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authorUwe Kleine-König <u.kleine-koenig@pengutronix.de>2024-03-03 10:24:09 +0100
committerAndrew Morton <akpm@linux-foundation.org>2024-03-12 13:09:22 -0700
commit8c86fb68ffcb0d8a5418e171c8d359d712912e9b (patch)
tree5d19b4a82cc54d1c036f9f855691fa7946bb56ca /lib
parent4bb7be96fc8871f37ff705e02443a0526fb4df8d (diff)
mul_u64_u64_div_u64: increase precision by conditionally swapping a and b
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>
Diffstat (limited to 'lib')
-rw-r--r--lib/math/div64.c15
1 files changed, 15 insertions, 0 deletions
diff --git a/lib/math/div64.c b/lib/math/div64.c
index 55a81782e271..191761b1b623 100644
--- a/lib/math/div64.c
+++ b/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, 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
*
* (b / c) * a +