diff --git a/libcxxabi/test/configs/cmake-bridge.cfg.in b/libcxxabi/test/configs/cmake-bridge.cfg.in
index edf80c8f2a732..1bdba32405ddd 100644
--- a/libcxxabi/test/configs/cmake-bridge.cfg.in
+++ b/libcxxabi/test/configs/cmake-bridge.cfg.in
@@ -27,6 +27,8 @@ config.test_exec_root = os.path.join('@LIBCXXABI_BINARY_DIR@', 'test')
 config.host_triple = '@LLVM_HOST_TRIPLE@'
 
 config.substitutions.append(('%{libcxx}', '@LIBCXXABI_LIBCXX_PATH@'))
+config.substitutions.append(('%{libcxxabi}', '@LIBCXXABI_SOURCE_DIR@'))
+config.substitutions.append(('%{llvm}', '@LLVM_MAIN_SRC_DIR@'))
 config.substitutions.append(('%{install-prefix}', '@LIBCXXABI_TESTING_INSTALL_PREFIX@'))
 config.substitutions.append(('%{include}', '@LIBCXXABI_TESTING_INSTALL_PREFIX@/include'))
 config.substitutions.append(('%{cxx-include}', '@LIBCXXABI_TESTING_INSTALL_PREFIX@/@LIBCXXABI_INSTALL_INCLUDE_DIR@'))
diff --git a/libcxxabi/test/itanium_demangle_matches_llvm.sh.test b/libcxxabi/test/itanium_demangle_matches_llvm.sh.test
new file mode 100644
index 0000000000000..31371d121fbdd
--- /dev/null
+++ b/libcxxabi/test/itanium_demangle_matches_llvm.sh.test
@@ -0,0 +1,6 @@
+# This test diffs the ItaniumDemangle.h header in libcxxabi and LLVM to ensure
+# that they are the same.
+
+# RUN: tail -n +3 %{libcxxabi}/src/demangle/ItaniumDemangle.h > %t.libcxxabi_demangle
+# RUN: tail -n +3 %{llvm}/include/llvm/Demangle/ItaniumDemangle.h > %t.llvm_demangle
+# RUN: diff %t.libcxxabi_demangle %t.llvm_demangle