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 13:14:37 -0800
From: Randy Dunlap <rdunlap@...radead.org>
To: Marc Kleine-Budde <mkl@...gutronix.de>,
 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 3/2/24 13:05, Marc Kleine-Budde wrote:
> 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

	s/loose/lose/

>> +		 * 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
>>
>>
>>
> 

-- 
#Randy