Index: compiler-rt/trunk/cmake/Modules/SanitizerUtils.cmake
===================================================================
--- compiler-rt/trunk/cmake/Modules/SanitizerUtils.cmake
+++ compiler-rt/trunk/cmake/Modules/SanitizerUtils.cmake
@@ -46,6 +46,17 @@
   endforeach()
 endmacro()
 
+# This function is only used on Darwin, where undefined symbols must be specified
+# in the linker invocation.
+function(add_weak_symbols libname linkflags)
+  file(STRINGS "${COMPILER_RT_SOURCE_DIR}/lib/${libname}/weak_symbols.txt" WEAK_SYMBOLS)
+  set(local_linkflags ${${linkflags}})
+  foreach(SYMBOL ${WEAK_SYMBOLS})
+    set(local_linkflags ${local_linkflags} -Wl,-U,${SYMBOL})
+  endforeach()
+  set(${linkflags} ${local_linkflags} PARENT_SCOPE)
+endfunction()
+
 macro(add_sanitizer_rt_version_list name)
   set(vers ${CMAKE_CURRENT_BINARY_DIR}/${name}.vers)
   cmake_parse_arguments(ARG "" "" "LIBS;EXTRA" ${ARGN})
Index: compiler-rt/trunk/lib/asan/CMakeLists.txt
===================================================================
--- compiler-rt/trunk/lib/asan/CMakeLists.txt
+++ compiler-rt/trunk/lib/asan/CMakeLists.txt
@@ -106,6 +106,10 @@
 add_compiler_rt_component(asan)
 
 if(APPLE)
+  add_weak_symbols("asan" WEAK_SYMBOL_LINKFLAGS)
+  add_weak_symbols("ubsan" WEAK_SYMBOL_LINKFLAGS)
+  add_weak_symbols("sanitizer_common" WEAK_SYMBOL_LINKFLAGS)
+
   add_compiler_rt_runtime(clang_rt.asan
     SHARED
     OS ${SANITIZER_COMMON_SUPPORTED_OS}
@@ -117,6 +121,7 @@
                 RTLSanCommon
                 RTUbsan
     CFLAGS ${ASAN_DYNAMIC_CFLAGS}
+    LINKFLAGS ${WEAK_SYMBOL_LINKFLAGS}
     DEFS ${ASAN_DYNAMIC_DEFINITIONS}
     PARENT_TARGET asan)
 else()
Index: compiler-rt/trunk/lib/asan/tests/CMakeLists.txt
===================================================================
--- compiler-rt/trunk/lib/asan/tests/CMakeLists.txt
+++ compiler-rt/trunk/lib/asan/tests/CMakeLists.txt
@@ -59,6 +59,11 @@
 if(APPLE)
   list(APPEND ASAN_UNITTEST_COMMON_CFLAGS ${DARWIN_osx_CFLAGS})
   list(APPEND ASAN_UNITTEST_COMMON_LINKFLAGS ${DARWIN_osx_LINKFLAGS})
+
+  add_weak_symbols("asan" WEAK_SYMBOL_LINKFLAGS)
+  add_weak_symbols("ubsan" WEAK_SYMBOL_LINKFLAGS)
+  add_weak_symbols("sanitizer_common" WEAK_SYMBOL_LINKFLAGS)
+  list(APPEND ASAN_UNITTEST_COMMON_LINKFLAGS ${WEAK_SYMBOL_LINKFLAGS})
 endif()
 
 if(MSVC)
Index: compiler-rt/trunk/lib/asan/weak_symbols.txt
===================================================================
--- compiler-rt/trunk/lib/asan/weak_symbols.txt
+++ compiler-rt/trunk/lib/asan/weak_symbols.txt
@@ -0,0 +1,3 @@
+___asan_default_options
+___asan_default_suppressions
+___asan_on_error
Index: compiler-rt/trunk/lib/sanitizer_common/sanitizer_internal_defs.h
===================================================================
--- compiler-rt/trunk/lib/sanitizer_common/sanitizer_internal_defs.h
+++ compiler-rt/trunk/lib/sanitizer_common/sanitizer_internal_defs.h
@@ -32,7 +32,7 @@
 # define SANITIZER_WEAK_ATTRIBUTE  __attribute__((weak))
 #endif
 
-#if (SANITIZER_LINUX || SANITIZER_WINDOWS) && !SANITIZER_GO
+#if (SANITIZER_LINUX || SANITIZER_MAC || SANITIZER_WINDOWS) && !SANITIZER_GO
 # define SANITIZER_SUPPORTS_WEAK_HOOKS 1
 #else
 # define SANITIZER_SUPPORTS_WEAK_HOOKS 0
Index: compiler-rt/trunk/lib/sanitizer_common/tests/CMakeLists.txt
===================================================================
--- compiler-rt/trunk/lib/sanitizer_common/tests/CMakeLists.txt
+++ compiler-rt/trunk/lib/sanitizer_common/tests/CMakeLists.txt
@@ -81,6 +81,9 @@
 if(APPLE)
   list(APPEND SANITIZER_TEST_CFLAGS_COMMON ${DARWIN_osx_CFLAGS})
   list(APPEND SANITIZER_TEST_LINK_FLAGS_COMMON ${DARWIN_osx_LINKFLAGS})
+
+  add_weak_symbols("sanitizer_common" WEAK_SYMBOL_LINKFLAGS)
+  list(APPEND SANITIZER_TEST_LINK_FLAGS_COMMON ${WEAK_SYMBOL_LINKFLAGS})
 endif()
 
 # MSVC linker is allocating 1M for the stack by default, which is not
Index: compiler-rt/trunk/lib/sanitizer_common/weak_symbols.txt
===================================================================
--- compiler-rt/trunk/lib/sanitizer_common/weak_symbols.txt
+++ compiler-rt/trunk/lib/sanitizer_common/weak_symbols.txt
@@ -0,0 +1,6 @@
+___sanitizer_free_hook
+___sanitizer_malloc_hook
+___sanitizer_symbolize_code
+___sanitizer_symbolize_data
+___sanitizer_symbolize_demangle
+___sanitizer_symbolize_flush
Index: compiler-rt/trunk/lib/stats/CMakeLists.txt
===================================================================
--- compiler-rt/trunk/lib/stats/CMakeLists.txt
+++ compiler-rt/trunk/lib/stats/CMakeLists.txt
@@ -5,8 +5,14 @@
 
 if(APPLE)
   set(STATS_LIB_FLAVOR SHARED)
+
+  add_weak_symbols("asan" WEAK_SYMBOL_LINKFLAGS)
+  add_weak_symbols("ubsan" WEAK_SYMBOL_LINKFLAGS)
+  add_weak_symbols("sanitizer_common" WEAK_SYMBOL_LINKFLAGS)
 else()
   set(STATS_LIB_FLAVOR STATIC)
+
+  set(WEAK_SYMBOL_LINKFLAGS)
 endif()
 
 add_compiler_rt_runtime(clang_rt.stats
@@ -17,6 +23,7 @@
   OBJECT_LIBS RTSanitizerCommon
               RTSanitizerCommonLibc
   CFLAGS ${SANITIZER_COMMON_CFLAGS}
+  LINKFLAGS ${WEAK_SYMBOL_LINKFLAGS}
   PARENT_TARGET stats)
 
 add_compiler_rt_runtime(clang_rt.stats_client
@@ -25,4 +32,5 @@
   OS ${SANITIZER_COMMON_SUPPORTED_OS}
   SOURCES stats_client.cc
   CFLAGS ${SANITIZER_COMMON_CFLAGS}
+  LINKFLAGS ${WEAK_SYMBOL_LINKFLAGS}
   PARENT_TARGET stats)
Index: compiler-rt/trunk/lib/tsan/CMakeLists.txt
===================================================================
--- compiler-rt/trunk/lib/tsan/CMakeLists.txt
+++ compiler-rt/trunk/lib/tsan/CMakeLists.txt
@@ -107,6 +107,10 @@
     # Pass ASM file directly to the C++ compiler.
     set_source_files_properties(${TSAN_ASM_SOURCES} PROPERTIES LANGUAGE C)
   endif()
+
+  add_weak_symbols("ubsan" WEAK_SYMBOL_LINKFLAGS)
+  add_weak_symbols("sanitizer_common" WEAK_SYMBOL_LINKFLAGS)
+
   add_compiler_rt_runtime(clang_rt.tsan
     SHARED
     OS ${TSAN_SUPPORTED_OS}
@@ -117,6 +121,7 @@
                 RTSanitizerCommonLibc
                 RTUbsan
     CFLAGS ${TSAN_RTL_CFLAGS}
+    LINKFLAGS ${WEAK_SYMBOL_LINKFLAGS}
     PARENT_TARGET tsan)
   add_compiler_rt_object_libraries(RTTsan_dynamic
     OS ${TSAN_SUPPORTED_OS}
Index: compiler-rt/trunk/lib/tsan/tests/CMakeLists.txt
===================================================================
--- compiler-rt/trunk/lib/tsan/tests/CMakeLists.txt
+++ compiler-rt/trunk/lib/tsan/tests/CMakeLists.txt
@@ -76,14 +76,18 @@
           ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR})
         list(APPEND TEST_OBJECTS lib${TSAN_TEST_RUNTIME}.a)
         list(APPEND TEST_DEPS ${TSAN_TEST_RUNTIME})
+
+        add_weak_symbols("ubsan" WEAK_SYMBOL_LINKFLAGS)
+        add_weak_symbols("sanitizer_common" WEAK_SYMBOL_LINKFLAGS)
+
         # Intentionally do *not* link with `-fsanitize=thread`. We already link
         # against a static version of the runtime, and we don't want the dynamic
         # one.
         add_compiler_rt_test(TsanUnitTests "${testname}-${arch}-Test"
                 OBJECTS ${TEST_OBJECTS}
                 DEPS ${TEST_DEPS}
-                LINK_FLAGS ${TARGET_LINK_FLAGS} ${DARWIN_osx_LINKFLAGS}
-                           -lc++)
+                LINK_FLAGS ${TARGET_LINK_FLAGS} ${DARWIN_osx_LINK_FLAGS}
+                           ${WEAK_SYMBOL_LINKFLAGS} -lc++)
       endif()
     endforeach()
   endif()
Index: compiler-rt/trunk/lib/ubsan/CMakeLists.txt
===================================================================
--- compiler-rt/trunk/lib/ubsan/CMakeLists.txt
+++ compiler-rt/trunk/lib/ubsan/CMakeLists.txt
@@ -56,6 +56,9 @@
       SOURCES ${UBSAN_STANDALONE_SOURCES}
       CFLAGS ${UBSAN_STANDALONE_CFLAGS})
 
+    add_weak_symbols("ubsan" WEAK_SYMBOL_LINKFLAGS)
+    add_weak_symbols("sanitizer_common" WEAK_SYMBOL_LINKFLAGS)
+
     add_compiler_rt_runtime(clang_rt.ubsan
       SHARED
       OS ${SANITIZER_COMMON_SUPPORTED_OS}
@@ -64,6 +67,7 @@
                   RTUbsan_standalone
                   RTSanitizerCommon
                   RTSanitizerCommonLibc
+      LINKFLAGS ${WEAK_SYMBOL_LINKFLAGS}
       PARENT_TARGET ubsan)
   endif()
 
Index: compiler-rt/trunk/lib/ubsan/weak_symbols.txt
===================================================================
--- compiler-rt/trunk/lib/ubsan/weak_symbols.txt
+++ compiler-rt/trunk/lib/ubsan/weak_symbols.txt
@@ -0,0 +1 @@
+___ubsan_default_options