[libcxx-commits] [libcxx] Reapply "[libc++] Optimize num_put integral functions" (#131613) (PR #133572)

[Nico Weber via libcxx-commits]

================
@@ -61,13 +61,21 @@ Improvements and New Features
 
 - Updated formatting library to Unicode 16.0.0.
 
+- The ``num_put::do_put`` integral overloads have been optimized, resulting in a performance improvement of up to 2.4x.
+
 Deprecations and Removals
 -------------------------
 
 - ``std::is_pod`` and ``std::is_pod_v`` are deprecated in C++20 and later.
 
 - ``std::is_trivial`` and ``std::is_trivial_v`` are deprecated in C++26 and later.
 
+Potentially breaking changes
+----------------------------
+
+- The implementation of ``num_put::do_put`` has been replaced to improve the performance, which can lead to different
+  output when printing pointers.
----------------
nico wrote:

It always leads to different output on Windows, right? libc++ never printed a `0x` prefix on pointers on Windows before, and now it always does (making it consistent with libc++ on other platforms, and inconsistent with Microsoft's STL as far as I know). Maybe the release notes could be more precise about this :)

https://github.com/llvm/llvm-project/pull/133572