Messages in this thread |  | | | Date | Thu, 08 May 2008 23:00:45 +0100 | | From | Jeremy Fitzhardinge <> | | Subject | Re: [PATCH] common implementation of iterative div/mod | |
Andrew Morton wrote:
> On Thu, 08 May 2008 16:16:41 +0100> Jeremy Fitzhardinge <jeremy@goop.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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