diff --git a/llvm/include/llvm/Support/ErrorHandling.h b/llvm/include/llvm/Support/ErrorHandling.h
--- a/llvm/include/llvm/Support/ErrorHandling.h
+++ b/llvm/include/llvm/Support/ErrorHandling.h
@@ -124,7 +124,9 @@
 
 /// Marks that the current location is not supposed to be reachable.
 /// In !NDEBUG builds, prints the message and location info to stderr.
-/// In NDEBUG builds, the behavior is controlled by the CMake flag
+/// In NDEBUG builds, if the platform does not support a builtin unreachable
+/// then we call an internal LLVM runtime function. Otherwise the behavior is
+/// controlled by the CMake flag
 ///   -DLLVM_UNREACHABLE_OPTIMIZE
 /// * When "ON" (default) llvm_unreachable() becomes an optimizer hint
 ///   that the current location is not supposed to be reachable: the hint
@@ -139,12 +141,12 @@
 #ifndef NDEBUG
 #define llvm_unreachable(msg) \
   ::llvm::llvm_unreachable_internal(msg, __FILE__, __LINE__)
-#elif !LLVM_UNREACHABLE_OPTIMIZE
-#define llvm_unreachable(msg) LLVM_BUILTIN_TRAP
-#elif defined(LLVM_BUILTIN_UNREACHABLE)
+#elif !defined(LLVM_BUILTIN_UNREACHABLE)
+#define llvm_unreachable(msg) ::llvm::llvm_unreachable_internal()
+#elif LLVM_UNREACHABLE_OPTIMIZE
 #define llvm_unreachable(msg) LLVM_BUILTIN_UNREACHABLE
 #else
-#define llvm_unreachable(msg) ::llvm::llvm_unreachable_internal()
+#define llvm_unreachable(msg) LLVM_BUILTIN_TRAP, LLVM_BUILTIN_UNREACHABLE
 #endif
 
 #endif