The patch titled
Subject: mul_u64_u64_div_u64: increase precision by conditionally swapping a and b
has been added to the -mm mm-nonmm-unstable branch. Its filename is
mul_u64_u64_div_u64-increase-precision-by-conditionally-swapping-a-and-b.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mul_u64_u64_div_u64-increase-precision-by-conditionally-swapping-a-and-b.patch
This patch will later appear in the mm-nonmm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxxxx>
Subject: mul_u64_u64_div_u64: increase precision by conditionally swapping a and b
Date: Sun, 3 Mar 2024 10:24:09 +0100
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@xxxxxxxxxxxxxx
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxxxx>
Tested-by: Biju Das <biju.das.jz@xxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
lib/math/div64.c | 15 +++++++++++++++
1 file changed, 15 insertions(+)
--- a/lib/math/div64.c~mul_u64_u64_div_u64-increase-precision-by-conditionally-swapping-a-and-b
+++ a/lib/math/div64.c
@@ -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 */
@@ -191,6 +192,20 @@ u64 mul_u64_u64_div_u64(u64 a, u64 b, u6
/* 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
*
* (b / c) * a +
_
Patches currently in -mm which might be from u.kleine-koenig@xxxxxxxxxxxxxx are
mul_u64_u64_div_u64-increase-precision-by-conditionally-swapping-a-and-b.patch
