Index: CMakeLists.txt
===================================================================
--- CMakeLists.txt
+++ CMakeLists.txt
@@ -43,6 +43,7 @@
 option(LIBCXX_ENABLE_CXX1Y "Enable -std=c++1y and use of c++1y language features if the compiler supports it." OFF)
 option(LIBCXX_ENABLE_SHARED "Build libc++ as a shared library." ON)
 option(LIBCXX_ENABLE_THREADS "Build libc++ with support for threads." ON)
+option(LIBCXX_ENABLE_UNSTABLE_ABI "Build with the unstable ABI changes." OFF)
 option(LIBCXX_BUILD_32_BITS "Build 32 bit libc++" OFF)
 option(LIBCXX_ENABLE_MONOTONIC_CLOCK
   "Build libc++ with support for a monotonic clock.
@@ -206,6 +207,8 @@
 if (NOT LIBCXX_ENABLE_SHARED)
   list(APPEND LIBCXX_COMPILE_FLAGS -D_LIBCPP_BUILD_STATIC)
 endif()
+# Unstable ABI
+append_if(LIBCXX_CXX_FLAGS LIBCXX_ENABLE_UNSTABLE_ABI -D_LIBCPP_ABI_UNSTABLE)
 
 if (CMAKE_SIZEOF_VOID_P EQUAL 8 AND NOT WIN32)
   if (LIBCXX_BUILD_32_BITS)
Index: include/__config
===================================================================
--- include/__config
+++ include/__config
@@ -581,6 +581,13 @@
 #define __has_builtin(__x) 0
 #endif
 
+#ifdef _LIBCPP_ALTERNATE_STRING_LAYOUT
+#define _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
+#endif
+#ifdef _LIBCPP_ABI_UNSTABLE
+#define _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
+#endif
+
 #if __has_feature(cxx_explicit_conversions) || defined(__IBMCPP__)
 #   define _LIBCPP_EXPLICIT explicit
 #else
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;};
 
@@ -1680,7 +1680,7 @@
     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
@@ -1698,7 +1698,7 @@
         {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
@@ -1716,7 +1716,7 @@
         {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: test/CMakeLists.txt
===================================================================
--- test/CMakeLists.txt
+++ test/CMakeLists.txt
@@ -32,6 +32,7 @@
   pythonize_bool(LIBCXX_ENABLE_SHARED)
   pythonize_bool(LIBCXX_BUILD_32_BITS)
   pythonize_bool(LIBCXX_ENABLE_THREADS)
+  pythonize_bool(LIBCXX_ENABLE_UNSTABLE_ABI)
   pythonize_bool(LIBCXX_ENABLE_MONOTONIC_CLOCK)
 
   set(AUTO_GEN_COMMENT "## Autogenerated by libcxx configuration.\n# Do not edit!")
Index: test/lit.cfg
===================================================================
--- test/lit.cfg
+++ test/lit.cfg
@@ -485,7 +485,10 @@
         elif not enable_monotonic_clock:
             self.lit_config.fatal('enable_monotonic_clock cannot be false when'
                                   ' enable_threads is true.')
-
+        # Configure the ABI flags.
+        enable_unstable_abi = self.get_lit_bool('enable_unstable_abi', False)
+        if enable_unstable_abi:
+            self.compile_flags += ['-D_LIBCPP_ABI_UNSTABLE']
 
     def configure_link_flags(self):
         # Configure library search paths
Index: test/lit.site.cfg.in
===================================================================
--- test/lit.site.cfg.in
+++ test/lit.site.cfg.in
@@ -10,6 +10,7 @@
 config.enable_32bit             = "@LIBCXX_BUILD_32_BITS@"
 config.enable_threads           = "@LIBCXX_ENABLE_THREADS@"
 config.enable_monotonic_clock   = "@LIBCXX_ENABLE_MONOTONIC_CLOCK@"
+config.enable_unstable_abi      = "@LIBCXX_ENABLE_UNSTABLE_ABI@"
 config.cxx_abi                  = "@LIBCXX_CXX_ABI_LIBNAME@"
 config.llvm_use_sanitizer       = "@LLVM_USE_SANITIZER@"
 config.abi_library_path         = "@LIBCXX_CXX_ABI_LIBRARY_PATH@"