3 files changed, 63 insertions, 4 deletions
diff --git a/libcxx/docs/Hardening.rst b/libcxx/docs/Hardening.rst
index 42aacfd..d399b94 100644
--- a/libcxx/docs/Hardening.rst
+++ b/libcxx/docs/Hardening.rst
@@ -311,7 +311,10 @@ ABI configuration.
 ABI options
 -----------
 
-Vendors can use the following ABI options to enable additional hardening checks:
+Vendors can use some ABI options at CMake configuration time (when building libc++
+itself) to enable additional hardening checks. This is done by passing these
+macros as ``-DLIBCXX_ABI_DEFINES="_LIBCPP_ABI_FOO;_LIBCPP_ABI_BAR;etc"`` at
+CMake configuration time. The available options are:
 
 - ``_LIBCPP_ABI_BOUNDED_ITERATORS`` -- changes the iterator type of select
   containers (see below) to a bounded iterator that keeps track of whether it's
@@ -341,7 +344,7 @@ Vendors can use the following ABI options to enable additional hardening checks:
 
   ABI impact: changes the iterator type of ``vector`` (except ``vector<bool>``).
 
-- ``_LIBCPP_ABI_BOUNDED_UNIQUE_PTR``` -- tracks the bounds of the array stored inside
+- ``_LIBCPP_ABI_BOUNDED_UNIQUE_PTR`` -- tracks the bounds of the array stored inside
   a ``std::unique_ptr<T[]>``, allowing it to trap when accessed out-of-bounds. This
   requires the ``std::unique_ptr`` to be created using an API like ``std::make_unique``
   or ``std::make_unique_for_overwrite``, otherwise the bounds information is not available
@@ -407,7 +410,7 @@ Hardened containers status
       - ✅
       - ❌
     * - ``forward_list``
-      - ❌
+      - ✅
       - ❌
     * - ``deque``
       - ✅
@@ -458,7 +461,7 @@ Hardened containers status
       - Partial
       - N/A
     * - ``bitset``
-      - ❌
+      - ✅
       - N/A
 
 Note: for ``vector`` and ``string``, the iterator does not check for
diff --git a/libcxx/docs/ReleaseNotes/20.rst b/libcxx/docs/ReleaseNotes/20.rst
index c8a07fb..ecfbaa5 100644
--- a/libcxx/docs/ReleaseNotes/20.rst
+++ b/libcxx/docs/ReleaseNotes/20.rst
@@ -73,6 +73,39 @@ Improvements and New Features
   optimized, resulting in a performance improvement of up to 2x for trivial element types (e.g., `std::vector<int>`),
   and up to 3.4x for non-trivial element types (e.g., `std::vector<std::vector<int>>`).
 
+- On Windows, ``<system_error>``'s ``std::system_category`` is now distinct from ``std::generic_category``. The behavior
+  on other operating systems is unchanged.
+
+  On Windows -- unlike on Unix systems -- the libc and system APIs use distinct error codes. The libc functions return
+  ``errno.h`` error codes via the ``errno`` global, while Win32 API functions return ``winerror.h`` error codes via
+  ``GetLastError()``.
+
+  The C++ standard's ``std::error_code`` and ``std::error_category`` functionality was designed to support multiple
+  error domains, precisely in order to handle situations such as this. However, libc++ formerly treated
+  ``generic_category()`` and ``system_category()`` as equivalent, even on Windows. It now implements the intended split,
+  where ``system_category`` represents native ``winerror.h`` error codes, and ``generic_category`` represents libc error
+  codes (and, equivalently, ``std::errc::*`` errors).
+
+  This change enables code like ``std::error_code(GetLastError(), std::system_category()) ==
+  std::errc::invalid_argument`` to function as desired: constructing an ``error_code`` with the Windows error number in
+  the "system" category, and then mapping it to a generic code with ``error_condition``, for comparison with the
+  ``std::errc`` constant.
+
+  This is an incompatible change: ``std::error_code(ENOSYS, std::system_category()) ==
+  std::errc::function_not_supported`` would formerly have returned true, but now returns false on Windows. Code
+  providing a number from the ``errno.h`` domain should be migrated to construct a ``generic_category`` error_code,
+  instead. (E.g., use ``std::error_code(ENOSYS, std::generic_category())``). The new behavior matches MSVC.
+
+- On Windows, the ``std::filesystem`` library now returns the Win32 ``system_category`` error codes, where it's feasible
+  to do so. This allows interrogation and reporting of the original error code, which is useful if multiple Windows
+  errors map to a single generic error (such as with ``std::errc::no_such_file_or_directory``).
+
+  This is also a slightly-incompatible API change: code inspecting the raw integer value from the returned error_code
+  expecting an integer from ``generic_category`` (e.g. ``err.value() == ENOTDIR``) will not work as desired. Instead,
+  such code should use the comparison operators which implicitly handle eror mappings, ``err ==
+  std::errc::not_a_directory``, or use ``err.default_error_condition()`` to map to an ``error_condition``, and then test
+  its ``value()`` and ``category()``.
+
 Deprecations and Removals
 -------------------------
 
diff --git a/libcxx/docs/TestingLibcxx.rst b/libcxx/docs/TestingLibcxx.rst
index cf092fa..e98b96b 100644
--- a/libcxx/docs/TestingLibcxx.rst
+++ b/libcxx/docs/TestingLibcxx.rst
@@ -459,6 +459,29 @@ we only want to make sure they don't rot. Do not rely on the results of benchmar
 run through ``check-cxx`` for anything, instead run the benchmarks manually using
 the instructions for running individual tests.
 
+If you want to compare the results of different benchmark runs, we recommend using the
+``libcxx-compare-benchmarks`` helper tool. First, configure CMake in a build directory
+and run the benchmark:
+
+.. code-block:: bash
+
+  $ cmake -S runtimes -B <build1> [...]
+  $ libcxx/utils/libcxx-lit <build1> libcxx/test/benchmarks/string.bench.cpp --param optimization=speed
+
+Then, do the same for the second configuration you want to test. Use a different build
+directory for that configuration:
+
+.. code-block:: bash
+
+  $ cmake -S runtimes -B <build2> [...]
+  $ libcxx/utils/libcxx-lit <build2> libcxx/test/benchmarks/string.bench.cpp --param optimization=speed
+
+Finally, use ``libcxx-compare-benchmarks`` to compare both:
+
+.. code-block:: bash
+
+  $ libcxx/utils/libcxx-compare-benchmarks <build1> <build2> libcxx/test/benchmarks/string.bench.cpp
+
 .. _`Google Benchmark`: https://github.com/google/benchmark
 
 .. _testing-hardening-assertions: