Index: clang/lib/Frontend/InitPreprocessor.cpp
===================================================================
--- clang/lib/Frontend/InitPreprocessor.cpp
+++ clang/lib/Frontend/InitPreprocessor.cpp
@@ -1317,11 +1317,10 @@
   llvm::raw_string_ostream Predefines(PredefineBuffer);
   MacroBuilder Builder(Predefines);
 
-  // Emit line markers for various builtin sections of the file.  We don't do
-  // this in asm preprocessor mode, because "# 4" is not a line marker directive
-  // in this mode.
-  if (!PP.getLangOpts().AsmPreprocessor)
-    Builder.append("# 1 \"<built-in>\" 3");
+  // Emit line markers for various builtin sections of the file. The 3 here
+  // marks <built-in> as being a system header, which suppresses warnings when
+  // the same macro is defined multiple times.
+  Builder.append("# 1 \"<built-in>\" 3");
 
   // Install things like __POWERPC__, __GNUC__, etc into the macro table.
   if (InitOpts.UsePredefines) {
@@ -1359,8 +1358,7 @@
 
   // Add on the predefines from the driver.  Wrap in a #line directive to report
   // that they come from the command line.
-  if (!PP.getLangOpts().AsmPreprocessor)
-    Builder.append("# 1 \"<command line>\" 1");
+  Builder.append("# 1 \"<command line>\" 1");
 
   // Process #define's and #undef's in the order they are given.
   for (unsigned i = 0, e = InitOpts.Macros.size(); i != e; ++i) {
@@ -1372,8 +1370,7 @@
   }
 
   // Exit the command line and go back to <built-in> (2 is LC_LEAVE).
-  if (!PP.getLangOpts().AsmPreprocessor)
-    Builder.append("# 1 \"<built-in>\" 2");
+  Builder.append("# 1 \"<built-in>\" 2");
 
   // If -imacros are specified, include them now.  These are processed before
   // any -include directives.
Index: clang/lib/Lex/PPDirectives.cpp
===================================================================
--- clang/lib/Lex/PPDirectives.cpp
+++ clang/lib/Lex/PPDirectives.cpp
@@ -1185,8 +1185,12 @@
                                     CurPPLexer->getConditionalStackDepth() > 0);
     return;
   case tok::numeric_constant:  // # 7  GNU line marker directive.
-    if (getLangOpts().AsmPreprocessor)
-      break;  // # 4 is not a preprocessor directive in .S files.
+    // In a .S file "# 4" may be a comment so don't treat it as a preprocessor
+    // directive. However do permit it in the predefines file, as we use line
+    // markers to mark the builtin macros as being in a system header.
+    if (getLangOpts().AsmPreprocessor &&
+        SourceMgr.getFileID(SavedHash.getLocation()) != getPredefinesFileID())
+      break;
     return HandleDigitDirective(Result);
   default:
     IdentifierInfo *II = Result.getIdentifierInfo();
Index: clang/test/Preprocessor/directives_asm.S
===================================================================
--- /dev/null
+++ clang/test/Preprocessor/directives_asm.S
@@ -0,0 +1,25 @@
+// RUN: %clang -c %s -o /dev/null 2>&1 | FileCheck %s
+
+// Check that preprocessor directives are recognised as such, but lines starting
+// with a # that aren't directives are instead treated as comments.
+
+#define MACRO .warning "This is a macro"
+        MACRO
+
+// CHECK: directives_asm.S:7:9: warning: This is a macro
+
+#not a preprocessing directive
+
+// CHECK-NOT: error: invalid preprocessing directive
+
+# 100
+
+        .warning "line number should not change"
+
+// CHECK: directives_asm.S:17:9: warning: line number should not change
+
+#line 100
+
+        .warning "line number should change"
+
+// CHECK: directives_asm.S:101:9: warning: line number should change
Index: clang/test/Preprocessor/macro_redefined.S
===================================================================
--- /dev/null
+++ clang/test/Preprocessor/macro_redefined.S
@@ -0,0 +1,10 @@
+// RUN: %clang %s -E -DCLI_MACRO=1 2>&1 | FileCheck %s
+
+#define CLI_MACRO
+// CHECK: macro_redefined.S{{.+}}: warning: 'CLI_MACRO' macro redefined
+// CHECK: <command line>{{.+}}: note: previous definition is here
+
+#define REGULAR_MACRO
+#define REGULAR_MACRO 1
+// CHECK: macro_redefined.S{{.+}}: warning: 'REGULAR_MACRO' macro redefined
+// CHECK: macro_redefined.S{{.+}}: note: previous definition is here