C使用SSE指令来比较大量的int向量

我有一个巨大的向量< vector< int>> (18M×128).通常我想要2行这个向量,并通过这个功能进行比较：

int getDiff(int indx1,int indx2) {
    int result = 0;
    int pplus,pminus,tmp;

    for (int k = 0; k < 128; k += 2) {
        pplus = nodeL[indx2][k] - nodeL[indx1][k];
        pminus = nodeL[indx1][k + 1] - nodeL[indx2][k + 1];

        tmp = max(pplus,pminus);
        if (tmp > result) {
            result = tmp;
        }
    }
    return result;
}

如你所见,函数循环通过两个行向量进行一些减法,最后返回一个最大值.这个功能将被使用一百万次,所以我想知道是否可以通过SSE指令加速.我使用Ubuntu 12.04和gcc.

当然这是微型优化,但是如果你能提供一些帮助,那么这是有帮助的,因为我对SSE一无所知.提前致谢

基准测试：

int nofTestCases = 10000000;

    vector<int> nodeIds(nofTestCases);
    vector<int> goalNodeIds(nofTestCases);
    vector<int> results(nofTestCases);

    for (int l = 0; l < nofTestCases; l++) {
        nodeIds[l] = randomNodeID(18000000);
        goalNodeIds[l] = randomNodeID(18000000);
    }



    double time,result;

    time = timestamp();
    for (int l = 0; l < nofTestCases; l++) {
        results[l] = getDiff2(nodeIds[l],goalNodeIds[l]);
    }
    result = timestamp() - time;
    cout << result / nofTestCases << "s" << endl;

    time = timestamp();
    for (int l = 0; l < nofTestCases; l++) {
        results[l] = getDiff(nodeIds[l],goalNodeIds[l]);
    }
    result = timestamp() - time;
    cout << result / nofTestCases << "s" << endl;

哪里

int randomNodeID(int n) {
    return (int) (rand() / (double) (RAND_MAX + 1.0) * n);
}

/** Returns a timestamp ('now') in seconds (incl. a fractional part). */
inline double timestamp() {
    struct timeval tp;
    gettimeofday(&tp,NULL);
    return double(tp.tv_sec) + tp.tv_usec / 1000000.;
}

解决方法

FWIW我将一个纯粹的SSE版本(SSE4.1)放在一起,似乎比Core i7上的原始标量码快了20％

#include <smmintrin.h>

int getDiff_SSE(int indx1,int indx2)
{
    int result[4] __attribute__ ((aligned(16))) = { 0 };

    const int * const p1 = &nodeL[indx1][0];
    const int * const p2 = &nodeL[indx2][0];

    const __m128i vke = _mm_set_epi32(0,-1,-1);
    const __m128i vko = _mm_set_epi32(-1,0);

    __m128i vresult = _mm_set1_epi32(0);

    for (int k = 0; k < 128; k += 4)
    {
        __m128i v1,v2,vmax;

        v1 = _mm_loadu_si128((__m128i *)&p1[k]);
        v2 = _mm_loadu_si128((__m128i *)&p2[k]);
        v1 = _mm_xor_si128(v1,vke);
        v2 = _mm_xor_si128(v2,vko);
        v1 = _mm_sub_epi32(v1,vke);
        v2 = _mm_sub_epi32(v2,vko);
        vmax = _mm_add_epi32(v1,v2);
        vresult = _mm_max_epi32(vresult,vmax);
    }
    _mm_store_si128((__m128i *)result,vresult);
    return max(max(max(result[0],result[1]),result[2]),result[3]);
}