Why are standard string functions faster than my custom string functions?

Question

I decided to find the speeds of 2 functions :

• strcmp - The standard comparison function defined in string.h
• xstrcmp- A function that has same parameters and does the same, just that I created it.

Here is my xstrcmp function :

int xstrlen(char *str)
{
    int i;
    for(i=0;;i++)
    {
        if(str[i]=='\0')
            break;
    }
    return i;
}

int xstrcmp(char *str1, char *str2)
{
    int i, k;
    if(xstrlen(str1)!=xstrlen(str2))
        return -1;
    k=xstrlen(str1)-1;
    for(i=0;i<=k;i++)
    {
        if(str1[i]!=str2[i])
            return -1;
    }
    return 0;
}

I didn't want to depend on strlen, since I want everything user-defined.

So, I found the results. strcmp did 364 comparisons per millisecond and my xstrcmp did just 20 comparisons per millisecond (atleast on my computer!)

Can anyone tell why this is so ? What does the xstrcmp function do to make itself so fast ?

Answer 1

if(xstrlen(str1)!=xstrlen(str2))    //computing length of str1
    return -1;                      
k=xstrlen(str1)-1;                  //computing length of str1 AGAIN!

You're computing the length of str1 TWICE. That is one reason why your function loses the game.

Also, your implemetation of xstrcmp is very naive compared to the ones defined in (most) Standard libraries. For example, your xstrcmp compares one byte at a time, when in fact it could compare multiple bytes in one go, taking advantage of proper alignment as well, or can do little preprocessing so as to align memory blocks, before actual comparison.