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SubjectRe: [KERNEL]: Avoid divide in IS_ALIGN
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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@gondor.apana.org.au>
>>
>> Cheers,
>> --
>> Visit Openswan at http://www.openswan.org/
>> Email: Herbert Xu ~{PmV>HI~} <herbert@gondor.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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