[ADT] Update RPOT to work with specializations of different types.

At the moment, ReversePostOrderTraversal performs a post-order walk on
the entry node of the passed in graph, rather than the graph type
itself.

If GT::NodeRef is the same as GraphT, everything works as expected and
this is the case for the current uses in-tree. But it does not work as
expected if GraphT != GT::NodeRef. In that case, we either fail to build
(if there is no GraphTrait specialization for GT:NodeRef) or we pick the
GraphTrait specialization for GT::NodeRef, instead of the specialization
of GraphT.

Both the depth-first and post-order iterators pick the expected
specalization and this patch updates ReversePostOrderTraversal to
delegate to po_begin & po_end to pick the right specialization, rather
than forcing using GraphTraits<GT::NodeRef>, by first getting the entry
node.

This makes `ReversePostOrderTraversal<Graph<6>> RPOT(G);` build and
work as expected in the test.

Reviewed By: dexonsmith

Differential Revision: https://reviews.llvm.org/D100169

include/llvm/ADT/PostOrderIterator.h

@@ -292,15 +292,15 @@ class ReversePostOrderTraversal {
 
   std::vector<NodeRef> Blocks; // Block list in normal PO order
 
-  void Initialize(NodeRef BB) {
+  void Initialize(const GraphT &G) {
-    std::copy(po_begin(BB), po_end(BB), std::back_inserter(Blocks));
+    std::copy(po_begin(G), po_end(G), std::back_inserter(Blocks));
   }
 
 public:
   using rpo_iterator = typename std::vector<NodeRef>::reverse_iterator;
   using const_rpo_iterator = typename std::vector<NodeRef>::const_reverse_iterator;
 
-  ReversePostOrderTraversal(GraphT G) { Initialize(GT::getEntryNode(G)); }
+  ReversePostOrderTraversal(const GraphT &G) { Initialize(G); }
 
   // Because we want a reverse post order, use reverse iterators from the vector
   rpo_iterator begin() { return Blocks.rbegin(); }

unittests/ADT/PostOrderIteratorTest.cpp

@@ -9,6 +9,8 @@
 #include "llvm/IR/BasicBlock.h"
 #include "llvm/IR/CFG.h"
 #include "gtest/gtest.h"
+#include "TestGraph.h"
+
 using namespace llvm;
 
 namespace {
@@ -33,4 +35,40 @@ TEST(PostOrderIteratorTest, Compiles) {
   auto PIExt = po_ext_end(NullBB, Ext);
   PIExt.insertEdge(Optional<BasicBlock *>(), NullBB);
 }
+
+// Test post-order and reverse post-order traversals for simple graph type.
+TEST(PostOrderIteratorTest, PostOrderAndReversePostOrderTraverrsal) {
+  Graph<6> G;
+  G.AddEdge(0, 1);
+  G.AddEdge(0, 2);
+  G.AddEdge(0, 3);
+  G.AddEdge(1, 4);
+  G.AddEdge(2, 5);
+  G.AddEdge(5, 2);
+  G.AddEdge(2, 4);
+  G.AddEdge(1, 4);
+
+  SmallVector<int> FromIterator;
+  for (auto N : post_order(G))
+    FromIterator.push_back(N->first);
+  EXPECT_EQ(6u, FromIterator.size());
+  EXPECT_EQ(4, FromIterator[0]);
+  EXPECT_EQ(1, FromIterator[1]);
+  EXPECT_EQ(5, FromIterator[2]);
+  EXPECT_EQ(2, FromIterator[3]);
+  EXPECT_EQ(3, FromIterator[4]);
+  EXPECT_EQ(0, FromIterator[5]);
+  FromIterator.clear();
+
+  ReversePostOrderTraversal<Graph<6>> RPOT(G);
+  for (auto N : RPOT)
+    FromIterator.push_back(N->first);
+  EXPECT_EQ(6u, FromIterator.size());
+  EXPECT_EQ(0, FromIterator[0]);
+  EXPECT_EQ(3, FromIterator[1]);
+  EXPECT_EQ(2, FromIterator[2]);
+  EXPECT_EQ(5, FromIterator[3]);
+  EXPECT_EQ(1, FromIterator[4]);
+  EXPECT_EQ(4, FromIterator[5]);
+}
 }