Index: libcxx/docs/DesignDocs/VisibilityMacros.rst
===================================================================
--- libcxx/docs/DesignDocs/VisibilityMacros.rst
+++ libcxx/docs/DesignDocs/VisibilityMacros.rst
@@ -61,6 +61,18 @@
   ABI, we should create a new _LIBCPP_HIDE_FROM_ABI_AFTER_XXX macro, and we can
   use it to start removing symbols from the ABI after that stable version.
 
+**_LIBCPP_HIDE_FROM_ABI_PER_TU**
+  This macro controls whether symbols hidden from the ABI are local to each
+  translation unit. When enabled, this means that translation units compiled
+  with different versions of libc++ can be linked together, since all non
+  ABI-facing functions are local to each translation unit. This allows static
+  archives built with different versions of libc++ to be linked together.
+
+  When the macro is not defined (the default), there is no guarantee that
+  translation units compiled with different versions of libc++ can interoperate.
+  However, this leads to code size improvements, since non ABI-facing functions
+  can be deduplicated across translation unit boundaries.
+
 **_LIBCPP_TYPE_VIS**
   Mark a type's typeinfo, vtable and members as having default visibility.
   This attribute cannot be used on class templates.
Index: libcxx/include/__config
===================================================================
--- libcxx/include/__config
+++ libcxx/include/__config
@@ -796,7 +796,11 @@
 #endif
 
 #ifndef _LIBCPP_HIDE_FROM_ABI
-#  define _LIBCPP_HIDE_FROM_ABI _LIBCPP_HIDDEN _LIBCPP_INTERNAL_LINKAGE
+#  ifdef _LIBCPP_HIDE_FROM_ABI_PER_TU
+#    define _LIBCPP_HIDE_FROM_ABI _LIBCPP_HIDDEN _LIBCPP_INTERNAL_LINKAGE
+#  else
+#    define _LIBCPP_HIDE_FROM_ABI _LIBCPP_HIDDEN _LIBCPP_ALWAYS_INLINE
+#  endif
 #endif
 
 #ifdef _LIBCPP_BUILDING_LIBRARY