多次元配列 in C

twoDimArray.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <assert.h>
#include <sys/time.h>

double getETime() {
    struct timeval tv;
    gettimeofday(&tv, NULL);
    return tv.tv_sec + (double) tv.tv_usec * 1e-6;
}

int main( int argv, char **argc ) {
    int NT = 10000;
    int NX = 1024;
    int NY = 1024;
    int x, y, t;
    double ts, te;
    
    
    // version1
    ts = getETime();
    double **u1, **p1;
    u1 = (double**)malloc(sizeof(double*)*(NY+2));
    p1 = (double**)malloc(sizeof(double*)*(NY+2));
    for (y = 0 ; y < NY+2; y++) {
        u1[y] = (double*)malloc(sizeof(double)*(NX+2));
        p1[y] = (double*)malloc(sizeof(double)*(NX+2));
    }
    for (y = 0; y < NY+2; y++) {
        for (x = 0; x < NX+2; x++) {
            u1[y][x] = x + y;
        }
    }
    for (t = 0; t < NT; t++) {
        for (y = 1; y < NY+1; y++) {
            for (x = 1; x < NX+1; x++) {
                p1[y][x] = u1[y][x] + u1[y+1][x] + u1[y-1][x] + u1[y][x+1] + u1[y][x-1];
            }
        }
    }
    te = getETime();
    double time1 = te - ts;
    printf("1: elapsed time %g\n", time1);
    
    
    // version2
    ts = getETime();
    double (*u2)[NX+2];
    double (*p2)[NX+2];
    u2 = (double(*)[NX+2])malloc(sizeof(double)*(NX+2)*(NY+2));
    p2 = (double(*)[NX+2])malloc(sizeof(double)*(NX+2)*(NY+2));
    for (y = 0; y < NY+2; y++) {
        for (x = 0; x < NX+2; x++) {
            u2[y][x] = x + y;
        }
    }
    for (t = 0; t < NT; t++) {
        for (y = 1; y < NY+1; y++) {
            for (x = 1; x < NX+1; x++) {
                p2[y][x] = u2[y][x] + u2[y+1][x] + u2[y-1][x] + u2[y][x+1] + u2[y][x-1];
            }
        }
    }
    te = getETime();
    double time2 = te - ts;
    printf("2: elapsed time %g\n", time2);
    
    
    // version3
    ts = getETime();
    double **u3, **p3;
    u3 = (double**)malloc(sizeof(double*)*(NY+2));
    p3 = (double**)malloc(sizeof(double*)*(NY+2));
    for (y = 0 ; y < NY+2; y++) {
        u3[y] = (double*)malloc(sizeof(double)*(NX+2));
        p3[y] = (double*)malloc(sizeof(double)*(NX+2));
    }
    for (y = 0; y < NY+2; y++) {
        for (x = 0; x < NX+2; x++) {
            u3[y][x] = x + y;
        }
    }
    for (t = 0; t < NT; t++) {
        for (y = 1; y < NY+1; y++) {
            for (x = 1; x < NX+1; x++) {
                p3[y][x] = u3[y][x] + u3[y+1][x] + u3[y-1][x] + u3[y][x+1] + u3[y][x-1];
            }
        }
    }
    te = getETime();
    double time3 = te - ts;
    printf("3: elapsed time %g\n", time3);
    
    
    // version4
    ts = getETime();
    int ind, ny;
    double *u4, *p4;
    u4 = (double*)malloc(sizeof(double)*(NY+2)*(NX+2));
    p4 = (double*)malloc(sizeof(double)*(NY+2)*(NX+2));
    for (y = 0; y < NY+2; y++) {
        ny = y * (NX+2);
        for (x = 0 ; x < NX+2; x++) {
            ind = ny + x;
            u4[ind] = x + y;
        }
    }
    int nyu;
    for (t = 0; t < NT; t++) {
        nyu = NX+2;
        for (y = 1; y < NY; y++) {
            for (x = 1; x < NX; x++) {
                ind = nyu + x;
                p4[ind] = u4[ind] + u4[ind+NX+2] + u4[ind-NX-2] + u4[ind+1] + u4[ind-1];
            }
            nyu += NX+2;
        }
    }
    te = getETime();
    double time4 = te - ts;
    printf("4: elapsed time %g\n", time4);
    
    
    // check
    nyu = NX+2;
    for (y = 1; y < NY; y++) {
        for (x = 1; x < NX; x++) {
            assert(p1[y][x] == p2[y][x]);
            assert(p1[y][x] == p3[y][x]);
            assert(p1[y][x] == p4[nyu+x]);
        }
        nyu += NX+2;
    }
    
    return 0;
}

version1: ポインタの配列へのポインタ（メモリはバラバラに確保）
version2: 配列へのポインタ
version3: ポインタの配列へのポインタ（メモリは一括確保）
version4: 一次元配列

1: elapsed time 21.5483
2: elapsed time 20.7368
3: elapsed time 21.0106
4: elapsed time 20.8277