diff --git a/llvm/lib/Support/regcomp.c b/llvm/lib/Support/regcomp.c
--- a/llvm/lib/Support/regcomp.c
+++ b/llvm/lib/Support/regcomp.c
@@ -537,7 +537,7 @@
 		break;
 	case '{':		/* okay as ordinary except if digit follows */
 		REQUIRE(!MORE() || !isdigit((uch)PEEK()), REG_BADRPT);
-		/* FALLTHROUGH */
+		__attribute__((fallthrough));
 	default:
 		ordinary(p, c);
 		break;
@@ -733,7 +733,7 @@
 		break;
 	case '*':
 		REQUIRE(starordinary, REG_BADRPT);
-		/* FALLTHROUGH */
+		__attribute__((fallthrough));
 	default:
 		ordinary(p, (char)c);
 		break;
@@ -1635,7 +1635,7 @@
 					return;
 				}
 			} while (OP(s) != O_QUEST && OP(s) != O_CH);
-			/* fallthrough */
+			__attribute__((fallthrough));
 		default:		/* things that break a sequence */
 			if (newlen > g->mlen) {		/* ends one */
 				start = newstart;