[IR] Add getArg() method to Function class

Adds a method which, when called with function.getArg(i), returns an
Argument* to the i'th argument.

Patch by Henry Wildermuth

Differential Revision: https://reviews.llvm.org/D64925

llvm-svn: 367576

include/llvm/IR/Function.h

@@ -710,6 +710,12 @@ public:
     return Arguments + NumArgs;
   }
 
+  Argument* getArg(unsigned i) const {
+    assert (i < NumArgs && "getArg() out of range!");
+    CheckLazyArguments();
+    return Arguments + i;
+  }
+
   iterator_range<arg_iterator> args() {
     return make_range(arg_begin(), arg_end());
   }

unittests/IR/FunctionTest.cpp

@@ -33,6 +33,14 @@ TEST(FunctionTest, hasLazyArguments) {
   // The argument list should be populated at first access.
   (void)F->arg_begin();
   EXPECT_FALSE(F->hasLazyArguments());
+
+  // Checking that getArg gets the arguments from F1 in the correct order.
+  unsigned i = 0;
+  for (Argument &A : F->args()) {
+    EXPECT_EQ(&A, F->getArg(i));
+    ++i;
+  }
+  EXPECT_FALSE(F->hasLazyArguments());
 }
 
 TEST(FunctionTest, stealArgumentListFrom) {