Index: docs/DesignDocs/ABIVersioning.rst
===================================================================
--- docs/DesignDocs/ABIVersioning.rst
+++ docs/DesignDocs/ABIVersioning.rst
@@ -15,3 +15,40 @@
 
 Any ABI-changing feature is placed under it's own macro, _LIBCPP_ABI_XXX, which is enabled
 based on the value of _LIBCPP_ABI_VERSION. _LIBCPP_ABI_UNSTABLE, if set, enables all features at once.
+
+========================
+Always_inline attributes
+========================
+
+To ensure ABI stability, libc++ attempts to tightly control the set of
+symbols exported from libc++.so.  This is achieved with a "trick" of
+marking all functions and class methods defined in the library headers
+on Linux/MacOSX with::
+
+  __attribute__((always_inline, visibility("hidden")))
+
+All direct calls to functions marked with the always_inline attribute
+are inlined and are never emitted as an external symbol
+reference. Hidden visibility removes the symbol from the shared
+library exports list.
+
+This approach has a number of drawbacks.
+
+* always_inline functions are not always inlined. Inlining is an
+  optimization, and there are multiple cases when a compiler can
+  decide against it.
+
+  * As of version 3.7, Clang can sometimes emit an **unreachable**
+    call to an always_inline function as an external symbol reference.
+  * Inlining may be impossible between functions with incompatible
+    attributes. For example, ``__attribute__((no_sanitize_address))``,
+    which suppresses AddressSanitizer instrumentation in a function,
+    prevents inlining of functions that do not have the same
+    attribute.
+
+* At -O0 optimization level indiscriminate inlining results in very
+  large stack frames. As a consequence, removing these attributes
+  speeds up libc++ test suite by 10%.
+
+These attributes are removed when building with
+LIBCXX_ABI_UNSTABLE=ON.
Index: include/__config
===================================================================
--- include/__config
+++ include/__config
@@ -31,6 +31,13 @@
 #define _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
 #endif
 
+#if defined(_LIBCPP_ABI_UNSTABLE)
+// The only purpose of the always_inline thing is ABI stability.
+// Disable it when building for unstable ABI.
+#define _LIBCPP_INLINE_VISIBILITY
+#define _LIBCPP_ALWAYS_INLINE
+#endif
+
 #define _LIBCPP_CONCAT1(_LIBCPP_X,_LIBCPP_Y) _LIBCPP_X##_LIBCPP_Y
 #define _LIBCPP_CONCAT(_LIBCPP_X,_LIBCPP_Y) _LIBCPP_CONCAT1(_LIBCPP_X,_LIBCPP_Y)