offsets_hdl = c10d::symmetric_memory::rendezvous(out_splits_offsets, group_name);
  auto in_splits_offsets_hdl = c10d::symmetric_memory::rendezvous(in_splits_offsets, group_name);
  int rank = input_hdl->get_rank();
  int world_size = input_hdl->get_world_size();
  constexpr int NUM_TILES = THREADS_PER_BLOCK / A2AV_TILE_SIZE;
  TORCH_CHECK(world_size <= NUM_TILES, "world_size must be smaller than NUM_TILES", NUM_TILES);

  int64_t major_align_val = 0;

  void* input_ptr = input.data_ptr();
  void* output_ptr = out.mutable_data_ptr();
  int64_t* out_splits_offsets_ptr = (int64_t*)(out_splits_offsets.mutable_data_ptr());
  int64_t* in_splits_offsets_ptr = (int64_t*)(in_splits_offsets.data_ptr());

  // Shape checks
  TORCH_CHECK(out_splits_offsets.is_contiguous()
      && in_splits_offsets.is_contiguous()
      && input.is_contiguous()
      && out.is_contiguous(),
      "input, out, in_splits_offsets and out_splits_offsets must be contiguous");
  auto out_split_shape = out_splits_offsets.sizes();
  auto in_split_shape = in_splits_offsets.sizes();
  TORCH_CHECK(in_split_shape.size() == 2
      && in_split_shape[0] == 2
      && in_split_shape[1] % world_size == 0,
      "in_splits_offsets must be 2D with 2 rows, "
      "each row must be a multiple of world_size");

  // Consistency checks
  TORCH_CHECK(input.dtype() == out.dtype()
      && input.stride(0) == out.stride(0),
      "input and out must have the same dtype and same stride at dim 0");
  TORCH_CHECK(out_splits_offsets.scalar_type() == at::kLong
      && in_splits_offsets.scalar_type() == at::kLong,
      "splits and offsets must be int64");

  // Number of experts per rank
  int ne = in_split_shape[1] / world_size;
  // TODO: number of experts is currently limited by the number of elements in a WarpScan.
  TORCH_CHECK(ne <= A2AV_TILE_SIZE, "Number of experts must be smaller than A2AV_TILE_SIZE", A2AV_TILE_SIZE);

  // Set device context for getting the stream and launching kernels below
  auto device = input.device();
  TORCH_CHECK(device.type() == at::DeviceType::CUDA &&
      out.device() == device &&
      in_splits_offsets.device() == device &&
      out_splits_offsets.device() == device,
      "all tensor arguments must be on the same CUDA device");
  c10::cuda::CUDAGuard guard(device);
  auto stream = at::cuda::getCurrentCUDAStream();
  auto& team_manager = TeamManager::get(device);
  auto team = team_manager.get_team(group_name, input_hdl->get_rank_to_global_rank());

  // Exchange output splits and source offsets
  auto input_dim0 = input.size(0);
  bool rank_is_row_in = false;
  // Use collective launch because kernel involves nvshmem barrier
  void* args0[] = {
      &in_splits_offsets_ptr,
      &out_splits_offsets_ptr,
      &team,
      &ne,
      &input_dim0,
      &rank_is_row_in};
  nvshmemx_collective_launch(
      (const void*)exchangeSplitAndOffset_2d<true>,  // true: input offsets provided
      dim3(1),
      dim3(THREADS_PER_BLOCK),
      args0,
      0,
      stream);

  // CTA Tuning
  // Naive for now, use 1 block per expert.
  // Total number of blocks is limited to 64 (intra-node) or 8 (inter-node).
  int num_blocks = std::min(world_size * ne, world_size > 8 ? 8 : 64);

  // Stride at dim 0
  size_t stride_bytes = input.stride(0) * input.element_size();
  bool rank_is_row_out = !rank_is_row_in;

  // All to all data exchange
  void* args1[] = {
      &input_ptr,
      &output_ptr,
      &in_splits_offsets_ptr,
      &out_splits_offsets_ptr,
      &stride_bytes,
      &ne,
      &world_size,
      &major_align_val,
      &rank_is_row_out,
      &team};
  nvshmemx_collective_launch(
      (const void*)allToAllV_2d,
      dim3(num_blocks),
      dim3(THREADS_PER_BLOCK),
      args1,
      0,
      stream);
}

/* Tiled Communication */

using Shape2D = nvshmemx::shape<int64_t, int64_t>;
using Stride2D = nvshmemx::stride<int64_t, int64_t>;

template <typename T>
__global__ void tile_reduce_kernel(
    T* src_ptr, T* dst_ptr, Shape2D shape, Stride2D strides, int64_t root, nvshmem_team_t* teams) {
#ifndef _NVSHMEM_DEVICELIB_SUPPORTED
  CUDA_KERNEL_ASSERT_MSG(false, "SM arch unsupported for NVSHMEM");
#else
  int bid = blockIdx.x;
  auto team = teams[bid];
  CUDA_KERNEL_ASSERT(team != NVSHMEM_TEAM_INVALID && " invalid team\n");

  // Global tile shape
  auto [rows, cols] = shape;
  auto [stride0, stride1] = strides;

  // Divide rows among CUDA blocks
  auto rows_per_block = at::ceil_div(rows, (int64_t)gridDim.x);
  auto block_start_row = rows_per_block * bid;
  auto block_shape = nvshmemx::make_shape(std::min(rows_per_block, rows - block_start_row), cols);
  auto block_layout = nvshmemx::make_layout(block_shape, strides);

  // Start pointer of each block's sub-tile
  auto block_src_ptr = src_ptr + stride0 * block_start_row;
  auto block_dst_ptr = dst_ptr + stride0 * block_start_row;
  auto block_src_tensor = nvshmemx::Tensor(block_src_ptr, block_layout);
  auto block_dst_tensor = nvshmemx::Tensor(block_dst_ptr, block_layout);

  // Making these empty to avoid nvshmemx::tile_sum_reduce_block() from doing
  // additional range checks
  auto start_coord = nvshmemx::make_shape();
  auto boundary = nvshmemx::make_shape();

  // Use one-shot pull to reduce the tile
  uint64_t flag = 0;
  constexpr auto algo = nvshmemx::tile_coll_algo_t::NVLS_ONE_SHOT_PULL_NBI;
  nvshmemx::tile_sum_reduce_block<decltype(block_src_tensor), decltype(block_dst_tensor), decltype(boundary), algo>(
      team, block_src_tensor, block_dst_tensor, start_coord, boundary, root, flag /* unused */);

  // Wait for the operation to complete
  nvshmemx::tile_collective_wait<algo>(team, flag /* unused */);
#endif
}

#define AT_DISPATCH_CASE_CONVERT(enum_type, scalar_type, ...)               \
  case enum_type: {                                                         \
    AT_PRIVATE_CHECK_SELECTIVE_BUILD(enum_type);                            \
    using scalar_t = scalar_type;                                           \
    return __VA_ARGS__();                                                   \
  }

#define AT_DISPATCH_NVSHMEM_FLOATS(scalar_type, name, ...)                  \
  AT_DISPATCH_SWITCH(                                                       \
      scalar_type, name,                                                    \
      AT_DISPATCH_CASE_CONVERT(at::kBFloat16, __nv_bfloat16, __VA_ARGS__);  \
      AT_DISPATCH_CASE_CONVERT(at::kHalf, __half, __VA_ARGS__);             \
      AT_DISPATCH_CASE(at::kFloat, __VA_ARGS__));

void tile_reduce(
    at::Tensor& in_tile,
    at::Tensor& out_tile,
    int64_t root,
    std::string group_name,
    std::string reduce_op) {
  /* Perform a tile reduce operation on the input tensor, with the root rank
   * receiving the reduced tensor. */
  TORCH_CHECK(reduce_op == "sum", "tile_reduce: only sum is supported for now");
  TORCH_CHECK(in_tile.dim() == 2 && out_tile.dim() == 2, "Only 2D tensors are supported");
  TORCH_CHECK_EQ(in_tile.dtype(), out_tile.dtype());
  TORCH_CHECK_EQ(in_tile.sizes(), out_tile.sizes());
  TORCH_CHECK_EQ(in_tile.strides(), out_tile.strides());
  TORCH_CHECK_EQ(in_tile.device(), out_tile.device());

  auto device = in_tile.device();
  c10::cuda::CUDAGuard guard(device);
  auto hdl = c10d::symmetric_memory::rendezvous(in_tile, group_name);
  c10d::symmetric_memory::rendezvous(out_tile, group_name);

  // Ideally 16 bytes per thread
  int nblocks = at::ceil_div(
      in_tile.numel() * in_tile.element_size(),
      (int64_t)THREADS_PER_BLOCK * 16);
  nblocks = std::min(nblocks, 24);

  // Need one team per block
  auto& team_manager = TeamManager::get(device);
  auto [teams, teams_dev] = team_manager.get_n_teams(
      group_name, hdl->get_rank_to_global_rank(), nblocks);
  TORCH_CHECK(
      root < nvshmem_team_n_pes(teams[0]),
      "root must be smaller than group size");
  auto stream = at::cuda::getCurrentCUDAStream();

  // Prepare launch parameters
  auto shape = nvshmemx::make_shape(in_tile.sizes()[0], in_tile.sizes()[1]);
  auto stride = nvshmemx::make_stride(in_tile.strides()[0], in_tile.strides()[1]);
  auto src_ptr = in_tile.const_data_ptr();
  auto dst_ptr = out_tile.mutable_data_ptr();
  void* args[] = {
      &src_ptr,
      &dst_ptr,
      &shape,
      &stride,
      &root,
      &teams_dev};

  AT_DISPATCH_NVSHMEM_FLOATS(in_tile.scalar_type(), "tile_reduce", [&]() {
    nvshmemx_collective_launch(
        (const void*)tile_reduce_kernel<scalar_t>,
        dim3(nblocks),
        dim3(THREADS_PER_BLOCK),
        args,
        0,
        stream);
    C10_CUDA_KERNEL_LAUNCH_CHECK();
  });
}

/* Multi-tile Communication */

void multi_root_tile_reduce(
    at::ArrayRef<at::Tensor> in_tiles,
    at::Tensor& out_tile,
    at::ArrayRef<int64_t> roots,
    std::string group_name,
    std::string reduce_op) {
  /* Perform multiple tile reductions concurrently, with each tile reduced to a separate root.
   Args:
     - `in_tiles` is a list of input tensors.
     - `out_tile` is the output tensor.
     - `roots` is a list of root ranks corresponding to each input tile, in the same order. A rank cannot be a root more than once.
     - `group_name` is the name of the group to use for the collective operation.
     - `reduce_op` is the reduction operation to perform. Currently only "sum" is supported.
   */
  TORCH_CHECK(reduce_op == "sum", "tile_reduce: only sum is supported for now");
  TORCH_CHECK(out_tile.dim() == 2, "Only 2D tensors are supported");
  TORCH_CHECK(roots.size() == in_tiles.size(), "Number of roots must match number of tiles");

  // Get device and stream
  auto device = out_tile.device();
  c10::cuda::CUDAGuard guard(device);
  auto stream = at::cuda::getCurrentCUDAStream();

  // Rendezvous all tensors, and find the tile "I" need to reduce
  auto hdl = c10d::symmetric_memory::rendezvous(out_tile, group_name);
  int rank = hdl->get_rank();
  int world_size = hdl->get_world_size();
  int i = 0, my_tile_idx = 0, root = world_size;
  // Note: if there is no tile for the current rank, my_tile_idx will remain
  // initial value 0, and root will remain `world_size`. This is OK. In
  // `nvshmemx::tile_sum_reduce_block`, this rank would skip the reduction
  // operation, but would still participate in the barrier.
  for (auto& in_tile : in_tiles) {
    TORCH_CHECK(in_tile.dim() == 2, "Only 2D tensors are supported");
    c10d::symmetric_memory::rendezvous(in_tile, group_name);
    TORCH_CHECK(roots[i] < world_size && roots[i] >= 0, "Invalid root");
    if (roots[i] == rank) {
      TORCH_CHECK(root == world_size, "Each rank can only be a root once");
      my_tile_idx = i;
      root = rank;
    }
    i++;
  }

  // Ideally 16 bytes per thread
  int nblocks = at::ceil_div(
      out_tile.numel() * out_tile.element_size(),
      (int64_t)THREADS_PER_BLOCK * 16);
  nblocks = std::min(nblocks, 24);

  // Need one team per block
  auto& team_manager = TeamManager::get(device);
  auto [teams, teams_dev] = team_manager.get_n_teams(
      group_name, hdl->get_rank_to_global_rank(), nblocks);

  // Prepare launch parameters
  auto shape = nvshmemx::make_shape(out_tile.sizes()[0], out_tile.sizes()[1]);
  auto stride = nvshmemx::make_stride(out_tile.strides()[0], out_tile.strides()[1]);
  auto in_tile_ptr = in_tiles[my_tile_idx].const_data_ptr();
  auto out_tile_ptr = out_tile.mutable_data_ptr();

  void* args[] = {
      &in_tile_ptr,
      &out_tile_ptr,
      &shape,
      &stride,
      &root,
      &teams_dev};

  AT_DISPATCH_NVSHMEM_FLOATS(out_tile.scalar_type(), "multi_root_tile_reduce", [&]() {
    nvshmemx_collective_launch(
        (const void*)tile_reduce_kernel<scalar_t>,
        dim3(nblocks),
        dim3(THREADS_PER_BLOCK),
        args,
        0,
        stream);
    C10_CUDA_KERNEL_LAUNCH_CHECK();
  });
}

} // namespace c10d::nvshmem_extension


TORCH_LIBRARY_IMPL(symm_mem, CUDA, m) {
  m.impl("nvshmem_broadcast", c10d::nvshmem_extension::nvshmem_broadcast);
  m.impl("nvshmem_put", c10d::nvshmem_extension::nvshmem_put);
  m.impl("nvshmem_get", c10d::nvshmem_extension::nvshmem_get);
  m.impl("nvshmem_wait_for_signal", c10d::nvshmem_extension::nvshmem_wait_for_signal);
  m.impl("nvshmem_put_with_signal", c10d::nvshmem_extension::nvshmem_put_with_signal);
  m.impl("nvshmem_all_to_all", c10d::nvshmem_extension::nvshmem_all_to_all);
  m.impl("all_to_all_vdev", c10d::nvshmem_extension::all_to_all_vdev);
  m.impl("all_to_all_vdev_2d", c10d::nvshmem_extension::all_to_all_vdev_2d);
  m.impl("all_to_all_vdev_2d_offset", c10d::nvshmem_extension::all_to_all_vdev_2d_offset);
  m.impl("tile_reduce", c10d::nvshmem_extension::tile_reduce);
  m.impl("multi_root_tile_reduce", c10d::nvshmem_extension::multi_root_tile_reduce);
}
