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

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Message-ID: <2c0942db0804301119r430b3b86t880437da1f7ce513@mail.gmail.com>
Date:	Wed, 30 Apr 2008 11:19:31 -0700
From:	"Ray Lee" <ray@...rabbit.org>
To:	"Soumyadip Das Mahapatra" <dip_kernel@...oo.co.in>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Improved Swaping Method In sort.c

On Wed, Apr 30, 2008 at 11:09 AM, Soumyadip Das Mahapatra
<dip_kernel@...oo.co.in> wrote:
> Thanks Jan,
>  Actually it should be
>  static void u32_swap(void *a, void *b)
>  {
>     *(u32 *)b ^= *(u32 *)a;
>     *(u32 *)a ^= *(u32 *)b;
>     *(u32 *)b ^= *(u32 *)a;
>  }
>  But this code saves memory equaling two integers. What about that ?

The above prevents gcc from storing the intermediary in a register. So
you're trading fast register access for slow memory access.

Really, you shouldn't be prosing optimizations unless you do two
things: check the output of gcc (on at least one platform, but really
should be several), and include timings of before/after.
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