Index: lib/Transforms/IPO/FunctionAttrs.cpp
===================================================================
--- lib/Transforms/IPO/FunctionAttrs.cpp
+++ lib/Transforms/IPO/FunctionAttrs.cpp
@@ -321,23 +321,19 @@
       return true;
     }
 
-    bool Found = false;
-    Function::arg_iterator AI = F->arg_begin(), AE = F->arg_end();
-    for (CallSite::arg_iterator PI = CS.arg_begin(), PE = CS.arg_end();
-         PI != PE; ++PI, ++AI) {
-      if (AI == AE) {
-        assert(F->isVarArg() && "More params than args in non-varargs call");
-        Captured = true;
-        return true;
-      }
-      if (PI == U) {
-        Uses.push_back(&*AI);
-        Found = true;
-        break;
-      }
+    unsigned UseIndex =
+      std::distance(const_cast<const Use *>(CS->op_begin()), U);
+
+    assert(UseIndex < CS.arg_size() && "Non-argument use?");
+    if (UseIndex >= F->arg_size()) {
+      assert(F->isVarArg() && "More params than args in non-varargs call");
+      Captured = true;
+      return true;
     }
-    assert(Found && "Capturing call-site captured nothing?");
-    (void)Found;
+
+    auto FormalArgIt = F->arg_begin();
+    std::advance(FormalArgIt, UseIndex);
+    Uses.push_back(FormalArgIt);
     return false;
   }