time: Improve performance of time64_to_tm()
The current implementation of time64_to_tm() contains unnecessary loops,
branches and look-up tables. The new one uses an arithmetic-based algorithm
appeared in [1] and is approximately 3x faster (YMMV).
The drawback is that the new code isn't intuitive and contains many 'magic
numbers' (not unusual for this type of algorithm). However, [1] justifies
all those numbers and, given this function's history, the code is unlikely
to need much maintenance, if any at all.
Add a KUnit test for it which checks every day in a 160,000 years interval
centered at 1970-01-01 against the expected result.
[1] Neri, Schneider, "Euclidean Affine Functions and Applications to
Calendar Algorithms". https://arxiv.org/abs/2102.06959
Signed-off-by: Cassio Neri <cassio.neri@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20210622213616.313046-1-cassio.neri@gmail.com
diff --git a/kernel/time/time_test.c b/kernel/time/time_test.c
new file mode 100644
index 0000000..341ebfa
--- /dev/null
+++ b/kernel/time/time_test.c
@@ -0,0 +1,98 @@
+// SPDX-License-Identifier: LGPL-2.1+
+
+#include <kunit/test.h>
+#include <linux/time.h>
+
+/*
+ * Traditional implementation of leap year evaluation.
+ */
+static bool is_leap(long year)
+{
+ return year % 4 == 0 && (year % 100 != 0 || year % 400 == 0);
+}
+
+/*
+ * Gets the last day of a month.
+ */
+static int last_day_of_month(long year, int month)
+{
+ if (month == 2)
+ return 28 + is_leap(year);
+ if (month == 4 || month == 6 || month == 9 || month == 11)
+ return 30;
+ return 31;
+}
+
+/*
+ * Advances a date by one day.
+ */
+static void advance_date(long *year, int *month, int *mday, int *yday)
+{
+ if (*mday != last_day_of_month(*year, *month)) {
+ ++*mday;
+ ++*yday;
+ return;
+ }
+
+ *mday = 1;
+ if (*month != 12) {
+ ++*month;
+ ++*yday;
+ return;
+ }
+
+ *month = 1;
+ *yday = 0;
+ ++*year;
+}
+
+/*
+ * Checks every day in a 160000 years interval centered at 1970-01-01
+ * against the expected result.
+ */
+static void time64_to_tm_test_date_range(struct kunit *test)
+{
+ /*
+ * 80000 years = (80000 / 400) * 400 years
+ * = (80000 / 400) * 146097 days
+ * = (80000 / 400) * 146097 * 86400 seconds
+ */
+ time64_t total_secs = ((time64_t) 80000) / 400 * 146097 * 86400;
+ long year = 1970 - 80000;
+ int month = 1;
+ int mdday = 1;
+ int yday = 0;
+
+ struct tm result;
+ time64_t secs;
+ s64 days;
+
+ for (secs = -total_secs; secs <= total_secs; secs += 86400) {
+
+ time64_to_tm(secs, 0, &result);
+
+ days = div_s64(secs, 86400);
+
+ #define FAIL_MSG "%05ld/%02d/%02d (%2d) : %ld", \
+ year, month, mdday, yday, days
+
+ KUNIT_ASSERT_EQ_MSG(test, year - 1900, result.tm_year, FAIL_MSG);
+ KUNIT_ASSERT_EQ_MSG(test, month - 1, result.tm_mon, FAIL_MSG);
+ KUNIT_ASSERT_EQ_MSG(test, mdday, result.tm_mday, FAIL_MSG);
+ KUNIT_ASSERT_EQ_MSG(test, yday, result.tm_yday, FAIL_MSG);
+
+ advance_date(&year, &month, &mdday, &yday);
+ }
+}
+
+static struct kunit_case time_test_cases[] = {
+ KUNIT_CASE(time64_to_tm_test_date_range),
+ {}
+};
+
+static struct kunit_suite time_test_suite = {
+ .name = "time_test_cases",
+ .test_cases = time_test_cases,
+};
+
+kunit_test_suite(time_test_suite);