linux-kernel - Re: [PATCH 1/4] lib: vsprintf: Optimize division by 10 for small integers.

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Date:	Sun, 23 Sep 2012 19:22:54 +0200
From:	Michal Nazarewicz <mpn@...gle.com>
To:	George Spelvin <linux@...izon.com>, vda.linux@...glemail.com
Cc:	hughd@...gle.com, linux-kernel@...r.kernel.org, linux@...izon.com
Subject: Re: [PATCH 1/4] lib: vsprintf: Optimize division by 10 for small integers.

On Fri, Aug 03 2012, George Spelvin <linux@...izon.com> wrote:
> Shrink the reciprocal approximations used in put_dec_full4
> based on the comments in put_dec_full9.
>
> Signed-off-by: George Spelvin <linux@...izon.com>
> Cc: Denys Vlasenko <vda.linux@...glemail.com>
> Cc: Michal Nazarewicz <mina86@...a86.com>

Have you verified that the comment is correct?

> ---
>  lib/vsprintf.c |    5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> I was looking over the code and noticed that the constants could be smaller.
>
> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> index c3f36d41..2f32fe8 100644
> --- a/lib/vsprintf.c
> +++ b/lib/vsprintf.c
> @@ -243,13 +243,14 @@ char *put_dec(char *buf, unsigned long long n)
>  
>  /* Second algorithm: valid only for 64-bit long longs */
>  
> +/* See comment in put_dec_full9 for choice of constants */
>  static noinline_for_stack
>  char *put_dec_full4(char *buf, unsigned q)
>  {
>  	unsigned r;
> -	r      = (q * 0xcccd) >> 19;
> +	r      = (q * 0xccd) >> 15;
>  	*buf++ = (q - 10 * r) + '0';
> -	q      = (r * 0x199a) >> 16;
> +	q      = (r * 0xcd) >> 11;
>  	*buf++ = (r - 10 * q)  + '0';
>  	r      = (q * 0xcd) >> 11;

If you are changing everything, this could also be changed to:

	r      = (q * 0x67) >> 10;

no?

>  	*buf++ = (q - 10 * r)  + '0';

-- 
Best regards,                                         _     _
.o. | Liege of Serenely Enlightened Majesty of      o' \,=./ `o
..o | Computer Science,  Michał “mina86” Nazarewicz    (o o)
ooo +----<email/xmpp: mpn@...gle.com>--------------ooO--(_)--Ooo--

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