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| author | Uwe Kleine-König <u.kleine-koenig@pengutronix.de> | 2024-03-03 10:24:09 +0100 |
|---|---|---|
| committer | Andrew Morton <akpm@linux-foundation.org> | 2024-03-12 13:09:22 -0700 |
| commit | 8c86fb68ffcb0d8a5418e171c8d359d712912e9b (patch) | |
| tree | 5d19b4a82cc54d1c036f9f855691fa7946bb56ca /lib | |
| parent | 4bb7be96fc8871f37ff705e02443a0526fb4df8d (diff) | |
| download | linux-stable-8c86fb68ffcb0d8a5418e171c8d359d712912e9b.tar.gz linux-stable-8c86fb68ffcb0d8a5418e171c8d359d712912e9b.tar.bz2 linux-stable-8c86fb68ffcb0d8a5418e171c8d359d712912e9b.zip | |
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.c | 15 |
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 + |