cuda_toolkit/include/cub/block/block_raking_layout.cuh

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/**
 * @file
 * cub::BlockRakingLayout provides a conflict-free shared memory layout abstraction for warp-raking
 * across thread block data.
 */

#pragma once

#include <cub/config.cuh>

#if defined(_CCCL_IMPLICIT_SYSTEM_HEADER_GCC)
#  pragma GCC system_header
#elif defined(_CCCL_IMPLICIT_SYSTEM_HEADER_CLANG)
#  pragma clang system_header
#elif defined(_CCCL_IMPLICIT_SYSTEM_HEADER_MSVC)
#  pragma system_header
#endif // no system header

#include <cub/util_type.cuh>

CUB_NAMESPACE_BEGIN

/**
 * @brief BlockRakingLayout provides a conflict-free shared memory layout abstraction for 1D raking
 *        across thread block data.    ![](raking.png)
 *
 * @ingroup BlockModule
 *
 * @par Overview
 * This type facilitates a shared memory usage pattern where a block of CUDA
 * threads places elements into shared memory and then reduces the active
 * parallelism to one "raking" warp of threads for serially aggregating consecutive
 * sequences of shared items.  Padding is inserted to eliminate bank conflicts
 * (for most data types).
 *
 * @tparam T
 *   The data type to be exchanged.
 *
 * @tparam BLOCK_THREADS
 *   The thread block size in threads.
 *
 * @tparam LEGACY_PTX_ARCH
 *   <b>[optional]</b> Unused.
 */
template <
    typename    T,
    int         BLOCK_THREADS,
    int         LEGACY_PTX_ARCH = 0>
struct BlockRakingLayout
{
    //---------------------------------------------------------------------
    // Constants and type definitions
    //---------------------------------------------------------------------

    enum
    {
        /// The total number of elements that need to be cooperatively reduced
        SHARED_ELEMENTS = BLOCK_THREADS,

        /// Maximum number of warp-synchronous raking threads
        MAX_RAKING_THREADS = CUB_MIN(BLOCK_THREADS, CUB_WARP_THREADS(0)),

        /// Number of raking elements per warp-synchronous raking thread (rounded up)
        SEGMENT_LENGTH = (SHARED_ELEMENTS + MAX_RAKING_THREADS - 1) / MAX_RAKING_THREADS,

        /// Never use a raking thread that will have no valid data (e.g., when BLOCK_THREADS is 62 and SEGMENT_LENGTH is 2, we should only use 31 raking threads)
        RAKING_THREADS = (SHARED_ELEMENTS + SEGMENT_LENGTH - 1) / SEGMENT_LENGTH,

        /// Whether we will have bank conflicts (technically we should find out if the GCD is > 1)
        HAS_CONFLICTS = (CUB_SMEM_BANKS(0) % SEGMENT_LENGTH == 0),

        /// Degree of bank conflicts (e.g., 4-way)
        CONFLICT_DEGREE = (HAS_CONFLICTS) ?
            (MAX_RAKING_THREADS * SEGMENT_LENGTH) / CUB_SMEM_BANKS(0) :
            1,

        /// Pad each segment length with one element if segment length is not relatively prime to warp size and can't be optimized as a vector load
        USE_SEGMENT_PADDING = ((SEGMENT_LENGTH & 1) == 0) && (SEGMENT_LENGTH > 2),

        /// Total number of elements in the raking grid
        GRID_ELEMENTS = RAKING_THREADS * (SEGMENT_LENGTH + USE_SEGMENT_PADDING),

        /// Whether or not we need bounds checking during raking (the number of reduction elements is not a multiple of the number of raking threads)
        UNGUARDED = (SHARED_ELEMENTS % RAKING_THREADS == 0),
    };


    /**
     * @brief Shared memory storage type
     */
    struct __align__(16) _TempStorage
    {
        T buff[BlockRakingLayout::GRID_ELEMENTS];
    };

    /// Alias wrapper allowing storage to be unioned
    struct TempStorage : Uninitialized<_TempStorage> {};


    /**
     * @brief Returns the location for the calling thread to place data into the grid
     */
    static __device__ __forceinline__ T* PlacementPtr(
        TempStorage &temp_storage,
        unsigned int linear_tid)
    {
        // Offset for partial
        unsigned int offset = linear_tid;

        // Add in one padding element for every segment
        if (USE_SEGMENT_PADDING > 0)
        {
            offset += offset / SEGMENT_LENGTH;
        }

        // Incorporating a block of padding partials every shared memory segment
        return temp_storage.Alias().buff + offset;
    }


    /**
     * @brief Returns the location for the calling thread to begin sequential raking
     */
    static __device__ __forceinline__ T* RakingPtr(
        TempStorage &temp_storage,
        unsigned int linear_tid)
    {
        return temp_storage.Alias().buff + (linear_tid * (SEGMENT_LENGTH + USE_SEGMENT_PADDING));
    }
};

CUB_NAMESPACE_END