From 8c86fb68ffcb0d8a5418e171c8d359d712912e9b Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Uwe=20Kleine-K=C3=B6nig?= Date: Sun, 3 Mar 2024 10:24:09 +0100 Subject: mul_u64_u64_div_u64: increase precision by conditionally swapping a and b MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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 Tested-by: Biju Das Signed-off-by: Andrew Morton --- lib/math/div64.c | 15 +++++++++++++++ 1 file changed, 15 insertions(+) (limited to 'lib') 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 #include #include +#include #include /* 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 * -- cgit v1.2.3