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@pengutronix.de>
---
Changes since v1:
- Make use of swap() (Thanks Marc)
- Fix a typo in a code comment (Thanks Randy)
- Fix a typo in the commit log (s/If/if/; noticed myself)
v1 got a Tested-by from Biju; I didn't add it here as the patch changed.
I'm optimistic that this v2 would pass his tests, too, but I don't wanna
assume stuff when adding tags.
Best regards
Uwe
lib/math/div64.c | 15 +++++++++++++++
1 file changed, 15 insertions(+)
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 */
@@ -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
*
base-commit: 1870cdc0e8dee32e3c221704a2977898ba4c10e8
--
2.43.0