Index: llvm/cmake/modules/SetTargetTriple.cmake
===================================================================
--- llvm/cmake/modules/SetTargetTriple.cmake
+++ llvm/cmake/modules/SetTargetTriple.cmake
@@ -1,9 +1,16 @@
 macro(set_llvm_target_triple)
-  set(LLVM_DEFAULT_TARGET_TRIPLE "${LLVM_DEFAULT_TARGET_TRIPLE_default}" CACHE STRING
-  "Default target for which LLVM will generate code." )
+  set(LLVM_DEFAULT_TARGET_TRIPLE
+    "${LLVM_DEFAULT_TARGET_TRIPLE_default}" CACHE STRING
+    "Default target for which LLVM will generate code." )
+  if (NOT LLVM_DEFAULT_TARGET_TRIPLE
+      MATCHES "^[0-9A-Za-z_]+-[0-9A-Za-z_]+-[0-9A-Za-z_.]+(-[0-9A-Za-z_]+)?\$")
+    message(FATAL_ERROR
+      "LLVM_DEFAULT_TARGET_TRIPLE '${LLVM_DEFAULT_TARGET_TRIPLE}' is malformed")
+  endif ()
   if (TARGET_TRIPLE)
-    message(WARNING "TARGET_TRIPLE is deprecated and will be removed in a future release. "
-    "Please use LLVM_DEFAULT_TARGET_TRIPLE instead.")
+    message(WARNING
+      "TARGET_TRIPLE is deprecated and will be removed in a future release. "
+      "Please use LLVM_DEFAULT_TARGET_TRIPLE instead.")
     set(LLVM_TARGET_TRIPLE "${TARGET_TRIPLE}")
   else()
     set(LLVM_TARGET_TRIPLE "${LLVM_DEFAULT_TARGET_TRIPLE}")