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Minor optimization: instead of converting between days and years using

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loops that start in 1970, assume most conversions are going to be for recent
dates and use a precomputed number of days through the end of 2016.
This commit is contained in:
Ian Lepore 2017-07-14 18:36:15 +00:00
parent 932d14c6e7
commit 32cd61b793
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=320997
1 changed files with 23 additions and 4 deletions
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27
sys/kern/subr_clock.c
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@ -97,6 +97,13 @@ static const int month_days[12] = {
31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31
};
/*
* Optimization: using a precomputed count of days between POSIX_BASE_YEAR and a
* recent year avoids lots of needless loop iterations in conversion.
* recent_base_days is the number of days through the end of recent_base_year.
*/
static const int recent_base_year = 2016;
static const int recent_base_days = 17167;
/*
* This inline avoids some unnecessary modulo operations
@ -157,8 +164,14 @@ clock_ct_to_ts(struct clocktime *ct, struct timespec *ts)
* Compute days since start of time
* First from years, then from months.
*/
days = 0;
for (i = POSIX_BASE_YEAR; i < year; i++)
if (year > recent_base_year) {
i = recent_base_year;
days = recent_base_days;
} else {
i = POSIX_BASE_YEAR;
days = 0;
}
for (; i < year; i++)
days += days_in_year(i);
/* Months */
@ -188,8 +201,14 @@ clock_ts_to_ct(struct timespec *ts, struct clocktime *ct)
ct->dow = day_of_week(days);
/* Subtract out whole years, counting them in i. */
for (year = POSIX_BASE_YEAR; days >= days_in_year(year); year++)
/* Subtract out whole years. */
if (days >= recent_base_days) {
year = recent_base_year + 1;
days -= recent_base_days;
} else {
year = POSIX_BASE_YEAR;
}
for (; days >= days_in_year(year); year++)
days -= days_in_year(year);
ct->year = year;