Re: Improved Swapping Method In sort.c

Sami Farin
2008-04-30T18:03:46Z

On Wed, Apr 30, 2008 at 19:33:42 +0200, Jan Engelhardt wrote:
>
> On Wednesday 2008-04-30 19:09, Soumyadip Das Mahapatra wrote:
>
> >Hello everybody,
> >The swapping method (in function void u32_swap() line no.. 14 to 16) in lib/sort.c can be improved by using the following code
> >*(u32 *)b ^= *(u32 *)a ^= *(u32 *)b ^= *(u32 *)a instead of the code given. This code
> >is not using third variable thus not consuming another memory. And I dont see any significance in
> >using *int size* argument. so the function should be
> >static void u32_swap(void *a, void *b)
> >{
> > *(u32 *)b ^= *(u32 *)a ^= *(u32 *)b ^= *(u32 *)a;
> >}
>
> That proposal looks like buggy.
>
> 19:32 yaguchi:/dev/shm > cat ui.c
> #include <stdint.h>
> #include <stdio.h>
>
> static void u32_swap(void *a, void *b)
> {
> *(uint32_t *)b ^= *(uint32_t *)a
> ^= *(uint32_t *)b
> ^= *(uint32_t *)a;
> }
>
> int main(void)
> {
> uint32_t x = 5, y = 7;
> printf("%u %u\n", x, y);
> u32_swap(&x, &y);
> printf("%u %u\n", x, y);
> }
> 19:32 yaguchi:/dev/shm > ./a.out
> 5 7
> 7 0

This happens because u32_swap has undefined behavior.

Try this instead:

static void u32_swap(void *a, void *b)
{
*(uint32_t *)a ^= *(uint32_t *)b;
*(uint32_t *)b ^= *(uint32_t *)a;
*(uint32_t *)a ^= *(uint32_t *)b;
}

But still, this change to Linux is not necessary,
it actually generates worse code.

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