diff --git a/libcxx/CMakeLists.txt b/libcxx/CMakeLists.txt
--- a/libcxx/CMakeLists.txt
+++ b/libcxx/CMakeLists.txt
@@ -71,6 +71,7 @@
 #===============================================================================
 # Setup CMake Options
 #===============================================================================
+include(CheckCXXSymbolExists)
 include(CMakeDependentOption)
 include(HandleCompilerRT)
 
@@ -310,6 +311,10 @@
 option(LIBCXX_HERMETIC_STATIC_LIBRARY
   "Do not export any symbols from the static library." OFF)
 
+check_cxx_symbol_exists(__SIZEOF_INT128__ "" LIBCXX_HAVE_INT128)
+cmake_dependent_option(LIBCXX_USE_INT128   "Use __int128 in the library" ON
+"LIBCXX_HAVE_INT128" OFF)
+
 #===============================================================================
 # Check option configurations
 #===============================================================================
@@ -897,6 +902,7 @@
 # easier to grep for target specific flags once the feature is complete.
 config_define_if_not(LIBCXX_ENABLE_INCOMPLETE_FEATURES _LIBCPP_HAS_NO_INCOMPLETE_FORMAT)
 config_define_if_not(LIBCXX_ENABLE_INCOMPLETE_FEATURES _LIBCPP_HAS_NO_INCOMPLETE_RANGES)
+config_define_if_not(LIBCXX_USE_INT128 _LIBCPP_HAS_NO_INT128)
 
 if (LIBCXX_ABI_DEFINES)
   set(abi_defines)
diff --git a/libcxx/include/__config b/libcxx/include/__config
--- a/libcxx/include/__config
+++ b/libcxx/include/__config
@@ -795,10 +795,6 @@
 typedef unsigned int   char32_t;
 #endif
 
-#ifndef __SIZEOF_INT128__
-#define _LIBCPP_HAS_NO_INT128
-#endif
-
 #ifdef _LIBCPP_CXX03_LANG
 # define static_assert(...) _Static_assert(__VA_ARGS__)
 # define decltype(...) __decltype(__VA_ARGS__)
diff --git a/libcxx/include/__config_site.in b/libcxx/include/__config_site.in
--- a/libcxx/include/__config_site.in
+++ b/libcxx/include/__config_site.in
@@ -33,6 +33,7 @@
 #cmakedefine _LIBCPP_HAS_NO_WIDE_CHARACTERS
 #cmakedefine _LIBCPP_HAS_NO_INCOMPLETE_FORMAT
 #cmakedefine _LIBCPP_HAS_NO_INCOMPLETE_RANGES
+#cmakedefine _LIBCPP_HAS_NO_INT128
 
 @_LIBCPP_ABI_DEFINES@
 
diff --git a/libcxx/lib/abi/CMakeLists.txt b/libcxx/lib/abi/CMakeLists.txt
--- a/libcxx/lib/abi/CMakeLists.txt
+++ b/libcxx/lib/abi/CMakeLists.txt
@@ -7,7 +7,7 @@
 # Right now, this is done by using the ABI identifier as the filename containing
 # the list of symbols exported by libc++ for that configuration, however we could
 # make it more sophisticated if the number of ABI-affecting parameters grew.
-function(cxx_abi_list_identifier result triple abi_library abi_version unstable exceptions new_delete_in_libcxx)
+function(cxx_abi_list_identifier result triple abi_library abi_version unstable exceptions new_delete_in_libcxx using_int128)
   set(abi_properties)
 
   if ("${triple}" MATCHES "darwin")
@@ -35,6 +35,11 @@
   else()
     list(APPEND abi_properties "no_new_in_libcxx")
   endif()
+  if (${using_int128})
+    list(APPEND abi_properties "int128")
+  else()
+    list(APPEND abi_properties "no_int128")
+  endif()
 
   list(JOIN abi_properties "." tmp)
   set(${result} "${tmp}" PARENT_SCOPE)
@@ -47,6 +52,7 @@
   "${LIBCXX_ABI_UNSTABLE}"
   "${LIBCXX_ENABLE_EXCEPTIONS}"
   "${LIBCXX_ENABLE_NEW_DELETE_DEFINITIONS}"
+  "${LIBCXX_USE_INT128}"
 )
 
 if (TARGET cxx_shared)
diff --git a/libcxx/lib/abi/arm64-apple-darwin.libcxxabi.v1.stable.exceptions.no_new_in_libcxx.abilist b/libcxx/lib/abi/arm64-apple-darwin.libcxxabi.v1.stable.exceptions.no_new_in_libcxx.int128.abilist
rename from libcxx/lib/abi/arm64-apple-darwin.libcxxabi.v1.stable.exceptions.no_new_in_libcxx.abilist
rename to libcxx/lib/abi/arm64-apple-darwin.libcxxabi.v1.stable.exceptions.no_new_in_libcxx.int128.abilist
diff --git a/libcxx/lib/abi/x86_64-apple-darwin.libcxxabi.v1.stable.exceptions.no_new_in_libcxx.abilist b/libcxx/lib/abi/x86_64-apple-darwin.libcxxabi.v1.stable.exceptions.no_new_in_libcxx.int128.abilist
rename from libcxx/lib/abi/x86_64-apple-darwin.libcxxabi.v1.stable.exceptions.no_new_in_libcxx.abilist
rename to libcxx/lib/abi/x86_64-apple-darwin.libcxxabi.v1.stable.exceptions.no_new_in_libcxx.int128.abilist
diff --git a/libcxx/lib/abi/x86_64-unknown-linux-gnu.libcxxabi.v1.stable.exceptions.no_new_in_libcxx.abilist b/libcxx/lib/abi/x86_64-unknown-linux-gnu.libcxxabi.v1.stable.exceptions.no_new_in_libcxx.int128.abilist
rename from libcxx/lib/abi/x86_64-unknown-linux-gnu.libcxxabi.v1.stable.exceptions.no_new_in_libcxx.abilist
rename to libcxx/lib/abi/x86_64-unknown-linux-gnu.libcxxabi.v1.stable.exceptions.no_new_in_libcxx.int128.abilist