Index: cmake/config-ix.cmake
===================================================================
--- cmake/config-ix.cmake
+++ cmake/config-ix.cmake
@@ -548,6 +548,16 @@
   filter_available_targets(SAFESTACK_SUPPORTED_ARCH
     ${ALL_SAFESTACK_SUPPORTED_ARCH})
   filter_available_targets(CFI_SUPPORTED_ARCH ${ALL_CFI_SUPPORTED_ARCH})
+
+  # Disable all tsan builds if the default target is not supported.
+  # This works around two problems:
+  # * A tsan'd libcxx is configured despite -fsanitize-thread not being
+  #   supported. This configure step then fails.
+  # * The check-tsan target tests the default target despite not having built it.
+  list(FIND TSAN_SUPPORTED_ARCH ${COMPILER_RT_DEFAULT_TARGET_ARCH} ARCH_INDEX)
+  if(ARCH_INDEX EQUAL -1)
+    set(TSAN_SUPPORTED_ARCH)
+  endif()
 endif()
 
 if (MSVC)