Comparing 32- and 64-bit

Question

A few days ago I saw this question on Stack Overflow.

I have tried to make a little demo to show the speed advantage of using 64-bit memory operations compared to 32-bit ones. So I wrote this code:

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

#define MEGABYTES 1024 * 1024
#define MEMORY_SIZE 100
#define LOOPS_START 100
#define LOOPS_STOP 10001

int main (int argc, const char * argv[])
{
    clock_t start, stop;
    int64_t* big_pile = malloc(MEGABYTES * MEMORY_SIZE * sizeof(char));

    for (int loops = LOOPS_START; loops < LOOPS_STOP; loops += 100)
    {
        start = clock();

        for (int i = 0; i < loops; i++)
        {
            for (int pos = 0; pos < MEGABYTES * MEMORY_SIZE * sizeof(char) / sizeof(int64_t); pos++) {
                big_pile[pos] = big_pile[pos] ^ big_pile[pos];
            }
        }

        stop = clock();
        double elapsed = (double)(stop - start) / CLOCKS_PER_SEC * 1000;
        double average = elapsed / loops;
        printf("Loops:\t%i\tElapsed time:\t%f\tAverage:\t%f\n", loops, elapsed, average);
    }

    free(big_pile);
}

I compile it under XCode on a MacBook Air running OSX Lion. This allow me to change the build target to 32- or 64-bit. When I run the program (and verify that the program is actually running in the correct bit-mode) I see no real difference in time.

Is this the right way to measure performance difference between 32- and 64-bit? Is XCode/the compiler playing tricks on me? Is there anything else wrong?

Note Sorry for the C. C isn't my native tongue ;-)

---

Solution findings

Writing 100 MB memory executing in 64-bit mode using 64-bit integer pointers takes ~47 seconds. Exceuting in 32-bit mode using 64-bit integer pointers takes ~70 seconds. Executing in 32-bit mode using 32-bit integer pointers takes ~90 seconds.

Answer 1

You are using int64_t, so most of your code gets compiled into the same machine code regardless of the bitness of the target. Also, A^A = 0, so the compiler might be optimizing your entire for loop with a single call to memset with zero.

I would recommend that you move different values around instead of xoring the same values together, and I would recommend that you use int32_t for the 32-bit version, and int64_t for the 64-bit version.