linux-kernel - Re: [PATCH] common implementation of iterative div/mod

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Message-ID: <4823780D.2000406@goop.org>
Date:	Thu, 08 May 2008 23:00:45 +0100
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	segher@...nel.crashing.org, hancockr@...w.ca,
	lists@...dbynature.de, linux-kernel@...r.kernel.org, mingo@...e.hu,
	tglx@...utronix.de, johnstul@...ibm.com
Subject: Re: [PATCH] common implementation of iterative div/mod

Andrew Morton wrote:
> On Thu, 08 May 2008 16:16:41 +0100
> Jeremy Fitzhardinge <jeremy@...p.org> wrote:
>
>   
>> We have a few instances of the open-coded iterative div/mod loop, used
>> when we don't expcet the dividend to be much bigger than the divisor.
>>     
If you get a chance, could you fix the tpyo.

>> Unfortunately modern gcc's have the tendency to strength "reduce" this
>> into a full mod operation, which isn't necessarily any faster, and
>> even if it were, doesn't exist if gcc implements it in libgcc.
>>
>> The workaround is to put a dummy asm statement in the loop to prevent
>> gcc from performing the transformation.
>>
>> This patch creates a single implementation of this loop, and uses it
>> to replace the open-coded versions I know about.
>>     
>
> Fair enough.  I'll plan on feeding this into 2.6.26 soon.
>
>   
>>  #endif /* BITS_PER_LONG == 32 */
>> +
>> +/*
>> + * Iterative div/mod for use when dividend is not expected to be much
>> + * bigger than divisor.
>> + */
>> +unsigned iter_div_u64_rem(u64 dividend, u32 divisor, u64 *remainder)
>> +{
>> +	unsigned ret = 0;
>> +
>> +	while(dividend >= divisor) {
>> +		/* The following asm() prevents the compiler from
>> +		   optimising this loop into a modulo operation.  */
>> +		asm("" : "+rm"(dividend));
>> +
>> +		dividend -= divisor;
>> +		ret++;
>> +	}
>> +
>> +	*remainder = dividend;
>> +
>> +	return ret;
>> +}
>> +EXPORT_SYMBOL(iter_div_u64_rem);
>>
>>     
>
> I think it would be better to do s/unsigned/u32/ here.  It's cosmetic, but
> all this sort of code is pretty formal about the sizes of the types which
> it uses, and it sure needs to be.
>   

OK.

    J

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