Index: lib/Sema/SemaExpr.cpp
===================================================================
--- lib/Sema/SemaExpr.cpp
+++ lib/Sema/SemaExpr.cpp
@@ -905,6 +905,16 @@
                           diag::err_call_incomplete_argument))
     return ExprError();
 
+  // If this is a literal zero that is narrower than a pointer, widen it to a
+  // pointer-sized integer. This allows passing NULL through variadic functions
+  // in pre-C++11 codebases where NULL is defined to 0 or 0L on LLP64 platforms.
+  if (E->getType()->isIntegerType() &&
+      Context.getTypeSize(E->getType()) <
+          Context.getTargetInfo().getPointerWidth(0) &&
+      E->isNullPointerConstant(Context, Expr::NPC_ValueDependentIsNotNull)) {
+    return ImpCastExprToType(E, Context.getIntPtrType(), CK_IntegralCast);
+  }
+
   return E;
 }
 
Index: test/CodeGen/variadic-null-llp64.c
===================================================================
--- /dev/null
+++ test/CodeGen/variadic-null-llp64.c
@@ -0,0 +1,14 @@
+// RUN: %clang_cc1 %s -emit-llvm -o - -triple x86_64-windows-msvc | FileCheck %s
+
+// 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);
+}
+// CHECK: define void @f(i8* %f)
+// CHECK: call void (i8*, ...)* @v(i8* {{.*}}, i32 1, i32 2, i32 3, i64 0)