Index: lib/CodeGen/CGCall.cpp
===================================================================
--- lib/CodeGen/CGCall.cpp	(revision 77045)
+++ lib/CodeGen/CGCall.cpp	(working copy)
@@ -505,6 +505,13 @@ void CodeGenModule::ConstructAttributeLi
 void CodeGenFunction::EmitFunctionProlog(const CGFunctionInfo &FI,
                                          llvm::Function *Fn,
                                          const FunctionArgList &Args) {
+
+  // If this is main, initialize the return value to zero.
+  // Implements C++ [basic.start.main]p5 and C99 5.1.2.2.3.
+  if (const FunctionDecl* FD = dyn_cast<FunctionDecl>(CurFuncDecl))
+    if (FD->isMain())
+      Builder.CreateStore(llvm::ConstantInt::get(LLVMIntTy, 0), ReturnValue);
+
   // FIXME: We no longer need the types from FunctionArgList; lift up and
   // simplify.