Index: cfe/trunk/lib/CodeGen/CGCall.cpp
===================================================================
--- cfe/trunk/lib/CodeGen/CGCall.cpp
+++ cfe/trunk/lib/CodeGen/CGCall.cpp
@@ -2728,6 +2728,24 @@
   args.add(EmitAnyExprToTemp(E), type);
 }
 
+QualType CodeGenFunction::getVarArgType(const Expr *Arg) {
+  // System headers on Windows define NULL to 0 instead of 0LL on Win64. MSVC
+  // implicitly widens null pointer constants that are arguments to varargs
+  // functions to pointer-sized ints.
+  if (!getTarget().getTriple().isOSWindows())
+    return Arg->getType();
+
+  if (Arg->getType()->isIntegerType() &&
+      getContext().getTypeSize(Arg->getType()) <
+          getContext().getTargetInfo().getPointerWidth(0) &&
+      Arg->isNullPointerConstant(getContext(),
+                                 Expr::NPC_ValueDependentIsNotNull)) {
+    return getContext().getIntPtrType();
+  }
+
+  return Arg->getType();
+}
+
 // In ObjC ARC mode with no ObjC ARC exception safety, tell the ARC
 // optimizer it can aggressively ignore unwind edges.
 void
@@ -3023,6 +3041,11 @@
         else
           V = Builder.CreateLoad(RV.getAggregateAddr());
 
+        // We might have to widen integers, but we should never truncate.
+        if (ArgInfo.getCoerceToType() != V->getType() &&
+            V->getType()->isIntegerTy())
+          V = Builder.CreateZExt(V, ArgInfo.getCoerceToType());
+
         // If the argument doesn't match, perform a bitcast to coerce it.  This
         // can happen due to trivial type mismatches.
         if (FirstIRArg < IRFuncTy->getNumParams() &&
Index: cfe/trunk/lib/CodeGen/CodeGenFunction.h
===================================================================
--- cfe/trunk/lib/CodeGen/CodeGenFunction.h
+++ cfe/trunk/lib/CodeGen/CodeGenFunction.h
@@ -2752,7 +2752,7 @@
 
     // If we still have any arguments, emit them using the type of the argument.
     for (; Arg != ArgEnd; ++Arg)
-      ArgTypes.push_back(Arg->getType());
+      ArgTypes.push_back(getVarArgType(*Arg));
 
     EmitCallArgs(Args, ArgTypes, ArgBeg, ArgEnd, CalleeDecl, ParamsToSkip,
                  ForceColumnInfo);
@@ -2765,6 +2765,8 @@
                     unsigned ParamsToSkip = 0, bool ForceColumnInfo = false);
 
 private:
+  QualType getVarArgType(const Expr *Arg);
+
   const TargetCodeGenInfo &getTargetHooks() const {
     return CGM.getTargetCodeGenInfo();
   }
Index: cfe/trunk/test/CodeGen/variadic-null-win64.c
===================================================================
--- cfe/trunk/test/CodeGen/variadic-null-win64.c
+++ cfe/trunk/test/CodeGen/variadic-null-win64.c
@@ -0,0 +1,17 @@
+// RUN: %clang_cc1 %s -emit-llvm -o - -triple x86_64-windows-msvc | FileCheck %s --check-prefix=WINDOWS
+// RUN: %clang_cc1 %s -emit-llvm -o - -triple x86_64-linux | FileCheck %s --check-prefix=LINUX
+
+// Make it possible to pass NULL through variadic functions on platforms where
+// NULL has an integer type that is more narrow than a pointer. On such
+// platforms we widen null pointer constants to a pointer-sized integer.
+
+#define NULL 0
+
+void v(const char *f, ...);
+void f(const char *f) {
+  v(f, 1, 2, 3, NULL);
+}
+// WINDOWS: define void @f(i8* %f)
+// WINDOWS: call void (i8*, ...)* @v(i8* {{.*}}, i32 1, i32 2, i32 3, i64 0)
+// LINUX: define void @f(i8* %f)
+// LINUX: call void (i8*, ...)* @v(i8* {{.*}}, i32 1, i32 2, i32 3, i32 0)