linux-kernel - Re: [PATCHv2 1/3] lib: vsprintf: optimised put_dec_trunc() and put_dec

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Message-ID: <87eie6w0m4.fsf@erwin.mina86.com>
Date:	Wed, 11 Aug 2010 00:42:43 +0200
From:	Michal Nazarewicz <mina86@...a86.com>
To:	Denys Vlasenko <vda.linux@...glemail.com>
Cc:	Michał Nazarewicz <m.nazarewicz@...sung.com>,
	linux-kernel@...r.kernel.org,
	"Douglas W. Jones" <jones@...uiowa.edu>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCHv2 1/3] lib: vsprintf: optimised put_dec_trunc() and  put_dec_full()

Denys Vlasenko <vda.linux@...glemail.com> writes:

> 2010/8/10 Michał Nazarewicz <m.nazarewicz@...sung.com>:
>>> On Sunday 08 August 2010 21:29, Michal Nazarewicz wrote:
>>>>
>>>> +       /*
>>>> +        * '(x * 0xcccd) >> 19' is an approximation of 'x / 10' that
>>>> +        * gives correct results for all x < 81920.  However, because
>>>> +        * intermediate result can be at most 32-bit we limit x to be
>>>> +        * 16-bit.
>>>> +        *
>>>> +        * Because of those, we check if we are dealing with a "big"
>>>> +        * number and if so, we make it smaller remembering to add to
>>>> +        * the most significant digit.
>>>> +        */
>>>> +       if (q >= 50000) {
>>>> +               a  = '5';
>>>> +               q -= 50000;
>>>
>>> ...
>>>>
>>>> +       /*
>>>> +        * 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@...glemail.com> wrote:
>>>
>>> You meant "need to check if q is < 81920"?
>>
>> No.  81920 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

Turns out something else was a problem ((x * 13) >> 7 works for x <
69).  I'll update comments in the next version.

-- 
Best regards,                                         _     _
 .o. | Liege of Serenly Enlightened Majesty of      o' \,=./ `o
 ..o | Computer Science,  Michal "mina86" Nazarewicz   (o o)
 ooo +--<mina86-tlen.pl>--<jid:mina86-jabber.org>--ooO--(_)--Ooo--
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