I find mktime cost too much time on __tz_convert which will read /etc/localtime every time to get current timezone.
However, if we assume timezone never change, if only take one call to __tz_convert to get current timezone, which I think can reduce much time.
However, I wonder how to achieve this? There are no field in std::tm that I can put my timezone into. And I do not want to re-implement mktime from scratch, because seems it has lots of corner case, such as DST.
EDIT
... OR is it safe to just call __mktime_internal with a special __localtime_r that does nothing?
EDIT2
I think I may wrong about why mktime is slow in my case.
In my benchmark, most time are cost in __tz_convert, for a lock __libc_lock_lock (tzset_lock);. So maybe it is not related to cat /etc/localtime?
