Re: Re: Flat Short of MEMORY_BLOCK

Brian Heilig:

> Because:
> 
> buffer[i] = a_byte;
> 
> is faster than:
> 
> memcpy (&buffer[i], &a_byte, 1);

Huh? Why should it be? It's essentially doing the 
same thing, and a C compiler should know how to optimise 
memcpy! As usual, you must do benchmarks before making 
such claims:

[email protected]:~$$ gcc -O2 arraymemcpy.c ; ./a.out 
array 130
memcpy 130

Test code:

#include <string.h>
#include <sys/times.h>
 
#define MAX 100000
#define REPEAT 5000

void test(int how)
{
        int r;
        int i;
        char a_byte = 12;
        char buffer[MAX];

        if (!how)
        {
                for (r=0;r<REPEAT;r++)
                {
                        for (i=0;i<MAX;i++)
                        {
                                buffer[i] = a_byte;
                        }
                }
        }
        else
        {
                for (r=0;r<REPEAT;r++)
                {
                        for (i=0;i<MAX;i++)
                        {
                                memcpy (&buffer[i], &a_byte, 1);
                        }
                }
        }
}

main ()
{
        struct tms before, after;

        times (&before);
        test (0);
        times (&after);

        printf ("array %d\n", after.tms_utime - before.tms_utime);

        times (&before);
        test (1);
        times (&after);

        printf ("memcpy %d\n", after.tms_utime - before.tms_utime);

}






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