diff --git a/libcxx/docs/UsingLibcxx.rst b/libcxx/docs/UsingLibcxx.rst
--- a/libcxx/docs/UsingLibcxx.rst
+++ b/libcxx/docs/UsingLibcxx.rst
@@ -429,3 +429,39 @@
 * ``identity::operator()``
 * ``to_integer``
 * ``to_underlying``
+
+
+Compiler intrinsics wrappers
+----------------------------
+
+The header ``__bits`` contains several compiler intrinsics wrapper functions.
+Unless specified otherwise those function are available with the following
+overloads:
+
+* ``unsigned``
+* ``unsigned long``
+* ``unsigned long long``
+
+All those functions are ``constexpr`` and don't throw exceptions.
+
+Starting with C++20 the Standard header ``<bit>`` is available. This header
+contains all functions available in  ``<__bits>``` under a different name.
+
+
+``__libcpp_ctz``
+~~~~~~~~~~~~~~~~
+
+Returns the number of trailing zero bits in the input.
+
+
+``__libcpp_clz``
+~~~~~~~~~~~~~~~~
+
+Returns the number of leading zero bits in the input.
+When the platform supports 128-bit values the overload ``__uint128_t`` is
+available, but not purely as an interinsic.
+
+``__libcpp_popcount``
+~~~~~~~~~~~~~~~~~~~~~
+
+Returns the number of bits set in the input.