linux-kernel - Re: [PATCH 3/4] lib: vsprintf: Optimize put_dec

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Date:	Sun, 23 Sep 2012 20:22:02 +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 3/4] lib: vsprintf: Optimize put_dec_trunc8

On Fri, Aug 03 2012, George Spelvin wrote:
> If you're going to have a conditional branch after
> each 32x32->64-bit multiply, might as well shrink the code
> and make it a loop.
>
> This also avoids using the long multiply for small integers.
>
> (This leaves the comments in a confusing state, but that's a separate
> patch to make review easier.)
>
> Signed-off-by: George Spelvin <linux@...izon.com>

NAK.

> ---
>  lib/vsprintf.c |   20 ++++++--------------
>  1 file changed, 6 insertions(+), 14 deletions(-)
>
> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> index a8e7392..3ca77b8 100644
> --- a/lib/vsprintf.c
> +++ b/lib/vsprintf.c
> @@ -174,20 +174,12 @@ char *put_dec_trunc8(char *buf, unsigned r)
>  	unsigned q;
>  
>  	/* Copy of previous function's body with added early returns */
> -	q      = (r * (uint64_t)0x1999999a) >> 32;
> -	*buf++ = (r - 10 * q) + '0'; /* 2 */
> -	if (q == 0)
> -		return buf;
> -	r      = (q * (uint64_t)0x1999999a) >> 32;
> -	*buf++ = (q - 10 * r) + '0'; /* 3 */
> -	if (r == 0)
> -		return buf;
> -	q      = (r * (uint64_t)0x1999999a) >> 32;
> -	*buf++ = (r - 10 * q) + '0'; /* 4 */
> -	if (q == 0)
> -		return buf;
> -	r      = (q * (uint64_t)0x1999999a) >> 32;
> -	*buf++ = (q - 10 * r) + '0'; /* 5 */
> +	while (r >= 10000) {
> +		q = r + '0';
> +		r  = (r * (uint64_t)0x1999999a) >> 32;
> +		*buf++ = q - 10*r;
> +	}

This loop looks nothing like the original code.  Why are you adding '0'
at the beginning?  Also, the original code switches the role of q and r,
the loop does not.

>  	if (r == 0)
>  		return buf;
>  	q      = (r * 0x199a) >> 16;

-- 
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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