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Subject[tip: timers/core] time: Optimize ns_to_timespec64()
The following commit has been merged into the timers/core branch of tip:

Commit-ID: 20d087368d38c7350a4519a3b316ef7eb2504692
Gitweb: https://git.kernel.org/tip/20d087368d38c7350a4519a3b316ef7eb2504692
Author: Arnd Bergmann <arnd@arndb.de>
AuthorDate: Fri, 08 Nov 2019 21:34:25 +01:00
Committer: Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Tue, 12 Nov 2019 08:15:15 +01:00

time: Optimize ns_to_timespec64()

ns_to_timespec64() calls div_s64_rem(), which is a rather slow function on
32-bit architectures, as it cannot take advantage of the do_div()
optimizations for constant arguments.

Open-code the div_s64_rem() function in ns_to_timespec64(), so a constant
divider can be passed into the optimized div_u64_rem() function.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20191108203435.112759-3-arnd@arndb.de
---
kernel/time/time.c | 21 ++++++++++++---------
1 file changed, 12 insertions(+), 9 deletions(-)

diff --git a/kernel/time/time.c b/kernel/time/time.c
index 5c54ca6..45a3589 100644
--- a/kernel/time/time.c
+++ b/kernel/time/time.c
@@ -550,18 +550,21 @@ EXPORT_SYMBOL(set_normalized_timespec64);
*/
struct timespec64 ns_to_timespec64(const s64 nsec)
{
- struct timespec64 ts;
+ struct timespec64 ts = { 0, 0 };
s32 rem;

- if (!nsec)
- return (struct timespec64) {0, 0};
-
- ts.tv_sec = div_s64_rem(nsec, NSEC_PER_SEC, &rem);
- if (unlikely(rem < 0)) {
- ts.tv_sec--;
- rem += NSEC_PER_SEC;
+ if (likely(nsec > 0)) {
+ ts.tv_sec = div_u64_rem(nsec, NSEC_PER_SEC, &rem);
+ ts.tv_nsec = rem;
+ } else if (nsec < 0) {
+ /*
+ * With negative times, tv_sec points to the earlier
+ * second, and tv_nsec counts the nanoseconds since
+ * then, so tv_nsec is always a positive number.
+ */
+ ts.tv_sec = -div_u64_rem(-nsec - 1, NSEC_PER_SEC, &rem) - 1;
+ ts.tv_nsec = NSEC_PER_SEC - rem - 1;
}
- ts.tv_nsec = rem;

return ts;
}
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