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

[MichaÅ Nazarewicz]

> 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@xxxxxxxxxxxxxx> 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,  MichaÅ "mina86" Nazarewicz       (o o)
+----[mina86*mina86.com]---[mina86*jabber.org]----ooO--(_)--Ooo--
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/