Index: llvm/trunk/include/llvm/Support/Compiler.h
===================================================================
--- llvm/trunk/include/llvm/Support/Compiler.h
+++ llvm/trunk/include/llvm/Support/Compiler.h
@@ -16,7 +16,9 @@
 
 #include "llvm/Config/llvm-config.h"
 
+#ifdef __cplusplus
 #include <new>
+#endif
 #include <stddef.h>
 
 #if defined(_MSC_VER)
@@ -252,10 +254,8 @@
 #define LLVM_FALLTHROUGH [[fallthrough]]
 #elif __has_cpp_attribute(gnu::fallthrough)
 #define LLVM_FALLTHROUGH [[gnu::fallthrough]]
-#elif !__cplusplus
-// Workaround for llvm.org/PR23435, since clang 3.6 and below emit a spurious
-// error when __has_cpp_attribute is given a scoped attribute in C mode.
-#define LLVM_FALLTHROUGH
+#elif !defined(__cplusplus) && __has_attribute(fallthrough)
+#define LLVM_FALLTHROUGH __attribute__((fallthrough))
 #elif __has_cpp_attribute(clang::fallthrough)
 #define LLVM_FALLTHROUGH [[clang::fallthrough]]
 #else
@@ -527,6 +527,7 @@
 #define LLVM_ENABLE_EXCEPTIONS 1
 #endif
 
+#ifdef __cplusplus
 namespace llvm {
 
 /// Allocate a buffer of memory with the given size and alignment.
@@ -569,4 +570,5 @@
 
 } // End namespace llvm
 
+#endif // __cplusplus
 #endif
Index: llvm/trunk/lib/Support/regcomp.c
===================================================================
--- llvm/trunk/lib/Support/regcomp.c
+++ llvm/trunk/lib/Support/regcomp.c
@@ -48,6 +48,7 @@
 #include "regex2.h"
 
 #include "llvm/Config/config.h"
+#include "llvm/Support/Compiler.h"
 
 /* character-class table */
 static struct cclass {
@@ -537,7 +538,7 @@
 		break;
 	case '{':		/* okay as ordinary except if digit follows */
 		REQUIRE(!MORE() || !isdigit((uch)PEEK()), REG_BADRPT);
-		/* fall through */
+		LLVM_FALLTHROUGH;
 	default:
 		ordinary(p, c);
 		break;
@@ -733,7 +734,7 @@
 		break;
 	case '*':
 		REQUIRE(starordinary, REG_BADRPT);
-		/* fall through */
+		LLVM_FALLTHROUGH;
 	default:
 		ordinary(p, (char)c);
 		break;
@@ -1635,7 +1636,7 @@
 					return;
 				}
 			} while (OP(s) != O_QUEST && OP(s) != O_CH);
-			/* fall through */
+			LLVM_FALLTHROUGH;
 		default:		/* things that break a sequence */
 			if (newlen > g->mlen) {		/* ends one */
 				start = newstart;