nccl.h

/*************************************************************************
 * Copyright (c) 2015-2021, NVIDIA CORPORATION. All rights reserved.
 * Modifications Copyright (c) 2019-2023 Advanced Micro Devices, Inc. All rights reserved.
 * Modifications Copyright (c) Microsoft Corporation. Licensed under the MIT License.
 *
 * See LICENSE.txt for license information
 ************************************************************************/

#ifndef NCCL_H_
#define NCCL_H_

#include <hip/hip_runtime.h>
#include <hip/hip_fp16.h>

#define NCCL_MAJOR 2
#define NCCL_MINOR 18
#define NCCL_PATCH 1
#define NCCL_SUFFIX ""

#define NCCL_VERSION_CODE 21801
#define NCCL_VERSION(X,Y,Z) (((X) <= 2 && (Y) <= 8) ? (X) * 1000 + (Y) * 100 + (Z) : (X) * 10000 + (Y) * 100 + (Z))

#define RCCL_BFLOAT16 1
#define RCCL_GATHER_SCATTER 1
#define RCCL_ALLTOALLV 1

#ifdef __cplusplus
extern "C" {
#endif

/*! @brief Opaque handle to communicator */
#include <limits.h>
typedef struct ncclComm* ncclComm_t;
#define NCCL_COMM_NULL NULL

#define NCCL_UNIQUE_ID_BYTES 128
typedef struct { char internal[NCCL_UNIQUE_ID_BYTES]; } ncclUniqueId;

/*! @brief Error type */
typedef enum { ncclSuccess                 =  0,
               ncclUnhandledCudaError      =  1,
               ncclSystemError             =  2,
               ncclInternalError           =  3,
               ncclInvalidArgument         =  4,
               ncclInvalidUsage            =  5,
               ncclRemoteError             =  6,
               ncclInProgress              =  7,
               ncclNumResults              =  8 } ncclResult_t;

#define NCCL_CONFIG_UNDEF_INT INT_MIN
#define NCCL_CONFIG_UNDEF_PTR NULL
#define NCCL_SPLIT_NOCOLOR -1

/* Communicator configuration. Users can assign value to attributes to specify the
 * behavior of a communicator. */
typedef struct ncclConfig_v21700 {
  /* attributes that users should never touch. */
  size_t size;
  unsigned int magic;
  unsigned int version;
  /* attributes that users are able to customize. */
  int blocking;
  int cgaClusterSize;
  int minCTAs;
  int maxCTAs;
  const char *netName;
  int splitShare;
} ncclConfig_t;

/* Config initializer must be assigned to initialize config structure when it is created.
 * Not initialized config will result in NCCL error. */
#define NCCL_CONFIG_INITIALIZER {                                       \
  sizeof(ncclConfig_t), /* size */                                      \
  0xcafebeef,           /* magic */                                     \
  NCCL_VERSION(NCCL_MAJOR, NCCL_MINOR, NCCL_PATCH), /* version */       \
  NCCL_CONFIG_UNDEF_INT,                    /* blocking */              \
  NCCL_CONFIG_UNDEF_INT,                    /* cgaClusterSize */        \
  NCCL_CONFIG_UNDEF_INT,                    /* minCTAs */               \
  NCCL_CONFIG_UNDEF_INT,                    /* maxCTAs */               \
  NCCL_CONFIG_UNDEF_PTR,                    /* netName */               \
  NCCL_CONFIG_UNDEF_INT                     /* splitShare */            \
}

/*! @brief Return the NCCL_VERSION_CODE of the NCCL library in the supplied integer.
 *
 * @details This integer is coded with the MAJOR, MINOR and PATCH level of the
 * NCCL library
 */
ncclResult_t  ncclGetVersion(int *version);
/// @cond include_hidden
ncclResult_t pncclGetVersion(int *version);
/// @endcond

/*! @brief Generates an ID for ncclCommInitRank

    @details
    Generates an ID to be used in ncclCommInitRank. ncclGetUniqueId should be
    called once and the Id should be distributed to all ranks in the
    communicator before calling ncclCommInitRank.

    @param[in]
    uniqueId     ncclUniqueId*
                 pointer to uniqueId

*/
ncclResult_t  ncclGetUniqueId(ncclUniqueId* uniqueId);
/// @cond include_hidden
ncclResult_t pncclGetUniqueId(ncclUniqueId* uniqueId);
/// @endcond

/*! @brief Create a new communicator (multi thread/process version) with a configuration
 * set by users. */
ncclResult_t  ncclCommInitRankConfig(ncclComm_t* comm, int nranks, ncclUniqueId commId, int rank, ncclConfig_t* config);
/// @cond include_hidden
ncclResult_t pncclCommInitRankConfig(ncclComm_t* comm, int nranks, ncclUniqueId commId, int rank, ncclConfig_t* config);
/// @endcond

/*! @brief Creates a new communicator (multi thread/process version).

    @details
    rank must be between 0 and nranks-1 and unique within a communicator clique.
    Each rank is associated to a CUDA device, which has to be set before calling
    ncclCommInitRank.
    ncclCommInitRank implicitly syncronizes with other ranks, so it must be
    called by different threads/processes or use ncclGroupStart/ncclGroupEnd.

    @param[in]
    comm        ncclComm_t*
                communicator struct pointer
    */
ncclResult_t  ncclCommInitRank(ncclComm_t* comm, int nranks, ncclUniqueId commId, int rank);
/// @cond include_hidden
ncclResult_t pncclCommInitRank(ncclComm_t* comm, int nranks, ncclUniqueId commId, int rank);
/// @endcond

/*! @brief Creates a clique of communicators (single process version).
 *
 * @details This is a convenience function to create a single-process communicator clique.
 * Returns an array of ndev newly initialized communicators in comm.
 * comm should be pre-allocated with size at least ndev*sizeof(ncclComm_t).
 * If devlist is NULL, the first ndev HIP devices are used.
 * Order of devlist defines user-order of processors within the communicator.
 * */
ncclResult_t  ncclCommInitAll(ncclComm_t* comm, int ndev, const int* devlist);
/// @cond include_hidden
ncclResult_t pncclCommInitAll(ncclComm_t* comm, int ndev, const int* devlist);
/// @endcond

/*! @brief Finalize a communicator.
 * @details ncclCommFinalize flushes all issued communications,
 * and marks communicator state as ncclInProgress. The state will change to ncclSuccess
 * when the communicator is globally quiescent and related resources are freed; then,
 * calling ncclCommDestroy can locally free the rest of the resources (e.g. communicator
 * itself) without blocking. */
ncclResult_t  ncclCommFinalize(ncclComm_t comm);
/// @cond include_hidden
ncclResult_t pncclCommFinalize(ncclComm_t comm);
/// @endcond

/*! @brief Frees local resources associated with communicator object. */

ncclResult_t  ncclCommDestroy(ncclComm_t comm);
/// @cond include_hidden
ncclResult_t pncclCommDestroy(ncclComm_t comm);
/// @endcond

/*! @brief Frees resources associated with communicator object and aborts any operations
 * that might still be running on the device. */
ncclResult_t  ncclCommAbort(ncclComm_t comm);
/// @cond include_hidden
ncclResult_t pncclCommAbort(ncclComm_t comm);
/// @endcond

/*! @brief Creates one or more communicators from an existing one.
 * Ranks with the same color will end up in the same communicator.
 * Within the new communicator, key will be used to order ranks.
 * NCCL_SPLIT_NOCOLOR as color will indicate the rank will not be part of any group
 * and will therefore return a NULL communicator.
 * If config is NULL, the new communicator will inherit the original communicator's
 * configuration*/
ncclResult_t  ncclCommSplit(ncclComm_t comm, int color, int key, ncclComm_t *newcomm, ncclConfig_t* config);
/// @cond include_hidden
ncclResult_t pncclCommSplit(ncclComm_t comm, int color, int key, ncclComm_t *newcomm, ncclConfig_t* config);
/// @endcond

/* Returns a string for each error code. */
/*! @brief Returns a string for each error code. */
const char*  ncclGetErrorString(ncclResult_t result);
/// @cond include_hidden
const char* pncclGetErrorString(ncclResult_t result);
/// @endcond

/*! @brief Returns a human-readable message of the last error that occurred.
 * comm is currently unused and can be set to NULL
 */
const char*  ncclGetLastError(ncclComm_t comm);
/// @cond include_hidden
const char* pncclGetLastError(ncclComm_t comm);
/// @endcond

/* Checks whether the comm has encountered any asynchronous errors */
ncclResult_t  ncclCommGetAsyncError(ncclComm_t comm, ncclResult_t *asyncError);
/// @cond include_hidden
ncclResult_t pncclCommGetAsyncError(ncclComm_t comm, ncclResult_t *asyncError);
/// @endcond

/*! @brief Gets the number of ranks in the communicator clique. */
ncclResult_t  ncclCommCount(const ncclComm_t comm, int* count);
/// @cond include_hidden
ncclResult_t pncclCommCount(const ncclComm_t comm, int* count);
/// @endcond

/*! @brief Returns the rocm device number associated with the communicator. */
ncclResult_t  ncclCommCuDevice(const ncclComm_t comm, int* device);
/// @cond include_hidden
ncclResult_t pncclCommCuDevice(const ncclComm_t comm, int* device);
/// @endcond

/*! @brief Returns the user-ordered "rank" associated with the communicator. */
ncclResult_t  ncclCommUserRank(const ncclComm_t comm, int* rank);
/// @cond include_hidden
ncclResult_t pncclCommUserRank(const ncclComm_t comm, int* rank);
/// @endcond

/*! @brief Reduction operation selector */
/* Reduction operation selector */
typedef enum { ncclNumOps_dummy = 5 } ncclRedOp_dummy_t;
typedef enum { ncclSum        = 0,
               ncclProd       = 1,
               ncclMax        = 2,
               ncclMin        = 3,
               ncclAvg        = 4,
               /* ncclNumOps: The number of built-in ncclRedOp_t values. Also
                * serves as the least possible value for dynamic ncclRedOp_t's
                * as constructed by ncclRedOpCreate*** functions. */
               ncclNumOps     = 5,
               /* ncclMaxRedOp: The largest valid value for ncclRedOp_t.
                * It is defined to be the largest signed value (since compilers
                * are permitted to use signed enums) that won't grow
                * sizeof(ncclRedOp_t) when compared to previous NCCL versions to
                * maintain ABI compatibility. */
               ncclMaxRedOp   = 0x7fffffff>>(32-8*sizeof(ncclRedOp_dummy_t))
             } ncclRedOp_t;

/*! @brief Data types */
typedef enum { ncclInt8       = 0, ncclChar       = 0,
               ncclUint8      = 1,
               ncclInt32      = 2, ncclInt        = 2,
               ncclUint32     = 3,
               ncclInt64      = 4,
               ncclUint64     = 5,
               ncclFloat16    = 6, ncclHalf       = 6,
               ncclFloat32    = 7, ncclFloat      = 7,
               ncclFloat64    = 8, ncclDouble     = 8,
               ncclBfloat16   = 9,
               ncclNumTypes   = 10 } ncclDataType_t;

/*! @brief ncclScalarResidence_t: Location and dereferencing logic for scalar arguments. */
typedef enum {
  /* ncclScalarDevice: The scalar is in device-visible memory and will be
   * dereferenced while the collective is running. */
  ncclScalarDevice = 0,

  /* ncclScalarHostImmediate: The scalar is in host-visible memory and will be
   * dereferenced before the ncclRedOpCreate***() function returns. */
  ncclScalarHostImmediate = 1
} ncclScalarResidence_t;

/*! @brief ncclRedOpCreatePreMulSum
 * Creates a new reduction operator which pre-multiplies input values by a given
 * scalar locally before reducing them with peer values via summation. For use
 * only with collectives launched against *comm* and *datatype*. The
 * *residence* argument indicates how/when the memory pointed to by *scalar*
 * will be dereferenced. Upon return, the newly created operator's handle
 * is stored in *op*.
 */
ncclResult_t  ncclRedOpCreatePreMulSum(ncclRedOp_t *op, void *scalar, ncclDataType_t datatype, ncclScalarResidence_t residence, ncclComm_t comm);
/// @cond include_hidden
ncclResult_t pncclRedOpCreatePreMulSum(ncclRedOp_t *op, void *scalar, ncclDataType_t datatype, ncclScalarResidence_t residence, ncclComm_t comm);
/// @endcond

/*! @brief ncclRedOpDestroy
 * @details Destroys the reduction operator *op*. The operator must have been created by
 * ncclRedOpCreatePreMul with the matching communicator *comm*. An operator may be
 * destroyed as soon as the last NCCL function which is given that operator returns.
 */
ncclResult_t ncclRedOpDestroy(ncclRedOp_t op, ncclComm_t comm);
/// @cond include_hidden
ncclResult_t pncclRedOpDestroy(ncclRedOp_t op, ncclComm_t comm);
/// @endcond

/*
 * Collective communication operations
 *
 * Collective communication operations must be called separately for each
 * communicator in a communicator clique.
 *
 * They return when operations have been enqueued on the CUDA stream.
 *
 * Since they may perform inter-CPU synchronization, each call has to be done
 * from a different thread or process, or need to use Group Semantics (see
 * below).
 */

/*!
 * @brief Reduce
 *
 * @details Reduces data arrays of length count in sendbuff into recvbuff using op
 * operation.
 * recvbuff may be NULL on all calls except for root device.
 * root is the rank (not the CUDA device) where data will reside after the
 * operation is complete.
 *
 * In-place operation will happen if sendbuff == recvbuff.
 */
ncclResult_t  ncclReduce(const void* sendbuff, void* recvbuff, size_t count, ncclDataType_t datatype,
    ncclRedOp_t op, int root, ncclComm_t comm, hipStream_t stream);
/// @cond include_hidden
ncclResult_t pncclReduce(const void* sendbuff, void* recvbuff, size_t count, ncclDataType_t datatype,
    ncclRedOp_t op, int root, ncclComm_t comm, hipStream_t stream);
/// @endcond

/*! @brief (deprecated) Broadcast (in-place)
 *
 * @details Copies count values from root to all other devices.
 * root is the rank (not the CUDA device) where data resides before the
 * operation is started.
 *
 * This operation is implicitely in place.
 */
ncclResult_t  ncclBcast(void* buff, size_t count, ncclDataType_t datatype, int root,
    ncclComm_t comm, hipStream_t stream);
/// @cond include_hidden
ncclResult_t pncclBcast(void* buff, size_t count, ncclDataType_t datatype, int root,
    ncclComm_t comm, hipStream_t stream);
/// @endcond

/*! @brief Broadcast
 *
 * @details Copies count values from root to all other devices.
 * root is the rank (not the HIP device) where data resides before the
 * operation is started.
 *
 * In-place operation will happen if sendbuff == recvbuff.
 */
ncclResult_t  ncclBroadcast(const void* sendbuff, void* recvbuff, size_t count, ncclDataType_t datatype, int root,
    ncclComm_t comm, hipStream_t stream);
/// @cond include_hidden
ncclResult_t pncclBroadcast(const void* sendbuff, void* recvbuff, size_t count, ncclDataType_t datatype, int root,
    ncclComm_t comm, hipStream_t stream);
/// @endcond

/*! @brief All-Reduce
 *
 * @details Reduces data arrays of length count in sendbuff using op operation, and
 * leaves identical copies of result on each recvbuff.
 *
 * In-place operation will happen if sendbuff == recvbuff.
 */
ncclResult_t  ncclAllReduce(const void* sendbuff, void* recvbuff, size_t count,
    ncclDataType_t datatype, ncclRedOp_t op, ncclComm_t comm, hipStream_t stream);
/// @cond include_hidden
ncclResult_t pncclAllReduce(const void* sendbuff, void* recvbuff, size_t count,
    ncclDataType_t datatype, ncclRedOp_t op, ncclComm_t comm, hipStream_t stream);
/// @endcond

/*!
 * @brief Reduce-Scatter
 *
 * @details Reduces data in sendbuff using op operation and leaves reduced result
 * scattered over the devices so that recvbuff on rank i will contain the i-th
 * block of the result.
 * Assumes sendcount is equal to nranks*recvcount, which means that sendbuff
 * should have a size of at least nranks*recvcount elements.
 *
 * In-place operations will happen if recvbuff == sendbuff + rank * recvcount.
 */
ncclResult_t  ncclReduceScatter(const void* sendbuff, void* recvbuff,
    size_t recvcount, ncclDataType_t datatype, ncclRedOp_t op, ncclComm_t comm,
    hipStream_t stream);
/// @cond include_hidden
ncclResult_t pncclReduceScatter(const void* sendbuff, void* recvbuff,
    size_t recvcount, ncclDataType_t datatype, ncclRedOp_t op, ncclComm_t comm,
    hipStream_t stream);
/// @endcond

/*! @brief All-Gather
 *
 * @details Each device gathers sendcount values from other GPUs into recvbuff,
 * receiving data from rank i at offset i*sendcount.
 * Assumes recvcount is equal to nranks*sendcount, which means that recvbuff
 * should have a size of at least nranks*sendcount elements.
 *
 * In-place operations will happen if sendbuff == recvbuff + rank * sendcount.
 */
ncclResult_t  ncclAllGather(const void* sendbuff, void* recvbuff, size_t sendcount,
    ncclDataType_t datatype, ncclComm_t comm, hipStream_t stream);
/// @cond include_hidden
ncclResult_t pncclAllGather(const void* sendbuff, void* recvbuff, size_t sendcount,
    ncclDataType_t datatype, ncclComm_t comm, hipStream_t stream);
/// @endcond

/*! @brief Send
 *
 * @details Send data from sendbuff to rank peer.
 * Rank peer needs to call ncclRecv with the same datatype and the same count from this
 * rank.
 *
 * This operation is blocking for the GPU. If multiple ncclSend and ncclRecv operations
 * need to progress concurrently to complete, they must be fused within a ncclGroupStart/
 * ncclGroupEnd section.
 */
ncclResult_t  ncclSend(const void* sendbuff, size_t count, ncclDataType_t datatype, int peer,
    ncclComm_t comm, hipStream_t stream);
/// @cond include_hidden
ncclResult_t pncclSend(const void* sendbuff, size_t count, ncclDataType_t datatype, int peer,
    ncclComm_t comm, hipStream_t stream);
/// @endcond

/*! @brief Receive
 *
 * @details Receive data from rank peer into recvbuff.
 * Rank peer needs to call ncclSend with the same datatype and the same count to this
 * rank.
 *
 * This operation is blocking for the GPU. If multiple ncclSend and ncclRecv operations
 * need to progress concurrently to complete, they must be fused within a ncclGroupStart/
 * ncclGroupEnd section.
 */
ncclResult_t  ncclRecv(void* recvbuff, size_t count, ncclDataType_t datatype, int peer,
    ncclComm_t comm, hipStream_t stream);
/// @cond include_hidden
ncclResult_t pncclRecv(void* recvbuff, size_t count, ncclDataType_t datatype, int peer,
    ncclComm_t comm, hipStream_t stream);
/// @endcond

/*! @brief Gather
 *
 * @details Root device gathers sendcount values from other GPUs into recvbuff,
 * receiving data from rank i at offset i*sendcount.
 *
 * Assumes recvcount is equal to nranks*sendcount, which means that recvbuff
 * should have a size of at least nranks*sendcount elements.
 *
 * In-place operations will happen if sendbuff == recvbuff + rank * sendcount.
 */
ncclResult_t  ncclGather(const void* sendbuff, void* recvbuff, size_t sendcount,
    ncclDataType_t datatype, int root, ncclComm_t comm, hipStream_t stream);
/// @cond include_hidden
ncclResult_t pncclGather(const void* sendbuff, void* recvbuff, size_t sendcount,
    ncclDataType_t datatype, int root, ncclComm_t comm, hipStream_t stream);
/// @endcond

/*! @brief Scatter
 *
 * @details Scattered over the devices so that recvbuff on rank i will contain the i-th
 * block of the data on root.
 *
 * Assumes sendcount is equal to nranks*recvcount, which means that sendbuff
 * should have a size of at least nranks*recvcount elements.
 *
 * In-place operations will happen if recvbuff == sendbuff + rank * recvcount.
 */
ncclResult_t  ncclScatter(const void* sendbuff, void* recvbuff,
    size_t recvcount, ncclDataType_t datatype, int root, ncclComm_t comm,
    hipStream_t stream);
/// @cond include_hidden
ncclResult_t pncclScatter(const void* sendbuff, void* recvbuff,
    size_t recvcount, ncclDataType_t datatype, int root, ncclComm_t comm,
    hipStream_t stream);
/// @endcond

/*! @brief All-To-All
 *
 * @details Device (i) send (j)th block of data to device (j) and be placed as (i)th
 * block. Each block for sending/receiving has count elements, which means
 * that recvbuff and sendbuff should have a size of nranks*count elements.
 *
 * In-place operation will happen if sendbuff == recvbuff.
 */
ncclResult_t  ncclAllToAll(const void* sendbuff, void* recvbuff, size_t count,
    ncclDataType_t datatype, ncclComm_t comm, hipStream_t stream);
/// @cond include_hidden
ncclResult_t pncclAllToAll(const void* sendbuff, void* recvbuff, size_t count,
    ncclDataType_t datatype, ncclComm_t comm, hipStream_t stream);
/// @endcond

/*! @brief All-To-Allv
 *
 * @details Device (i) sends sendcounts[j] of data from offset sdispls[j]
 * to device (j). In the same time, device (i) receives recvcounts[j] of data
 * from device (j) to be placed at rdispls[j].

 * sendcounts, sdispls, recvcounts and rdispls are all measured in the units
 * of datatype, not bytes.
 *
 * In-place operation will happen if sendbuff == recvbuff.
 */
ncclResult_t  ncclAllToAllv(const void *sendbuff, const size_t sendcounts[],
    const size_t sdispls[], void *recvbuff, const size_t recvcounts[],
    const size_t rdispls[], ncclDataType_t datatype, ncclComm_t comm, hipStream_t stream);
/// @cond include_hidden
ncclResult_t pncclAllToAllv(const void *sendbuff, const size_t sendcounts[],
    const size_t sdispls[], void *recvbuff, const size_t recvcounts[],
    const size_t rdispls[], ncclDataType_t datatype, ncclComm_t comm, hipStream_t stream);
/// @endcond

/*! @brief Opaque handle to MSCCL algorithm */
typedef int mscclAlgoHandle_t;

/*! @brief MSCCL Load Algorithm
 *
 * @details Load MSCCL algorithm file specified in mscclAlgoFilePath and return
 * its handle via mscclAlgoHandle. This API is expected to be called by MSCCL
 * scheduler instead of end users.
 */
ncclResult_t  mscclLoadAlgo(const char *mscclAlgoFilePath, mscclAlgoHandle_t *mscclAlgoHandle, int rank);
ncclResult_t pmscclLoadAlgo(const char *mscclAlgoFilePath, mscclAlgoHandle_t *mscclAlgoHandle, int rank);

/*! @brief MSCCL Run Algorithm
 *
 * @details Run MSCCL algorithm specified by mscclAlgoHandle. The parameter
 * list merges all possible parameters required by different operations as this
 * is a general-purposed API. This API is expected to be called by MSCCL
 * scheduler instead of end users.
 */
ncclResult_t  mscclRunAlgo(
    const void* sendBuff, const size_t sendCounts[], const size_t sDisPls[],
    void* recvBuff, const size_t recvCounts[], const size_t rDisPls[],
    size_t count, ncclDataType_t dataType, int root, int peer, ncclRedOp_t op,
    mscclAlgoHandle_t mscclAlgoHandle, ncclComm_t comm, hipStream_t stream);
ncclResult_t pmscclRunAlgo(
    const void* sendBuff, const size_t sendCounts[], const size_t sDisPls[],
    void* recvBuff, const size_t recvCounts[], const size_t rDisPls[],
    size_t count, ncclDataType_t dataType, int root, int peer, ncclRedOp_t op,
    mscclAlgoHandle_t mscclAlgoHandle, ncclComm_t comm, hipStream_t stream);

/*! @brief MSCCL Load Algorithm
 *
 * @details Unload MSCCL algorithm previous loaded using its handle. This API
 * is expected to be called by MSCCL scheduler instead of end users.
 */
ncclResult_t  mscclUnloadAlgo(mscclAlgoHandle_t mscclAlgoHandle);
ncclResult_t pmscclUnloadAlgo(mscclAlgoHandle_t mscclAlgoHandle);

/*
 * Group semantics
 *
 * When managing multiple GPUs from a single thread, and since NCCL collective
 * calls may perform inter-CPU synchronization, we need to "group" calls for
 * different ranks/devices into a single call.
 *
 * Grouping NCCL calls as being part of the same collective operation is done
 * using ncclGroupStart and ncclGroupEnd. ncclGroupStart will enqueue all
 * collective calls until the ncclGroupEnd call, which will wait for all calls
 * to be complete. Note that for collective communication, ncclGroupEnd only
 * guarantees that the operations are enqueued on the streams, not that
 * the operation is effectively done.
 *
 * Both collective communication and ncclCommInitRank can be used in conjunction
 * of ncclGroupStart/ncclGroupEnd, but not together.
 *
 * Group semantics also allow to fuse multiple operations on the same device
 * to improve performance (for aggregated collective calls), or to permit
 * concurrent progress of multiple send/receive operations.
 */

/*! @brief Group Start
 *
 * Start a group call. All calls to NCCL until ncclGroupEnd will be fused into
 * a single NCCL operation. Nothing will be started on the CUDA stream until
 * ncclGroupEnd.
 */
ncclResult_t  ncclGroupStart();
/// @cond include_hidden
ncclResult_t pncclGroupStart();
/// @endcond

/*! @brief Group End
 *
 * End a group call. Start a fused NCCL operation consisting of all calls since
 * ncclGroupStart. Operations on the CUDA stream depending on the NCCL operations
 * need to be called after ncclGroupEnd.
 */
ncclResult_t  ncclGroupEnd();
/// @cond include_hidden
ncclResult_t pncclGroupEnd();
/// @endcond

#ifdef __cplusplus
} // end extern "C"
#endif

#endif // end include guard