linux-kernel - Re: [KERNEL]: Avoid divide in IS

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Message-ID: <Pine.LNX.4.61.0711210800140.25473@chaos.analogic.com>
Date:	Wed, 21 Nov 2007 08:06:36 -0500
From:	"linux-os \(Dick Johnson\)" <linux-os@...logic.com>
To:	"Herbert Xu" <herbert@...dor.apana.org.au>
Cc:	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>
Subject: Re: [KERNEL]: Avoid divide in IS_ALIGN


On Tue, 20 Nov 2007, Richard B. Johnson wrote:

> On Tue, 20 Nov 2007, Herbert Xu wrote:
>
>> Hi:
>> 
>> [KERNEL]: Avoid divide in IS_ALIGN
>> 
>> I was happy to discover the brand new IS_ALIGN macro and quickly
>> used it in my code.  To my dismay I found that the generated code
>> used division to perform the test.
>> 
>> This patch fixes it by changing the % test to an &.  This avoids
>> the division.
>> 
>> Signed-off-by: Herbert Xu <herbert@...dor.apana.org.au>
>> 
>> Cheers,
>> -- 
>> Visit Openswan at http://www.openswan.org/
>> Email: Herbert Xu ~{PmV>HI~} <herbert@...dor.apana.org.au>
>> Home Page: http://gondor.apana.org.au/~herbert/
>> PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
>> --
>> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
>> index 94bc996..39b3fa6 100644
>> --- a/include/linux/kernel.h
>> +++ b/include/linux/kernel.h
>> @@ -35,7 +35,7 @@ extern const char linux_proc_banner[];
>> #define ALIGN(x,a)		__ALIGN_MASK(x,(typeof(x))(a)-1)
>> #define __ALIGN_MASK(x,mask)	(((x)+(mask))&~(mask))
>> #define PTR_ALIGN(p, a)		((typeof(p))ALIGN((unsigned long)(p), 
>> (a)))
>> -#define IS_ALIGNED(x,a)		(((x) % ((typeof(x))(a))) == 0)
>> +#define IS_ALIGNED(x, a)		(((x) & ((typeof(x))(a) - 1)) == 0)
>> 
>
> Your macro modification is wrong.

There was a brain fart on the explaination of why it's wrong.
Here is a better explaination.

Executing this script.............

cat <<EOF >/tmp/xxx.c
#include <stdio.h>

#define IS_ALIGNED(x,a)		(((x) % ((typeof(x))(a))) == 0)
#define _IS_ALIGNED(x, a)	(((x) & ((typeof(x))(a) - 1)) == 0)

int main()
{
    int i;
    long p = 0x12345678;
    for(i=1; i< 0x11; i++)
        printf("Old = %d, new = %d\n", IS_ALIGNED(p, i), _IS_ALIGNED(p, i));
     return 0;
}
EOF
gcc -Wall -o /tmp/xxx /tmp/xxx.c
/tmp/xxx

... produces:
Old = 1, new = 1
Old = 1, new = 1
Old = 1, new = 1
Old = 1, new = 1
Old = 0, new = 1
Old = 1, new = 1
Old = 0, new = 1
Old = 1, new = 1
Old = 1, new = 0
Old = 0, new = 0
Old = 0, new = 0
Old = 1, new = 0
Old = 0, new = 0
Old = 0, new = 0
Old = 0, new = 0
Old = 0, new = 0

Which is clearly different.


Cheers,
Dick Johnson
Penguin : Linux version 2.6.22.1 on an i686 machine (5588.30 BogoMips).
My book : http://www.AbominableFirebug.com/
_


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