cuda-samples/Samples/2_Concepts_and_Techniques/scalarProd/scalarProd_kernel.cuh

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#include <cooperative_groups.h>

namespace cg = cooperative_groups;

///////////////////////////////////////////////////////////////////////////////
// On G80-class hardware 24-bit multiplication takes 4 clocks per warp
// (the same as for floating point  multiplication and addition),
// whereas full 32-bit multiplication takes 16 clocks per warp.
// So if integer multiplication operands are  guaranteed to fit into 24 bits
// (always lie within [-8M, 8M - 1] range in signed case),
// explicit 24-bit multiplication is preferred for performance.
///////////////////////////////////////////////////////////////////////////////
#define IMUL(a, b) __mul24(a, b)

///////////////////////////////////////////////////////////////////////////////
// Calculate scalar products of VectorN vectors of ElementN elements on GPU
// Parameters restrictions:
// 1) ElementN is strongly preferred to be a multiple of warp size to
//    meet alignment constraints of memory coalescing.
// 2) ACCUM_N must be a power of two.
///////////////////////////////////////////////////////////////////////////////
#define ACCUM_N 1024
__global__ void scalarProdGPU(float *d_C, float *d_A, float *d_B, int vectorN, int elementN)
{
    // Handle to thread block group
    cg::thread_block cta = cg::this_thread_block();
    // Accumulators cache
    __shared__ float accumResult[ACCUM_N];

    ////////////////////////////////////////////////////////////////////////////
    // Cycle through every pair of vectors,
    // taking into account that vector counts can be different
    // from total number of thread blocks
    ////////////////////////////////////////////////////////////////////////////
    for (int vec = blockIdx.x; vec < vectorN; vec += gridDim.x) {
        int vectorBase = IMUL(elementN, vec);
        int vectorEnd  = vectorBase + elementN;

        ////////////////////////////////////////////////////////////////////////
        // Each accumulator cycles through vectors with
        // stride equal to number of total number of accumulators ACCUM_N
        // At this stage ACCUM_N is only preferred be a multiple of warp size
        // to meet memory coalescing alignment constraints.
        ////////////////////////////////////////////////////////////////////////
        for (int iAccum = threadIdx.x; iAccum < ACCUM_N; iAccum += blockDim.x) {
            float sum = 0;

            for (int pos = vectorBase + iAccum; pos < vectorEnd; pos += ACCUM_N)
                sum += d_A[pos] * d_B[pos];

            accumResult[iAccum] = sum;
        }

        ////////////////////////////////////////////////////////////////////////
        // Perform tree-like reduction of accumulators' results.
        // ACCUM_N has to be power of two at this stage
        ////////////////////////////////////////////////////////////////////////
        for (int stride = ACCUM_N / 2; stride > 0; stride >>= 1) {
            cg::sync(cta);

            for (int iAccum = threadIdx.x; iAccum < stride; iAccum += blockDim.x)
                accumResult[iAccum] += accumResult[stride + iAccum];
        }

        cg::sync(cta);

        if (threadIdx.x == 0)
            d_C[vec] = accumResult[0];
    }
}