Re: [PATCHv2 1/3] lib: vsprintf: optimised put_dec_trunc() and put_dec_full()

[Denys Vlasenko]

2010/8/10 MichaÅ Nazarewicz <m.nazarewicz@xxxxxxxxxxx>:
>> On Sunday 08 August 2010 21:29, Michal Nazarewicz wrote:
>>>
>>> + Â Â Â /*
>>> + Â Â Â Â* '(x * 0xcccd) >> 19' is an approximation of 'x / 10' that
>>> + Â Â Â Â* gives correct results for all x < 81920. ÂHowever, because
>>> + Â Â Â Â* intermediate result can be at most 32-bit we limit x to be
>>> + Â Â Â Â* 16-bit.
>>> + Â Â Â Â*
>>> + Â Â Â Â* Because of those, we check if we are dealing with a "big"
>>> + Â Â Â Â* number and if so, we make it smaller remembering to add to
>>> + Â Â Â Â* the most significant digit.
>>> + Â Â Â Â*/
>>> + Â Â Â if (q >= 50000) {
>>> + Â Â Â Â Â Â Â a Â= '5';
>>> + Â Â Â Â Â Â Â q -= 50000;
>>
>> ...
>>>
>>> + Â Â Â /*
>>> + Â Â Â Â* We need to check if q is < 65536 so we might as well check
>
> On Tue, 10 Aug 2010 05:17:48 +0200, Denys Vlasenko
> <vda.linux@xxxxxxxxxxxxxx> wrote:
>>
>> You meant "need to check if q is < 81920"?
>
> No. Â81920 is a 17 bit number and when we multiply it by 0xcccd we lose
> the most significant bit.
>  Therefore we cannot use the '(x * 0xcccd) >>
> 19' approximation for numbers which are higher then 65535.

No. All x up to (exclusive) 81920 can be multiplied by 0xcccd
and result still fits into 32 bits. Proof:

# printf "%x\n" $((81919 * 0xcccd))
ffff7333

-- 
vda
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