Index: cfe/trunk/include/clang/Driver/CC1Options.td
===================================================================
--- cfe/trunk/include/clang/Driver/CC1Options.td
+++ cfe/trunk/include/clang/Driver/CC1Options.td
@@ -687,7 +687,7 @@
 def version : Flag<["-"], "version">,
   HelpText<"Print the compiler version">;
 def main_file_name : Separate<["-"], "main-file-name">,
-  HelpText<"Main file name to use for debug info">;
+  HelpText<"Main file name to use for debug info and source if missing">;
 def split_dwarf_output : Separate<["-"], "split-dwarf-output">,
   HelpText<"File name to use for split dwarf debug info output">;
 
Index: cfe/trunk/lib/CodeGen/ModuleBuilder.cpp
===================================================================
--- cfe/trunk/lib/CodeGen/ModuleBuilder.cpp
+++ cfe/trunk/lib/CodeGen/ModuleBuilder.cpp
@@ -65,6 +65,13 @@
   private:
     SmallVector<FunctionDecl *, 8> DeferredInlineMemberFuncDefs;
 
+    static llvm::StringRef ExpandModuleName(llvm::StringRef ModuleName,
+                                            const CodeGenOptions &CGO) {
+      if (ModuleName == "-" && !CGO.MainFileName.empty())
+        return CGO.MainFileName;
+      return ModuleName;
+    }
+
   public:
     CodeGeneratorImpl(DiagnosticsEngine &diags, llvm::StringRef ModuleName,
                       const HeaderSearchOptions &HSO,
@@ -73,7 +80,8 @@
                       CoverageSourceInfo *CoverageInfo = nullptr)
         : Diags(diags), Ctx(nullptr), HeaderSearchOpts(HSO),
           PreprocessorOpts(PPO), CodeGenOpts(CGO), HandlingTopLevelDecls(0),
-          CoverageInfo(CoverageInfo), M(new llvm::Module(ModuleName, C)) {
+          CoverageInfo(CoverageInfo),
+          M(new llvm::Module(ExpandModuleName(ModuleName, CGO), C)) {
       C.setDiscardValueNames(CGO.DiscardValueNames);
     }
 
@@ -121,7 +129,7 @@
     llvm::Module *StartModule(llvm::StringRef ModuleName,
                               llvm::LLVMContext &C) {
       assert(!M && "Replacing existing Module?");
-      M.reset(new llvm::Module(ModuleName, C));
+      M.reset(new llvm::Module(ExpandModuleName(ModuleName, CodeGenOpts), C));
       Initialize(*Ctx);
       return M.get();
     }
Index: cfe/trunk/test/Frontend/stdin-input.c
===================================================================
--- cfe/trunk/test/Frontend/stdin-input.c
+++ cfe/trunk/test/Frontend/stdin-input.c
@@ -0,0 +1,7 @@
+// RUN: cat %s | %clang -emit-llvm -g -S \
+// RUN: -Xclang -main-file-name -Xclang test/foo.c -x c - -o - | FileCheck %s
+// CHECK: ; ModuleID = 'test/foo.c'
+// CHECK: source_filename = "test/foo.c"
+// CHECK: !1 = !DIFile(filename: "test/foo.c"
+
+int main() {}