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mul_u64_u64_div_u64: increase precision by conditionally swapping a and b

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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.

Link: https://lkml.kernel.org/r/20240303092408.662449-2-u.kleine-koenig@pengutronix.de
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Tested-by: Biju Das <biju.das.jz@bp.renesas.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Uwe Kleine-König 2024-03-03 10:24:09 +01:00 committed by Andrew Morton
parent 4bb7be96fc
commit 8c86fb68ff
1 changed files with 15 additions and 0 deletions
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15
lib/math/div64.c
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@ -22,6 +22,7 @@
#include <linux/export.h>
#include <linux/math.h>
#include <linux/math64.h>
#include <linux/minmax.h>
#include <linux/log2.h>
/* Not needed on 64bit architectures */
@ -190,6 +191,20 @@ 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 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)
swap(a, b);
/*
* (b * a) / c is equal to
*