diff --git a/llvm/lib/Support/ErrorHandling.cpp b/llvm/lib/Support/ErrorHandling.cpp
index f4b591e777eb..9425445a853a 100644
--- a/llvm/lib/Support/ErrorHandling.cpp
+++ b/llvm/lib/Support/ErrorHandling.cpp
@@ -96,4 +96,9 @@ void llvm::llvm_unreachable_internal(const char *msg, const char *file,
     dbgs() << " at " << file << ":" << line;
   dbgs() << "!\n";
   abort();
+#ifdef LLVM_BUILTIN_UNREACHABLE
+  // Windows systems and possibly others don't declare abort() to be noreturn,
+  // so use the unreachable builtin to avoid a Clang self-host warning.
+  LLVM_BUILTIN_UNREACHABLE;
+#endif
 }
diff --git a/llvm/lib/Support/Windows/Path.inc b/llvm/lib/Support/Windows/Path.inc
index 5787636aa687..5a62a609b830 100644
--- a/llvm/lib/Support/Windows/Path.inc
+++ b/llvm/lib/Support/Windows/Path.inc
@@ -756,7 +756,6 @@ error_code mapped_file_region::init(int FD, bool CloseFD, uint64_t Offset) {
   case readonly:  flprotect = PAGE_READONLY; break;
   case readwrite: flprotect = PAGE_READWRITE; break;
   case priv:      flprotect = PAGE_WRITECOPY; break;
-  default: llvm_unreachable("invalid mapping mode");
   }
 
   FileMappingHandle = ::CreateFileMapping(FileHandle,
@@ -780,7 +779,6 @@ error_code mapped_file_region::init(int FD, bool CloseFD, uint64_t Offset) {
   case readonly:  dwDesiredAccess = FILE_MAP_READ; break;
   case readwrite: dwDesiredAccess = FILE_MAP_WRITE; break;
   case priv:      dwDesiredAccess = FILE_MAP_COPY; break;
-  default: llvm_unreachable("invalid mapping mode");
   }
   Mapping = ::MapViewOfFile(FileMappingHandle,
                             dwDesiredAccess,