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authorCassio Neri <cassio.neri@gmail.com>2021-06-23 00:36:16 +0300
committerThomas Gleixner <tglx@linutronix.de>2021-06-24 12:51:59 +0300
commit276010551664f73b6f1616dde471d6f0d63a73ba (patch)
tree36bdb68afbbc7088b00c1ea0c9c55e8693b16960 /kernel/time/timeconv.c
parent4e82d2e20f3b11f253bc5c6e92f05ed3694a1ae3 (diff)
downloadlinux-276010551664f73b6f1616dde471d6f0d63a73ba.tar.xz
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
Diffstat (limited to 'kernel/time/timeconv.c')
-rw-r--r--kernel/time/timeconv.c128
1 files changed, 70 insertions, 58 deletions
diff --git a/kernel/time/timeconv.c b/kernel/time/timeconv.c
index 62e3b46717a6..59b922c826e7 100644
--- a/kernel/time/timeconv.c
+++ b/kernel/time/timeconv.c
@@ -22,47 +22,16 @@
/*
* Converts the calendar time to broken-down time representation
- * Based on code from glibc-2.6
*
* 2009-7-14:
* Moved from glibc-2.6 to kernel by Zhaolei<zhaolei@cn.fujitsu.com>
+ * 2021-06-02:
+ * Reimplemented by Cassio Neri <cassio.neri@gmail.com>
*/
#include <linux/time.h>
#include <linux/module.h>
-
-/*
- * Nonzero if YEAR is a leap year (every 4 years,
- * except every 100th isn't, and every 400th is).
- */
-static int __isleap(long year)
-{
- return (year) % 4 == 0 && ((year) % 100 != 0 || (year) % 400 == 0);
-}
-
-/* do a mathdiv for long type */
-static long math_div(long a, long b)
-{
- return a / b - (a % b < 0);
-}
-
-/* How many leap years between y1 and y2, y1 must less or equal to y2 */
-static long leaps_between(long y1, long y2)
-{
- long leaps1 = math_div(y1 - 1, 4) - math_div(y1 - 1, 100)
- + math_div(y1 - 1, 400);
- long leaps2 = math_div(y2 - 1, 4) - math_div(y2 - 1, 100)
- + math_div(y2 - 1, 400);
- return leaps2 - leaps1;
-}
-
-/* How many days come before each month (0-12). */
-static const unsigned short __mon_yday[2][13] = {
- /* Normal years. */
- {0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334, 365},
- /* Leap years. */
- {0, 31, 60, 91, 121, 152, 182, 213, 244, 274, 305, 335, 366}
-};
+#include <linux/kernel.h>
#define SECS_PER_HOUR (60 * 60)
#define SECS_PER_DAY (SECS_PER_HOUR * 24)
@@ -77,9 +46,11 @@ static const unsigned short __mon_yday[2][13] = {
*/
void time64_to_tm(time64_t totalsecs, int offset, struct tm *result)
{
- long days, rem, y;
+ u32 u32tmp, day_of_century, year_of_century, day_of_year, month, day;
+ u64 u64tmp, udays, century, year;
+ bool is_Jan_or_Feb, is_leap_year;
+ long days, rem;
int remainder;
- const unsigned short *ip;
days = div_s64_rem(totalsecs, SECS_PER_DAY, &remainder);
rem = remainder;
@@ -103,27 +74,68 @@ void time64_to_tm(time64_t totalsecs, int offset, struct tm *result)
if (result->tm_wday < 0)
result->tm_wday += 7;
- y = 1970;
-
- while (days < 0 || days >= (__isleap(y) ? 366 : 365)) {
- /* Guess a corrected year, assuming 365 days per year. */
- long yg = y + math_div(days, 365);
-
- /* Adjust DAYS and Y to match the guessed year. */
- days -= (yg - y) * 365 + leaps_between(y, yg);
- y = yg;
- }
-
- result->tm_year = y - 1900;
-
- result->tm_yday = days;
-
- ip = __mon_yday[__isleap(y)];
- for (y = 11; days < ip[y]; y--)
- continue;
- days -= ip[y];
-
- result->tm_mon = y;
- result->tm_mday = days + 1;
+ /*
+ * The following algorithm is, basically, Proposition 6.3 of Neri
+ * and Schneider [1]. In a few words: it works on the computational
+ * (fictitious) calendar where the year starts in March, month = 2
+ * (*), and finishes in February, month = 13. This calendar is
+ * mathematically convenient because the day of the year does not
+ * depend on whether the year is leap or not. For instance:
+ *
+ * March 1st 0-th day of the year;
+ * ...
+ * April 1st 31-st day of the year;
+ * ...
+ * January 1st 306-th day of the year; (Important!)
+ * ...
+ * February 28th 364-th day of the year;
+ * February 29th 365-th day of the year (if it exists).
+ *
+ * After having worked out the date in the computational calendar
+ * (using just arithmetics) it's easy to convert it to the
+ * corresponding date in the Gregorian calendar.
+ *
+ * [1] "Euclidean Affine Functions and Applications to Calendar
+ * Algorithms". https://arxiv.org/abs/2102.06959
+ *
+ * (*) The numbering of months follows tm more closely and thus,
+ * is slightly different from [1].
+ */
+
+ udays = ((u64) days) + 2305843009213814918ULL;
+
+ u64tmp = 4 * udays + 3;
+ century = div64_u64_rem(u64tmp, 146097, &u64tmp);
+ day_of_century = (u32) (u64tmp / 4);
+
+ u32tmp = 4 * day_of_century + 3;
+ u64tmp = 2939745ULL * u32tmp;
+ year_of_century = upper_32_bits(u64tmp);
+ day_of_year = lower_32_bits(u64tmp) / 2939745 / 4;
+
+ year = 100 * century + year_of_century;
+ is_leap_year = year_of_century ? !(year_of_century % 4) : !(century % 4);
+
+ u32tmp = 2141 * day_of_year + 132377;
+ month = u32tmp >> 16;
+ day = ((u16) u32tmp) / 2141;
+
+ /*
+ * Recall that January 1st is the 306-th day of the year in the
+ * computational (not Gregorian) calendar.
+ */
+ is_Jan_or_Feb = day_of_year >= 306;
+
+ /* Convert to the Gregorian calendar and adjust to Unix time. */
+ year = year + is_Jan_or_Feb - 6313183731940000ULL;
+ month = is_Jan_or_Feb ? month - 12 : month;
+ day = day + 1;
+ day_of_year += is_Jan_or_Feb ? -306 : 31 + 28 + is_leap_year;
+
+ /* Convert to tm's format. */
+ result->tm_year = (long) (year - 1900);
+ result->tm_mon = (int) month;
+ result->tm_mday = (int) day;
+ result->tm_yday = (int) day_of_year;
}
EXPORT_SYMBOL(time64_to_tm);