linux-kernel - Re: [PATCH] mul_u64_u64_div_u64: Increase precision by conditionally swapping a and b

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Date: Sat, 2 Mar 2024 22:05:09 +0100
From: Marc Kleine-Budde <mkl@...gutronix.de>
To: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
Cc: Andrew Morton <akpm@...ux-foundation.org>, 
	Biju Das <biju.das.jz@...renesas.com>, linux-kernel@...r.kernel.org, kernel@...gutronix.de
Subject: Re: [PATCH] mul_u64_u64_div_u64: Increase precision by conditionally
 swapping a and b

On 02.03.2024 21:54:27, Uwe Kleine-König wrote:
> As indicated in the added comment, the algorithm works better if b is
> big. As multiplication is commutative, a and b can be swapped. Do this
> If a is bigger than b.
> 
> Signed-off-by: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
> ---
>  lib/math/div64.c | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/lib/math/div64.c b/lib/math/div64.c
> index 55a81782e271..baf6f8681907 100644
> --- a/lib/math/div64.c
> +++ b/lib/math/div64.c
> @@ -190,6 +190,23 @@ u64 mul_u64_u64_div_u64(u64 a, u64 b, u64 c)
>  
>  	/* can a * b overflow ? */
>  	if (ilog2(a) + ilog2(b) > 62) {
> +		/*
> +		 * Note that the algorithm after the if block below might loose
> +		 * some precision and the result is more exact for b > a. So
> +		 * exchange a and b if a is bigger than b.
> +		 *
> +		 * For example with a = 43980465100800, b = 100000000, c = 1000000000
> +		 * the below calculation doesn't modify b at all because div == 0
> +		 * and then shift becomes 45 + 26 - 62 = 9 and so the result
> +		 * becomes 4398035251080. However with a and b swapped the exact
> +		 * result is calculated (i.e. 4398046510080).
> +		 */
> +		if (a > b) {
> +			u64 tmp = a;
> +			a = b;
> +			b = tmp;

You can use swap() from linux/minmax.h here.

Marc

> +		}
> +
>  		/*
>  		 * (b * a) / c is equal to
>  		 *
> 
> base-commit: 1870cdc0e8dee32e3c221704a2977898ba4c10e8
> -- 
> 2.43.0
> 
> 
> 

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