[libc++] Avoid template instantiations in the duration aliases (llvm#178182)

philnik777 · web-flow · commit f537b4e51822 · 2026-02-04T10:26:48.000+01:00
These templates are instantiated whenever `&lt;__chrono/duration.h&gt;` is
included, making these calculations quite costly. I also don't think
that folding the calculations decreases readability here (IMO it's
actually easier to read now), so I don't think there is a reason we
shouldn't do this.

`&lt;__chrono/duration.h&gt;` is currently (transitively) included in at least
(I stopped checking) `&lt;algorithm&gt;`, `&lt;atomic&gt;`, `&lt;barrier&gt;`, `&lt;chrono&gt;`,
`&lt;condition_variable&gt;`, `&lt;future&gt;`, `&lt;iomanip&gt;`, `&lt;ios&gt;`, `&lt;iostream&gt;`,
`&lt;istream&gt;`, `&lt;latch&gt;`, `&lt;locale&gt;`, `&lt;mutex&gt;`, `&lt;ostream&gt;`,
`&lt;semaphore&gt;`, `&lt;shared_mutex&gt;`, `&lt;syncstream&gt;` and `&lt;thread&gt;`
diff --git a/libcxx/include/__chrono/duration.h b/libcxx/include/__chrono/duration.h
@@ -291,13 +291,13 @@ typedef duration<long long, nano> nanoseconds;
 typedef duration<long long, micro> microseconds;
 typedef duration<long long, milli> milliseconds;
 typedef duration<long long > seconds;
-typedef duration< long, ratio< 60> > minutes;
-typedef duration< long, ratio<3600> > hours;
+typedef duration<long, ratio<60> > minutes;
+typedef duration<long, ratio<60 * 60> > hours;
 #if _LIBCPP_STD_VER >= 20
-typedef duration< int, ratio_multiply<ratio<24>, hours::period>> days;
-typedef duration< int, ratio_multiply<ratio<7>, days::period>> weeks;
-typedef duration< int, ratio_multiply<ratio<146097, 400>, days::period>> years;
-typedef duration< int, ratio_divide<years::period, ratio<12>>> months;
+typedef duration<int, ratio<60 * 60 * 24>> days;
+typedef duration<int, ratio<60 * 60 * 24 * 7>> weeks;
+typedef duration<int, ratio<static_cast<int>(365.2425 * 60 * 60 * 24)>> years;
+typedef duration<int, ratio<static_cast<int>(365.2425 * 60 * 60 * 24) / 12>> months;
 #endif
 // Duration ==
 
