[Dominator] Add a way to dump a `dot` representation of the dominator tree

[davide]

Summary

This is very similar to what we do for CFG and SelectionDAG.
I find it very useful when I'm in VisualStudio (or any other debugger). I can just print DT.viewGraph() and that pops a new window with the dom displayed. As far as I can tell the only other way to dump the dom is manually using DT.print(dbgs()), which for large dominator trees is not that readable.

[• dberlin]

So, i use -view-dom-tree, which does this.
But i can understand wanting to do it programmatically, so LGTM.

[davide]

(

In D32145#732369, @dberlin wrote:

So, i use -view-dom-tree, which does this.
But i can understand wanting to do it programmatically, so LGTM.

Thanks!

include/llvm/IR/Dominators.h

public:
   /// \brief Provide an overload for a Use.
   bool isReachableFromEntry(const Use &U) const;
 
   /// \brief Verify the correctness of the domtree by re-computing it.
   ///
   /// This should only be used for debugging as it aborts the program if the
   /// verification fails.
   void verifyDomTree() const;
+
+  // Pop up a GraphViz/gv window with the Dominator Tree rendered using `dot`.
+  void viewGraph(const Twine &Name, const Twine &Title);
+  void viewGraph();
 };
 
 //===-------------------------------------
 // DominatorTree GraphTraits specializations so the DominatorTree can be
 // iterable by generic graph iterators.
 
 template <class Node, class ChildIterator> struct DomTreeGraphTraitsBase {
   typedef Node *NodeRef;

lib/Analysis/DomPrinter.cpp

struct DOTGraphTraits<PostDominatorTree*>
   }
 
   std::string getNodeLabel(DomTreeNode *Node, PostDominatorTree *G ) {
     return DOTGraphTraits<DomTreeNode*>::getNodeLabel(Node, G->getRootNode());
   }
 };
 }
 
+void DominatorTree::viewGraph(const Twine &Name, const Twine &Title) {
+#ifndef NDEBUG
+  ViewGraph(this, Name, false, Title);
+#else
+  errs() << "build with NDEBUG\n";
+#endif  // NDEBUG
+}
+
+void DominatorTree::viewGraph() {
+#ifndef NDEBUG
+  this->viewGraph("my dom", "dom");
+#else
+  errs() << "build with NDEBUG\n";
+#endif  // NDEBUG
+}
+
 namespace {
 struct DominatorTreeWrapperPassAnalysisGraphTraits {
   static DominatorTree *getGraph(DominatorTreeWrapperPass *DTWP) {
     return &DTWP->getDomTree();
   }
 };
 
 struct DomViewer : public DOTGraphTraitsViewer<