diff --git a/libcxx/CMakeLists.txt b/libcxx/CMakeLists.txt
--- a/libcxx/CMakeLists.txt
+++ b/libcxx/CMakeLists.txt
@@ -24,6 +24,7 @@
 set(LIBCXX_BINARY_INCLUDE_DIR "${LIBCXX_BINARY_DIR}/include/c++build")
 
 include(GNUInstallDirs)
+include(WarningFlags)
 
 # Require out of source build.
 include(MacroEnsureOutOfSourceBuild)
@@ -563,71 +564,6 @@
   target_compile_options(${target} PUBLIC "${LIBCXX_ADDITIONAL_COMPILE_FLAGS}")
 endfunction()
 
-# Warning flags ===============================================================
-function(cxx_add_warning_flags target)
-  target_compile_definitions(${target} PUBLIC -D_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
-  if (MSVC)
-    # -W4 is the cl.exe/clang-cl equivalent of -Wall. (In cl.exe and clang-cl,
-    # -Wall is equivalent to -Weverything in GCC style compiler drivers.)
-    target_add_compile_flags_if_supported(${target} PRIVATE -W4)
-  else()
-    target_add_compile_flags_if_supported(${target} PRIVATE -Wall)
-  endif()
-  target_add_compile_flags_if_supported(${target} PRIVATE -Wextra
-                                                          -W
-                                                          -Wwrite-strings
-                                                          -Wno-unused-parameter
-                                                          -Wno-long-long
-                                                          -Werror=return-type
-                                                          -Wextra-semi
-                                                          -Wundef
-                                                          -Wunused-template
-                                                          -Wformat-nonliteral)
-  if ("${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang")
-    target_add_compile_flags_if_supported(${target} PRIVATE
-      -Wno-user-defined-literals
-      -Wno-covered-switch-default
-      -Wno-suggest-override
-    )
-    if (LIBCXX_TARGETING_CLANG_CL)
-      target_add_compile_flags_if_supported(${target} PRIVATE
-        -Wno-c++98-compat
-        -Wno-c++98-compat-pedantic
-        -Wno-c++11-compat
-        -Wno-undef
-        -Wno-reserved-id-macro
-        -Wno-gnu-include-next
-        -Wno-gcc-compat # For ignoring "'diagnose_if' is a clang extension" warnings
-        -Wno-zero-as-null-pointer-constant # FIXME: Remove this and fix all occurrences.
-        -Wno-deprecated-dynamic-exception-spec # For auto_ptr
-        -Wno-sign-conversion
-        -Wno-old-style-cast
-        -Wno-deprecated # FIXME: Remove this and fix all occurrences.
-        -Wno-shift-sign-overflow # FIXME: Why do we need this with clang-cl but not clang?
-        -Wno-double-promotion # FIXME: remove me
-      )
-    endif()
-  elseif("${CMAKE_CXX_COMPILER_ID}" MATCHES "GNU")
-    target_add_compile_flags_if_supported(${target} PRIVATE
-      -Wno-attributes
-      -Wno-literal-suffix
-      -Wno-c++14-compat
-      -Wno-noexcept-type
-      -Wno-suggest-override)
-  endif()
-  if (LIBCXX_ENABLE_WERROR)
-    target_add_compile_flags_if_supported(${target} PRIVATE -Werror)
-    target_add_compile_flags_if_supported(${target} PRIVATE -WX)
-  else()
-    # TODO(EricWF) Remove this. We shouldn't be suppressing errors when -Werror is
-    # added elsewhere.
-    target_add_compile_flags_if_supported(${target} PRIVATE -Wno-error)
-  endif()
-  if (LIBCXX_ENABLE_PEDANTIC)
-    target_add_compile_flags_if_supported(${target} PRIVATE -pedantic)
-  endif()
-endfunction()
-
 # Exception flags =============================================================
 function(cxx_add_exception_flags target)
   if (LIBCXX_ENABLE_EXCEPTIONS)
@@ -910,7 +846,7 @@
 # Setup all common build flags =================================================
 function(cxx_add_common_build_flags target)
   cxx_add_basic_build_flags(${target})
-  cxx_add_warning_flags(${target})
+  cxx_add_warning_flags(${target} ${LIBCXX_ENABLE_WERROR} ${LIBCXX_ENABLE_PEDANTIC})
   cxx_add_windows_flags(${target})
   cxx_add_exception_flags(${target})
   cxx_add_rtti_flags(${target})
diff --git a/libcxxabi/CMakeLists.txt b/libcxxabi/CMakeLists.txt
--- a/libcxxabi/CMakeLists.txt
+++ b/libcxxabi/CMakeLists.txt
@@ -44,7 +44,7 @@
   "Provide support for exceptions in the runtime.
   When disabled, libc++abi does not support stack unwinding and other exceptions-related features." ON)
 option(LIBCXXABI_ENABLE_ASSERTIONS "Enable assertions independent of build mode." ON)
-option(LIBCXXABI_ENABLE_PEDANTIC "Compile with pedantic enabled." ON)
+option(LIBCXXABI_ENABLE_PEDANTIC "Compile with pedantic enabled." OFF)
 option(LIBCXXABI_ENABLE_WERROR "Fail and stop if a warning is triggered." OFF)
 option(LIBCXXABI_USE_LLVM_UNWINDER "Build and use the LLVM unwinder." OFF)
 option(LIBCXXABI_ENABLE_STATIC_UNWINDER "Statically link the LLVM unwinder." OFF)
@@ -279,41 +279,6 @@
 # it is being built as part of libcxx.
 add_definitions(-D_LIBCPP_BUILDING_LIBRARY)
 
-add_compile_flags_if_supported(-Werror=return-type)
-
-# Get warning flags
-add_compile_flags_if_supported(-W)
-add_compile_flags_if_supported(-Wall)
-add_compile_flags_if_supported(-Wchar-subscripts)
-add_compile_flags_if_supported(-Wconversion)
-add_compile_flags_if_supported(-Wmismatched-tags)
-add_compile_flags_if_supported(-Wmissing-braces)
-add_compile_flags_if_supported(-Wnewline-eof)
-add_compile_flags_if_supported(-Wunused-function)
-add_compile_flags_if_supported(-Wshadow)
-add_compile_flags_if_supported(-Wshorten-64-to-32)
-add_compile_flags_if_supported(-Wsign-compare)
-add_compile_flags_if_supported(-Wsign-conversion)
-add_compile_flags_if_supported(-Wstrict-aliasing=2)
-add_compile_flags_if_supported(-Wstrict-overflow=4)
-add_compile_flags_if_supported(-Wunused-parameter)
-add_compile_flags_if_supported(-Wunused-variable)
-add_compile_flags_if_supported(-Wwrite-strings)
-add_compile_flags_if_supported(-Wundef)
-
-add_compile_flags_if_supported(-Wno-suggest-override)
-
-if (LIBCXXABI_ENABLE_WERROR)
-  add_compile_flags_if_supported(-Werror)
-  add_compile_flags_if_supported(-WX)
-else()
-  add_compile_flags_if_supported(-Wno-error)
-  add_compile_flags_if_supported(-WX-)
-endif()
-if (LIBCXXABI_ENABLE_PEDANTIC)
-  add_compile_flags_if_supported(-pedantic)
-endif()
-
 # Get feature flags.
 add_compile_flags_if_supported(-fstrict-aliasing)
 
diff --git a/libcxxabi/src/CMakeLists.txt b/libcxxabi/src/CMakeLists.txt
--- a/libcxxabi/src/CMakeLists.txt
+++ b/libcxxabi/src/CMakeLists.txt
@@ -158,8 +158,11 @@
   message(STATUS "Could not find ParallelSTL, libc++abi will not attempt to use it but the build may fail if the libc++ in use needs it to be available.")
 endif()
 
+include(WarningFlags)
+
 # Build the shared library.
 add_library(cxxabi_shared_objects OBJECT EXCLUDE_FROM_ALL ${LIBCXXABI_SOURCES} ${LIBCXXABI_HEADERS})
+cxx_add_warning_flags(cxxabi_shared_objects ${LIBCXXABI_ENABLE_WERROR} ${LIBCXXABI_ENABLE_PEDANTIC})
 if (LIBCXXABI_USE_LLVM_UNWINDER)
   if (LIBCXXABI_STATICALLY_LINK_UNWINDER_IN_SHARED_LIBRARY)
     target_link_libraries(cxxabi_shared_objects PUBLIC unwind_shared_objects) # propagate usage requirements
@@ -246,6 +249,7 @@
 
 # Build the static library.
 add_library(cxxabi_static_objects OBJECT EXCLUDE_FROM_ALL ${LIBCXXABI_SOURCES} ${LIBCXXABI_HEADERS})
+cxx_add_warning_flags(cxxabi_static_objects ${LIBCXXABI_ENABLE_WERROR} ${LIBCXXABI_ENABLE_PEDANTIC})
 if (LIBCXXABI_USE_LLVM_UNWINDER AND LIBCXXABI_STATICALLY_LINK_UNWINDER_IN_STATIC_LIBRARY)
   target_link_libraries(cxxabi_static_objects PUBLIC unwind_static_objects) # propagate usage requirements
   target_sources(cxxabi_static_objects PUBLIC $<TARGET_OBJECTS:unwind_static_objects>)
diff --git a/libcxxabi/src/demangle/ItaniumDemangle.h b/libcxxabi/src/demangle/ItaniumDemangle.h
--- a/libcxxabi/src/demangle/ItaniumDemangle.h
+++ b/libcxxabi/src/demangle/ItaniumDemangle.h
@@ -19,6 +19,7 @@
 #include "DemangleConfig.h"
 #include "StringView.h"
 #include "Utility.h"
+#include <__cxxabi_config.h>
 #include <algorithm>
 #include <cassert>
 #include <cctype>
@@ -30,6 +31,11 @@
 #include <type_traits>
 #include <utility>
 
+#ifdef _LIBCXXABI_COMPILER_CLANG
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wunused-template"
+#endif
+
 DEMANGLE_NAMESPACE_BEGIN
 
 template <class T, size_t N> class PODSmallVector {
@@ -5498,4 +5504,8 @@
 
 DEMANGLE_NAMESPACE_END
 
+#ifdef _LIBCXXABI_COMPILER_CLANG
+#pragma clang diagnostic pop
+#endif
+
 #endif // DEMANGLE_ITANIUMDEMANGLE_H
diff --git a/libunwind/CMakeLists.txt b/libunwind/CMakeLists.txt
--- a/libunwind/CMakeLists.txt
+++ b/libunwind/CMakeLists.txt
@@ -169,28 +169,6 @@
   endif()
 endif()
 
-# Get warning flags
-add_compile_flags_if_supported(-W)
-add_compile_flags_if_supported(-Wall)
-add_compile_flags_if_supported(-Wchar-subscripts)
-add_compile_flags_if_supported(-Wconversion)
-add_compile_flags_if_supported(-Wmismatched-tags)
-add_compile_flags_if_supported(-Wmissing-braces)
-add_compile_flags_if_supported(-Wnewline-eof)
-add_compile_flags_if_supported(-Wno-unused-function)
-add_compile_flags_if_supported(-Wshadow)
-add_compile_flags_if_supported(-Wshorten-64-to-32)
-add_compile_flags_if_supported(-Wsign-compare)
-add_compile_flags_if_supported(-Wsign-conversion)
-add_compile_flags_if_supported(-Wstrict-aliasing=2)
-add_compile_flags_if_supported(-Wstrict-overflow=4)
-add_compile_flags_if_supported(-Wunused-parameter)
-add_compile_flags_if_supported(-Wunused-variable)
-add_compile_flags_if_supported(-Wwrite-strings)
-add_compile_flags_if_supported(-Wundef)
-
-add_compile_flags_if_supported(-Wno-suggest-override)
-
 if (WIN32)
   # The headers lack matching dllexport attributes (_LIBUNWIND_EXPORT);
   # silence the warning instead of cluttering the headers (which aren't
@@ -199,18 +177,6 @@
   add_compile_flags_if_supported(-Wno-dll-attribute-on-redeclaration)
 endif()
 
-if (LIBUNWIND_ENABLE_WERROR)
-  add_compile_flags_if_supported(-Werror)
-  add_compile_flags_if_supported(-WX)
-else()
-  add_compile_flags_if_supported(-Wno-error)
-  add_compile_flags_if_supported(-WX-)
-endif()
-
-if (LIBUNWIND_ENABLE_PEDANTIC)
-  add_compile_flags_if_supported(-pedantic)
-endif()
-
 # Get feature flags.
 # Exceptions
 # Catches C++ exceptions only and tells the compiler to assume that extern C
diff --git a/libunwind/src/CMakeLists.txt b/libunwind/src/CMakeLists.txt
--- a/libunwind/src/CMakeLists.txt
+++ b/libunwind/src/CMakeLists.txt
@@ -135,8 +135,11 @@
 # ease, but does not rely on C++ at runtime.
 set(CMAKE_CXX_IMPLICIT_LINK_LIBRARIES "")
 
+include(WarningFlags)
+
 # Build the shared library.
 add_library(unwind_shared_objects OBJECT EXCLUDE_FROM_ALL ${LIBUNWIND_SOURCES} ${LIBUNWIND_HEADERS})
+cxx_add_warning_flags(unwind_shared_objects ${LIBUNWIND_ENABLE_WERROR} ${LIBUNWIND_ENABLE_PEDANTIC})
 if(CMAKE_C_COMPILER_ID STREQUAL MSVC)
   target_compile_options(unwind_shared_objects PRIVATE /GR-)
 else()
@@ -174,6 +177,7 @@
 
 # Build the static library.
 add_library(unwind_static_objects OBJECT EXCLUDE_FROM_ALL ${LIBUNWIND_SOURCES} ${LIBUNWIND_HEADERS})
+cxx_add_warning_flags(unwind_static_objects ${LIBUNWIND_ENABLE_WERROR} ${LIBUNWIND_ENABLE_PEDANTIC})
 if(CMAKE_C_COMPILER_ID STREQUAL MSVC)
   target_compile_options(unwind_static_objects PRIVATE /GR-)
 else()
diff --git a/runtimes/cmake/Modules/WarningFlags.cmake b/runtimes/cmake/Modules/WarningFlags.cmake
new file mode 100644
--- /dev/null
+++ b/runtimes/cmake/Modules/WarningFlags.cmake
@@ -0,0 +1,78 @@
+include(HandleFlags)
+
+# Warning flags ===============================================================
+function(cxx_add_warning_flags target enable_werror enable_pedantic)
+  target_compile_definitions(${target} PUBLIC -D_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+  if (MSVC)
+    # -W4 is the cl.exe/clang-cl equivalent of -Wall. (In cl.exe and clang-cl,
+    # -Wall is equivalent to -Weverything in GCC style compiler drivers.)
+    target_add_compile_flags_if_supported(${target} PRIVATE -W4)
+  else()
+    target_add_compile_flags_if_supported(${target} PRIVATE -Wall)
+  endif()
+  # TODO: Should -Wconversion be enabled?
+  target_add_compile_flags_if_supported(${target} PRIVATE
+      -Wall
+      -Wextra
+      -Wnewline-eof
+      -Wshadow
+      -Wwrite-strings
+      -Wno-unused-parameter
+      -Wno-long-long
+      -Werror=return-type
+      -Wextra-semi
+      -Wundef
+      -Wunused-template
+      -Wformat-nonliteral
+      )
+
+  if ("${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang")
+    target_add_compile_flags_if_supported(${target} PRIVATE
+      -Wno-user-defined-literals
+      -Wno-covered-switch-default
+      -Wno-suggest-override
+    )
+    if (LIBCXX_TARGETING_CLANG_CL)
+      target_add_compile_flags_if_supported(${target} PRIVATE
+        -Wno-c++98-compat
+        -Wno-c++98-compat-pedantic
+        -Wno-c++11-compat
+        -Wno-undef
+        -Wno-reserved-id-macro
+        -Wno-gnu-include-next
+        -Wno-gcc-compat # For ignoring "'diagnose_if' is a clang extension" warnings
+        -Wno-zero-as-null-pointer-constant # FIXME: Remove this and fix all occurrences.
+        -Wno-deprecated-dynamic-exception-spec # For auto_ptr
+        -Wno-sign-conversion
+        -Wno-old-style-cast
+        -Wno-deprecated # FIXME: Remove this and fix all occurrences.
+        -Wno-shift-sign-overflow # FIXME: Why do we need this with clang-cl but not clang?
+        -Wno-double-promotion # FIXME: remove me
+      )
+    endif()
+
+  elseif("${CMAKE_CXX_COMPILER_ID}" MATCHES "GNU")
+
+    target_add_compile_flags_if_supported(${target} PRIVATE
+      -Wstrict-aliasing=2
+      -Wstrict-overflow=4
+      -Wno-attributes
+      -Wno-literal-suffix
+      -Wno-c++14-compat
+      -Wno-noexcept-type
+      -Wno-suggest-override
+      )
+
+  endif()
+  if (${enable_werror})
+    target_add_compile_flags_if_supported(${target} PRIVATE -Werror)
+    target_add_compile_flags_if_supported(${target} PRIVATE -WX)
+  else()
+    # TODO(EricWF) Remove this. We shouldn't be suppressing errors when -Werror is
+    # added elsewhere.
+    target_add_compile_flags_if_supported(${target} PRIVATE -Wno-error)
+  endif()
+  if (${enable_pedantic})
+    target_add_compile_flags_if_supported(${target} PRIVATE -pedantic)
+  endif()
+endfunction()