使用-ffast-math编译时AVX代码段错误?

时间:2016-04-29 15:50:29

标签: c linux gcc simd fast-math

我正在尝试使用GCC内置的simd支持编写几个内核。我有这个代码对AVX点产品内核进行基准测试:

#include <time.h>
#include <stdio.h>
#include <assert.h>
#include <stdint.h>
#include <stdlib.h>
#include <unistd.h>

// define rtdsc instruction
static __inline__ uint64_t tick(void) {
    uint32_t hi, lo;
    __asm__ __volatile__ ("rdtsc" : "=a"(lo), "=d"(hi));
    return ( (uint64_t)lo)|( ((uint64_t)hi)<<32 );
}

// AVX dot product 
float avx_dot(float* __restrict__ ans, float* __restrict__ A, float* __restrict__ B, int N, ssize_t nprod, ssize_t shift) {
    assert(N % 32 == 0 && "N not divisible by 32");
    const int VECTOR_SIZE = 8;

    typedef float vec
        __attribute__ ((vector_size (sizeof(float) * VECTOR_SIZE)));

    N /= VECTOR_SIZE;

    for (ssize_t ii=0; ii < nprod; ii++) {
        vec *Av = (vec*)A;
        vec *Bv = (vec*)(B + ii*shift);

        vec temp[4] = {0,0,0,0};
        for(int jj = 0; jj < N; jj += 4) {
            temp[0] += Av[jj+0] * Bv[jj+0];
            temp[1] += Av[jj+1] * Bv[jj+1];
            temp[2] += Av[jj+2] * Bv[jj+2];
            temp[3] += Av[jj+3] * Bv[jj+3];
        }

        union {
            vec   tempv;
            float tempf[VECTOR_SIZE];
        };

        tempv = temp[0] + temp[1] + temp[2] + temp[3];

        ans[ii] = 0;
        for(int jj = 0; jj < VECTOR_SIZE; ++jj) {
            ans[ii] += tempf[jj];
        }
    }
}

int main(int argc, const char *argv[]) {
    const ssize_t NITER   = 1000;
    const ssize_t DECIM   = atoi(argv[2]);
    const ssize_t DOTPROD = atoi(argv[3]);
    ssize_t size = atoi(argv[1]);

    float* A; posix_memalign((void**)&A, 128, size*sizeof(float));
    float* B; posix_memalign((void**)&B, 128, (size+(DOTPROD-1)*DECIM)*sizeof(float));

    srand(time(NULL));
    for (ssize_t ii=0; ii < size;                   ii++) A[ii] = rand();
    for (ssize_t ii=0; ii < size+(DOTPROD-1)*DECIM; ii++) B[ii] = rand();

    printf("# size: %i  nproducts: %i  shift: %i\n", size, DOTPROD, DECIM);
    printf("# iter  answer  cycles  seconds  samprate\n");
    float results[DOTPROD];
    for (ssize_t ii=0; ii < NITER; ii++) {
        uint64_t beg = tick();
        avx_dot(results, A, B, size, DOTPROD, DECIM);
        uint64_t end = tick();

        float ans = 0;
        for (ssize_t jj=0; jj < DOTPROD; jj++) {
            ans += results[jj];
        }

        double    CLOCK   = 3300e6; 
        uint64_t cycles   = end-beg;
        double   seconds  = (double)cycles/CLOCK;
        double   samprate = (size*DOTPROD)/seconds;

        printf("%-5zd %f %lli %.3e %e\n", ii, ans, (unsigned long long)cycles, seconds, samprate);
    }

    return 0;
}

奇怪的是,编译时使用:

g++ -O3 -march=corei7-avx dotprod.cc -ffast-math -o dotprod

我第一次访问avx_dot中的temp时遇到段错误。但是,编译时使用:

g++ -O3 -march=corei7-avx dotprod.cc -o dotprod

IE,没有-ffast-math,它运行正常。我很困惑,因为快速数学不应该影响内存访问我相信,所以我不知道段错误来自哪里。

我正在继续:

CentOS Linux release 7.2.1511
gcc version 4.8.5 20150623 (Red Hat 4.8.5-4) (GCC)

任何人都可以在他们的机器上确认这种行为,并对正在发生的事情有所了解吗?

1 个答案:

答案 0 :(得分:3)

我的随机猜测是数据对齐,考虑到它无法加载数据(失败指令是...... vmovaps(%rcx),%ymm4 ...%rcx = 0x603228而Bv位于0x603228,并且阅读文档该指令显示16字节对齐的要求。

进一步调查:

当Bv偏移8个字节到B时出现问题,由于这一行(并且AVX需要16byte对齐):

vec *Bv = (vec*)(B + ii*shift);


./dotprod-fast 64 10 10
A=0x1125080
B=0x1125200
# size: 64  nproducts: 10  shift: 10
# iter  answer  cycles  seconds  samprate
Av=0x1125080
Bv=0x1125200
Av=0x1125080
Bv=0x1125228
Segmentation fault (core dumped)