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

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Date:	Tue, 10 Aug 2010 09:39:46 +0200
From:	Michał Nazarewicz <m.nazarewicz@...sung.com>
To:	Michal Nazarewicz <mina86@...a86.com>,
	Denys Vlasenko <vda.linux@...glemail.com>
Cc:	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()

> 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.

>> +	 * if we can just call the _full version of this function.
>> +	 */
>> +	if (q > 9999)
>> +		return put_dec_full(buf, q);

-- 
Best regards,                                        _     _
| Humble Liege of Serenely Enlightened Majesty of  o' \,=./ `o
| Computer Science,  Michał "mina86" Nazarewicz       (o o)
+----[mina86*mina86.com]---[mina86*jabber.org]----ooO--(_)--Ooo--
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