libstdc++: Improve performance of chrono::utc_clock::now()

We can use an array instead of a std::vector, and we can avoid the
binary search for the common case of a time point after the most recent
leap second. On one system where I tested this, utc_clock::now() now
takes about 16ns instead of 31ns.

libstdc++-v3/ChangeLog:

	* include/std/chrono (get_leap_second_info): Optimize.

diff --git a/libstdc++-v3/include/std/chrono b/libstdc++-v3/include/std/chrono
index 90b73f8198e434b6fdc4accca8d1314e6779b28c..2468023f6c5eec242cef185dd54c387a176c8841 100644 (file)
--- a/libstdc++-v3/include/std/chrono
+++ b/libstdc++-v3/include/std/chrono
@@ -2747,9 +2747,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       {
        if constexpr (is_same_v<_Duration, seconds>)
          {
-           // TODO move this function into the library and get leaps from tzdb.
-           vector<seconds::rep> __leaps
-           {
+           const seconds::rep __leaps[] {
                78796800, // 1 Jul 1972
                94694400, // 1 Jan 1973
               126230400, // 1 Jan 1974
@@ -2778,12 +2776,31 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
              1435708800, // 1 Jul 2015
              1483228800, // 1 Jan 2017
            };
+           // The list above is known to be valid until 2023-06-28 00:00:00 UTC
+           const seconds::rep __expires = 1687910400;
+           const seconds::rep __s = __ut.time_since_epoch().count();
+
+           const seconds::rep* __first = std::begin(__leaps);
+           const seconds::rep* __last = std::end(__leaps);
+
+           if (__s > __expires)
+             {
+               // TODO: use updated leap_seconds from tzdb
+#if 0
+               auto __db = get_tzdb_list().begin();
+               __first = __db->leap_seconds.data();
+               __last = __first + __db->leap_seconds.size();
+#endif
+             }
+
+           // Don't bother searching the list if we're after the last one.
+           if (__s > __last[-1])
+             return { false, seconds(__last - __first) };
 
-           auto __s = __ut.time_since_epoch().count();
-           auto __pos = std::upper_bound(__leaps.begin(), __leaps.end(), __s);
+           auto __pos = std::upper_bound(__first, __last, __s);
            return {
-             __pos != __leaps.begin() && __pos[-1] == __s,
-             seconds{__pos - __leaps.begin()}
+             __pos != begin(__leaps) && __pos[-1] == __s,
+             seconds{__pos - __first}
            };
          }
        else