linux-kernel - Re: Improved Swapping Method In sort.c

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Message-ID: <20080430180335.fcv7ft5lgbsbjmoj@m.safari.iki.fi>
Date:	Wed, 30 Apr 2008 21:03:35 +0300
From:	Sami Farin <safari-kernel@...ari.iki.fi>
To:	linux-kernel@...r.kernel.org
Subject: 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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