Index: include/llvm/IR/Argument.h
===================================================================
--- include/llvm/IR/Argument.h
+++ include/llvm/IR/Argument.h
@@ -32,13 +32,14 @@
 class Argument : public Value, public ilist_node<Argument> {
   virtual void anchor();
   Function *Parent;
+  unsigned ArgNo;
 
   friend class SymbolTableListTraits<Argument>;
   void setParent(Function *parent);
 
 public:
   /// Argument constructor.
-  explicit Argument(Type *Ty, const Twine &Name = "");
+  explicit Argument(Type *Ty, const Twine &Name = "", unsigned ArgNo = 0);
 
   inline const Function *getParent() const { return Parent; }
   inline       Function *getParent()       { return Parent; }
@@ -46,7 +47,7 @@
   /// Return the index of this formal argument in its containing function.
   ///
   /// For example in "void foo(int a, float b)" a is 0 and b is 1.
-  unsigned getArgNo() const;
+  unsigned getArgNo() const { return ArgNo; }
 
   /// Return true if this argument has the nonnull attribute. Also returns true
   /// if at least one byte is known to be dereferenceable and the pointer is in
Index: lib/IR/Function.cpp
===================================================================
--- lib/IR/Function.cpp
+++ lib/IR/Function.cpp
@@ -39,8 +39,8 @@
 
 void Argument::anchor() { }
 
-Argument::Argument(Type *Ty, const Twine &Name)
-    : Value(Ty, Value::ArgumentVal), Parent(nullptr) {
+Argument::Argument(Type *Ty, const Twine &Name, unsigned ArgNo)
+    : Value(Ty, Value::ArgumentVal), Parent(nullptr), ArgNo(ArgNo) {
   setName(Name);
 }
 
@@ -48,18 +48,6 @@
   Parent = parent;
 }
 
-unsigned Argument::getArgNo() const {
-  const Function *F = getParent();
-  assert(F && "Argument is not in a function");
-
-  Function::const_arg_iterator AI = F->arg_begin();
-  unsigned ArgIdx = 0;
-  for (; &*AI != this; ++AI)
-    ++ArgIdx;
-
-  return ArgIdx;
-}
-
 bool Argument::hasNonNullAttr() const {
   if (!getType()->isPointerTy()) return false;
   if (getParent()->getAttributes().
@@ -240,7 +228,8 @@
   for (unsigned i = 0, e = FT->getNumParams(); i != e; ++i) {
     assert(!FT->getParamType(i)->isVoidTy() &&
            "Cannot have void typed arguments!");
-    ArgumentList.push_back(new Argument(FT->getParamType(i)));
+    ArgumentList.push_back(
+        new Argument(FT->getParamType(i), "", i));
   }
 
   // Clear the lazy arguments bit.