In file stdlib/qsort.c, why is swapping elements code so complex?

[Amit]

> >
> > The complex logic you're complaining about *is* an improvement.  Lots of
> > effort has gone in to making qsort faster and more robust.
>
>
> There is always a tradeoff between speed and code complexity. I prefer
> code complexity when the performance gains are significant (more than
> 50% performance gain). Here, in qsort(), the complex code will not be
> much more efficient than swapping bytes using a temporary variable.
>


I did an experiment. Swapping integers is only around 13% faster than
swapping 4 bytes/characters one by one.

""""So, in my opinion, 13% performance improvement doesn't warrant
such complex swapping elements code in qsort() function.""""

The code that I wrote for the experiment is below:

---------------
swap_int.c
---------------

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

void swap_int(int *iptr, int *jptr)
{

    int temp = 0;

    temp = *iptr;
    *iptr = *jptr;
    *jptr = temp;

} // end of function swap_int()

int main(void)
{

    int i = 0;
    int j = 0;
    int k = 0;
    int max = INT_MAX/4;

    srand((unsigned int)(time(NULL)));

    for (k = 0; k < max; k++) {
        i = rand();
        j = rand();
        swap_int(&i, &j);
    } // end of for loop

} // end of function main()

-----------------
swap_char.c
-----------------

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

void swap_char(char *iptr, char *jptr)
{

    int a = 0;
    int num_bytes = sizeof(int);
    unsigned char temp = 0;

    for (a = 0; a < num_bytes; a++) {
        temp = iptr[a];
        iptr[a] = jptr[a];
        jptr[a] = temp;
    }

} // end of function swap_char()

int main(void)
{

    int i = 0;
    int j = 0;
    int k = 0;
    int max = INT_MAX/4;

    srand((unsigned int)(time(NULL)));

    for (k = 0; k < max; k++) {
        i = rand();
        j = rand();
        swap_char((char *)(&i), (char *)(&j));
    } // end of for loop

} // end of function main()

""""And gcc didn't optimize the functions away because I didn't use
any optimization flag. You can check yourself by generating the
assembly code of the programs that I gave above and you will see that
the functions are present in the assembly code.""""

I compiled the programs using the following command: gcc swap_int.c -o
swap_int.out (and similarly for swap_char.c).

I generated the assembly code of the programs using the following
command: gcc -S swap_int.c (and similarly for swap_char.c).

Amit