Index: CMakeLists.txt
===================================================================
--- CMakeLists.txt
+++ CMakeLists.txt
@@ -58,6 +58,8 @@
 option(LIBCXX_INSTALL_HEADERS "Install the libc++ headers." ON)
 option(LIBCXX_INSTALL_LIBRARY "Install the libc++ library." ON)
 option(LIBCXX_INSTALL_SUPPORT_HEADERS "Install libc++ support headers." ON)
+set(LIBCXX_ABI_VERSION 1 CACHE STRING "ABI version of libc++.")
+option(LIBCXX_ABI_UNSTABLE "Unstable ABI of libc++." OFF)
 
 # ABI Library options ---------------------------------------------------------
 set(LIBCXX_CXX_ABI "${LIBCXX_CXX_ABI}" CACHE STRING
@@ -298,6 +300,11 @@
 endif()
 
 # Configuration file flags =====================================================
+if (NOT LIBCXX_ABI_VERSION EQUAL "1")
+  config_define(${LIBCXX_ABI_VERSION} _LIBCPP_ABI_VERSION)
+endif()
+config_define_if(LIBCXX_ABI_UNSTABLE _LIBCPP_ABI_UNSTABLE)
+
 config_define_if_not(LIBCXX_ENABLE_GLOBAL_FILESYSTEM_NAMESPACE _LIBCPP_HAS_NO_GLOBAL_FILESYSTEM_NAMESPACE)
 config_define_if_not(LIBCXX_ENABLE_STDIN _LIBCPP_HAS_NO_STDIN)
 config_define_if_not(LIBCXX_ENABLE_STDOUT _LIBCPP_HAS_NO_STDOUT)
Index: cmake/Modules/HandleLibcxxFlags.cmake
===================================================================
--- cmake/Modules/HandleLibcxxFlags.cmake
+++ cmake/Modules/HandleLibcxxFlags.cmake
@@ -65,6 +65,12 @@
   endif()
 endmacro()
 
+macro(config_define value def)
+  set(${def} ${value})
+  add_definitions(-D${def}=${value})
+  set(LIBCXX_NEEDS_SITE_CONFIG ON)
+endmacro()
+
 # Add a specified list of flags to both 'LIBCXX_COMPILE_FLAGS' and
 # 'LIBCXX_LINK_FLAGS'.
 macro(add_flags)
Index: docs/Abi.rst
===================================================================
--- /dev/null
+++ docs/Abi.rst
@@ -0,0 +1,17 @@
+
+====================
+Libc++ ABI stability
+====================
+
+Libc++ aims to preserve stable ABI to avoid subtle bugs when code built to the old ABI
+is linked with the code build to the new ABI. At the same time, libc++ allows ABI-breaking
+improvements and bugfixes for the scenarios when ABI change is not a issue.
+
+To support both cases, libc++ allows specifying the ABI version at the
+build time.  The version is defined with a cmake option
+LIBCXX_ABI_VERSION. Another option LIBCXX_ABI_UNSTABLE can be used to
+include all present ABI breaking features. These options translate
+into C++ macro definitions _LIBCPP_ABI_VERSION, _LIBCPP_ABI_UNSTABLE.
+
+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.
Index: docs/BuildingLibcxx.rst
===================================================================
--- docs/BuildingLibcxx.rst
+++ docs/BuildingLibcxx.rst
@@ -193,6 +193,25 @@
 
   Build libc++ with run time type information.
 
+
+libc++ Feature options
+----------------------
+
+The following options allow building libc++ for a different ABI version.
+
+.. option:: LIBCXX_ABI_VERSION:STRING
+
+  **Default**: ``1``
+
+  Defines the target ABI version of libc++.
+
+.. option:: LIBCXX_ABI_UNSTABLE:BOOL
+
+  **Default**: ``OFF``
+
+  Build the "unstable" ABI version of libc++. Includes all ABI changing features
+  on top of the current stable version.
+
 .. _LLVM-specific variables:
 
 LLVM-specific options
Index: include/__config
===================================================================
--- include/__config
+++ include/__config
@@ -23,7 +23,13 @@
 
 #define _LIBCPP_VERSION 3800
 
+#ifndef _LIBCPP_ABI_VERSION
 #define _LIBCPP_ABI_VERSION 1
+#endif
+
+#if defined(_LIBCPP_ABI_UNSTABLE) || _LIBCPP_ABI_VERSION >= 2
+#define _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
+#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)
@@ -240,9 +246,12 @@
 
 #if defined(__clang__)
 
-#if defined(__APPLE__) && !defined(__i386__) && !defined(__x86_64__) &&        \
-    !defined(__arm__)
-#define _LIBCPP_ALTERNATE_STRING_LAYOUT
+// _LIBCPP_ALTERNATE_STRING_LAYOUT is an old name for
+// _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT left here for backward compatibility.
+#if (defined(__APPLE__) && !defined(__i386__) && !defined(__x86_64__) &&       \
+     !defined(__arm__)) ||                                                     \
+    defined(_LIBCPP_ALTERNATE_STRING_LAYOUT)
+#define _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
 #endif
 
 #if __has_feature(cxx_alignas)
Index: include/__config_site.in
===================================================================
--- include/__config_site.in
+++ include/__config_site.in
@@ -10,6 +10,8 @@
 #ifndef _LIBCPP_CONFIG_SITE
 #define _LIBCPP_CONFIG_SITE
 
+#cmakedefine _LIBCPP_ABI_VERSION @_LIBCPP_ABI_VERSION@
+#cmakedefine _LIBCPP_ABI_UNSTABLE
 #cmakedefine _LIBCPP_HAS_NO_GLOBAL_FILESYSTEM_NAMESPACE
 #cmakedefine _LIBCPP_HAS_NO_STDIN
 #cmakedefine _LIBCPP_HAS_NO_STDOUT
Index: include/string
===================================================================
--- include/string
+++ include/string
@@ -1185,7 +1185,7 @@
 #pragma warning( pop )
 #endif // _LIBCPP_MSVC
 
-#ifdef _LIBCPP_ALTERNATE_STRING_LAYOUT
+#ifdef _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
 
 template <class _CharT, size_t = sizeof(_CharT)>
 struct __padding
@@ -1198,7 +1198,7 @@
 {
 };
 
-#endif  // _LIBCPP_ALTERNATE_STRING_LAYOUT
+#endif  // _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
 
 template<class _CharT, class _Traits, class _Allocator>
 class _LIBCPP_TYPE_VIS_ONLY basic_string
@@ -1234,7 +1234,7 @@
 
 private:
 
-#ifdef _LIBCPP_ALTERNATE_STRING_LAYOUT
+#ifdef _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
 
     struct __long
     {
@@ -1294,7 +1294,7 @@
         value_type __data_[__min_cap];
     };
 
-#endif  // _LIBCPP_ALTERNATE_STRING_LAYOUT
+#endif  // _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
 
     union __ulx{__long __lx; __short __lxx;};
 
@@ -1698,7 +1698,7 @@
     const allocator_type& __alloc() const _NOEXCEPT
         {return __r_.second();}
 
-#ifdef _LIBCPP_ALTERNATE_STRING_LAYOUT
+#ifdef _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
 
     _LIBCPP_INLINE_VISIBILITY
     void __set_short_size(size_type __s) _NOEXCEPT
@@ -1716,7 +1716,7 @@
         {return __r_.first().__s.__size_;}
 #   endif
 
-#else  // _LIBCPP_ALTERNATE_STRING_LAYOUT
+#else  // _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
 
     _LIBCPP_INLINE_VISIBILITY
     void __set_short_size(size_type __s) _NOEXCEPT
@@ -1734,7 +1734,7 @@
         {return __r_.first().__s.__size_ >> 1;}
 #   endif
 
-#endif  // _LIBCPP_ALTERNATE_STRING_LAYOUT
+#endif  // _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
 
     _LIBCPP_INLINE_VISIBILITY
     void __set_long_size(size_type __s) _NOEXCEPT
Index: lib/CMakeLists.txt
===================================================================
--- lib/CMakeLists.txt
+++ lib/CMakeLists.txt
@@ -129,8 +129,8 @@
     COMPILE_FLAGS "${LIBCXX_COMPILE_FLAGS}"
     LINK_FLAGS    "${LIBCXX_LINK_FLAGS}"
     OUTPUT_NAME   "c++"
-    VERSION       "1.0"
-    SOVERSION     "1"
+    VERSION       "${LIBCXX_ABI_VERSION}.0"
+    SOVERSION     "${LIBCXX_ABI_VERSION}"
   )
 
 if (LIBCXX_INSTALL_LIBRARY)
Index: test/CMakeLists.txt
===================================================================
--- test/CMakeLists.txt
+++ test/CMakeLists.txt
@@ -13,6 +13,7 @@
 pythonize_bool(LIBCXX_ENABLE_RTTI)
 pythonize_bool(LIBCXX_ENABLE_SHARED)
 pythonize_bool(LIBCXX_BUILD_32_BITS)
+pythonize_bool(LIBCXX_ABI_UNSTABLE)
 pythonize_bool(LIBCXX_ENABLE_GLOBAL_FILESYSTEM_NAMESPACE)
 pythonize_bool(LIBCXX_ENABLE_STDIN)
 pythonize_bool(LIBCXX_ENABLE_STDOUT)
Index: test/libcxx/test/config.py
===================================================================
--- test/libcxx/test/config.py
+++ test/libcxx/test/config.py
@@ -387,6 +387,7 @@
         # Configure feature flags.
         self.configure_compile_flags_exceptions()
         self.configure_compile_flags_rtti()
+        self.configure_compile_flags_abi_version()
         self.configure_compile_flags_no_global_filesystem_namespace()
         self.configure_compile_flags_no_stdin()
         self.configure_compile_flags_no_stdout()
@@ -440,6 +441,15 @@
             self.config.available_features.add('libcpp-no-rtti')
             self.cxx.compile_flags += ['-fno-rtti', '-D_LIBCPP_NO_RTTI']
 
+    def configure_compile_flags_abi_version(self):
+        abi_version = self.get_lit_conf('abi_version', '').strip()
+        abi_unstable = self.get_lit_bool('abi_unstable')
+        if abi_version:
+          self.cxx.compile_flags += ['-D_LIBCPP_ABI_VERSION=' + abi_version]
+        if abi_unstable:
+          self.config.available_features.add('libcpp-abi-unstable')
+          self.cxx.compile_flags += ['-D_LIBCPP_ABI_UNSTABLE']
+
     def configure_compile_flags_no_global_filesystem_namespace(self):
         enable_global_filesystem_namespace = self.get_lit_bool(
             'enable_global_filesystem_namespace', True)
Index: test/lit.site.cfg.in
===================================================================
--- test/lit.site.cfg.in
+++ test/lit.site.cfg.in
@@ -7,6 +7,8 @@
 config.enable_rtti              = "@LIBCXX_ENABLE_RTTI@"
 config.enable_shared            = "@LIBCXX_ENABLE_SHARED@"
 config.enable_32bit             = "@LIBCXX_BUILD_32_BITS@"
+config.abi_version              = "@LIBCXX_ABI_VERSION@"
+config.abi_unstable             = "@LIBCXX_ABI_UNSTABLE@"
 config.enable_global_filesystem_namespace = "@LIBCXX_ENABLE_GLOBAL_FILESYSTEM_NAMESPACE@"
 config.enable_stdin             = "@LIBCXX_ENABLE_STDIN@"
 config.enable_stdout            = "@LIBCXX_ENABLE_STDOUT@"