Re: Improved Swapping Method In sort.c

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.

--
Do what you love because life is too short for anything else.

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