2d_array_test.c

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

void *alloc_2d_array(size_t size, int length, int width) {
	int i;
	size_t offset = length * sizeof(void *);
	char **block = malloc(offset + size * length * width);
	*block = (char *)(block + offset);
	for (i = 1; i < length; i++)
		block[i] = block[i - 1] + size * width;
	return block;
}

#define N 0x2000

#define SHUF(i) ((rand() & 0x1fff) ^ i)

int main() {
	int *flat_arr = malloc(N * N * sizeof(int));
	int **ptr_arr = alloc_2d_array(sizeof(int), N, N);
	int i, j, sum;
	clock_t time;

	printf("N = %d\n", N);

	time = clock();
	for (i=0; i<N; i++)
		for(j=0; j<N; j++)
			sum += flat_arr[SHUF(i) * N + SHUF(j)];
	printf("Muladd time: %.4f\n", (double)(clock() - time) / CLOCKS_PER_SEC);

	time = clock();
	for (i=0; i<N; i++)
		for(j=0; j<N; j++)
			sum += ptr_arr[SHUF(i)][SHUF(j)];
	printf("Twoptr time: %.4f\n", (double)(clock() - time) / CLOCKS_PER_SEC);
	
	free(flat_arr);
	free(ptr_arr);
	exit(0);
}

stdout.txt

N = 8192
Muladd time: 1.7231
Twoptr time: 1.4388