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

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Message-Id: <201008100517.48231.vda.linux@googlemail.com>
Date:	Tue, 10 Aug 2010 05:17:48 +0200
From:	Denys Vlasenko <vda.linux@...glemail.com>
To:	Michal Nazarewicz <mina86@...a86.com>
Cc:	linux-kernel@...r.kernel.org, m.nazarewicz@...sung.com,
	"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:
> -- Speed --------------------------------------------------------
> orig_put_dec_full   :   1.054570   1.356214   1.732636   1.725760  Original
> mod1_put_dec_full   :   1.000000   1.017216   1.255518   1.116559
> mod3_put_dec_full   :   1.018222   1.000000   1.000000   1.000000  Proposed
> 
> orig_put_dec_trunc  :   1.137903   1.216017   1.850478   1.662370  Original
> mod1_put_dec_trunc  :   1.000000   1.078154   1.355635   1.400637
> mod3_put_dec_trunc  :   1.025989   1.000000   1.000000   1.000000  Proposed
> -- Size ---------------------------------------------------------
> orig_put_dec_full   :   1.212766   1.310345   1.355372   1.355372  Original
> mod1_put_dec_full   :   1.021277   1.000000   1.000000   1.000000
> mod3_put_dec_full   :   1.000000   1.172414   1.049587   1.049587  Proposed
> 
> orig_put_dec_trunc  :   1.363636   1.317365   1.784000   1.784000  Original
> mod1_put_dec_trunc  :   1.181818   1.275449   1.400000   1.400000
> mod3_put_dec_trunc  :   1.000000   1.000000   1.000000   1.000000  Proposed

In my testing on Phenom II the speed gain is smaller,
but it is indeed faster. And smaller!


> +	/*
> +	 * '(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

You meant "need to check if q is < 81920"?

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

-- 
vda
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