ICC减少缓慢并产生错误的结果

Question

我正在尝试使用英特尔编译器（ICC）为Xeon Phi协处理器编写一个简单的缩减代码。但是，我的代码有两个问题：第一个问题是它产生错误的结果并且它比串行解决方案慢。我用这个选项编译了代码（mpi_reduce.c是文件名）：

icc mpi_reduce.c -V -openmp -o mpi_reduce.out

这是我的代码：

#include <omp.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/time.h>


float * _Cilk_shared data; //pointer to “shared” memory

_Cilk_shared float MIC_OMPReduction(int size)
{
    int i;
    #ifdef __MIC__
    float Result;
    int nThreads = 32;
    omp_set_num_threads(nThreads);
    #pragma omp parallel for reduction(+:Result)
    for (i=0; i<size; ++i)
    {
        Result += data[i];
    }
    return Result;
    #else
        printf("Intel(R) Xeon Phi(TM) Coprocessor not available\n");
    #endif
    return 0.0f;
}

float reduction_serial(int size)
{
    float ret = 0;
    int i;
    for ( i=0; i<size; ++i)
    {
        ret += data[i];
    }
    return ret;
}


int main()
{
    struct timeval tv1, tv2;
    int sec, usec;
    int i;
    float result_serial, result_parallel;
    size_t size = 1*1e6;
    int n_bytes = size*sizeof(float);
    data = (_Cilk_shared float *)_Offload_shared_malloc (n_bytes);
    printf("begin computation for size: %d \n",size);
    for (i=0; i<size; ++i)
    {
        data[i] = i%10;
    }

    gettimeofday(&tv1,NULL);
    result_serial = reduction_serial(size);
    gettimeofday(&tv2,NULL);
    sec = (int) (tv2.tv_sec-tv1.tv_sec);
    usec = (int) (tv2.tv_usec-tv1.tv_usec);
    if (usec < 0){
        sec--;
        usec += 1000000;
    }
    printf("reduction_serial: %f sec\n",sec+usec/1000000.0);
    printf("reduction_serial result = %f \n",result_serial);

    gettimeofday(&tv1,NULL);
    result_parallel = _Cilk_offload MIC_OMPReduction(size);
    gettimeofday(&tv2,NULL);
    sec = (int) (tv2.tv_sec-tv1.tv_sec);
    usec = (int) (tv2.tv_usec-tv1.tv_usec);
    if (usec < 0){
        sec--;
        usec += 1000000;
    }
    printf("reduction_parallel: %f sec\n",sec+usec/1000000.0);
    printf("reduction_parallel result = %f \n",result_parallel);

    _Offload_shared_free(data);

    return 0;
}

这是我代码的输出：

begin computation for size: 1000000
reduction_serial: 0.000239 sec
reduction_serial result = 4500000.000000
reduction_parallel: 0.461872 sec
reduction_parallel result = 4513334.000000

我注意到当我在没有-openmp选项的情况下编译代码时，并行代码的结果是正确的，但是使用-openmp选项时结果是错误的。