Replace custom written DFS walk with depth first iterator

Summary:
GenericDomTreeConstruction had its own written DFS walk.
It is basically identical to the DFS walk df_* is doing in the iterators.
the one difference is that df_iterator uses an internal visited set.
The GenericDomTreeConstruction one reused a field in an existing densemap lookup.

Time-wise, this way is actually more cache-friendly (the previous way has a random store
into a successor's info, the new way does that store at the same time and in the same place
as other stores to the same info)

It costs some very small amount of memory to do this, and one we pay in some other part of
dom tree construction *anyway*, so we aren't really increasing dom tree constructions's
peak memory usage.

It could still be changed to use the old field with a little work on df_ext_* if we care
(and if someone find performance regressions)

Reviewers: chandlerc

Reviewed By: chandlerc

Subscribers: Eugene.Zelenko, llvm-commits

Differential Revision: https://reviews.llvm.org/D8932

llvm-svn: 294339

llvm/include/llvm/Support/GenericDomTree.h

@@ -683,6 +683,10 @@ template <class NodeT> class DominatorTreeBase : public DominatorBase<NodeT> {
   Eval(DominatorTreeBaseByGraphTraits<GraphT> &DT, typename GraphT::NodeRef V,
        unsigned LastLinked);
 
+  template <class GraphT>
+  friend unsigned ReverseDFSPass(DominatorTreeBaseByGraphTraits<GraphT> &DT,
+                                 typename GraphT::NodeRef V, unsigned N);
+
   template <class GraphT>
   friend unsigned DFSPass(DominatorTreeBaseByGraphTraits<GraphT> &DT,
                           typename GraphT::NodeRef V, unsigned N);

llvm/include/llvm/Support/GenericDomTreeConstruction.h

@@ -24,82 +24,77 @@
 #ifndef LLVM_SUPPORT_GENERICDOMTREECONSTRUCTION_H
 #define LLVM_SUPPORT_GENERICDOMTREECONSTRUCTION_H
 
+#include "llvm/ADT/DepthFirstIterator.h"
 #include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/Support/GenericDomTree.h"
 
 namespace llvm {
 
-template <class GraphT>
-unsigned DFSPass(DominatorTreeBaseByGraphTraits<GraphT> &DT,
-                 typename GraphT::NodeRef V, unsigned N) {
-  // This is more understandable as a recursive algorithm, but we can't use the
-  // recursive algorithm due to stack depth issues.  Keep it here for
-  // documentation purposes.
-#if 0
-  InfoRec &VInfo = DT.Info[DT.Roots[i]];
-  VInfo.DFSNum = VInfo.Semi = ++N;
-  VInfo.Label = V;
-
-  Vertex.push_back(V);        // Vertex[n] = V;
-
-  for (succ_iterator SI = succ_begin(V), E = succ_end(V); SI != E; ++SI) {
-    InfoRec &SuccVInfo = DT.Info[*SI];
-    if (SuccVInfo.Semi == 0) {
-      SuccVInfo.Parent = V;
-      N = DTDFSPass(DT, *SI, N);
-    }
+// External storage for depth first iterator that reuses the info lookup map
+// domtree already has.  We don't have a set, but a map instead, so we are
+// converting the one argument insert calls.
+template <class NodeRef, class InfoType> struct df_iterator_dom_storage {
+public:
+  typedef DenseMap<NodeRef, InfoType> BaseSet;
+  df_iterator_dom_storage(BaseSet &Storage) : Storage(Storage) {}
+
+  typedef typename BaseSet::iterator iterator;
+  std::pair<iterator, bool> insert(NodeRef N) {
+    return Storage.insert({N, InfoType()});
   }
-#else
-  bool IsChildOfArtificialExit = (N != 0);
+  void completed(NodeRef) {}
 
-  SmallVector<
-      std::pair<typename GraphT::NodeRef, typename GraphT::ChildIteratorType>,
-      32>
-      Worklist;
-  Worklist.push_back(std::make_pair(V, GraphT::child_begin(V)));
-  while (!Worklist.empty()) {
-    typename GraphT::NodeRef BB = Worklist.back().first;
-    typename GraphT::ChildIteratorType NextSucc = Worklist.back().second;
+private:
+  BaseSet &Storage;
+};
 
+template <class GraphT>
+unsigned ReverseDFSPass(DominatorTreeBaseByGraphTraits<GraphT> &DT,
+                        typename GraphT::NodeRef V, unsigned N) {
+  df_iterator_dom_storage<
+      typename GraphT::NodeRef,
+      typename DominatorTreeBaseByGraphTraits<GraphT>::InfoRec>
+      DFStorage(DT.Info);
+  bool IsChildOfArtificialExit = (N != 0);
+  for (auto I = idf_ext_begin(V, DFStorage), E = idf_ext_end(V, DFStorage);
+       I != E; ++I) {
+    typename GraphT::NodeRef BB = *I;
     auto &BBInfo = DT.Info[BB];
+    BBInfo.DFSNum = BBInfo.Semi = ++N;
+    BBInfo.Label = BB;
+    // Set the parent to the top of the visited stack.  The stack includes us,
+    // and is 1 based, so we subtract to account for both of these.
+    if (I.getPathLength() > 1)
+      BBInfo.Parent = DT.Info[I.getPath(I.getPathLength() - 2)].DFSNum;
+    DT.Vertex.push_back(BB); // Vertex[n] = V;
 
-    // First time we visited this BB?
-    if (NextSucc == GraphT::child_begin(BB)) {
-      BBInfo.DFSNum = BBInfo.Semi = ++N;
-      BBInfo.Label = BB;
-
-      DT.Vertex.push_back(BB);       // Vertex[n] = V;
-
-      if (IsChildOfArtificialExit)
-        BBInfo.Parent = 1;
-
-      IsChildOfArtificialExit = false;
-    }
-
-    // store the DFS number of the current BB - the reference to BBInfo might
-    // get invalidated when processing the successors.
-    unsigned BBDFSNum = BBInfo.DFSNum;
-
-    // If we are done with this block, remove it from the worklist.
-    if (NextSucc == GraphT::child_end(BB)) {
-      Worklist.pop_back();
-      continue;
-    }
-
-    // Increment the successor number for the next time we get to it.
-    ++Worklist.back().second;
-
-    // Visit the successor next, if it isn't already visited.
-    typename GraphT::NodeRef Succ = *NextSucc;
+    if (IsChildOfArtificialExit)
+      BBInfo.Parent = 1;
 
-    auto &SuccVInfo = DT.Info[Succ];
-    if (SuccVInfo.Semi == 0) {
-      SuccVInfo.Parent = BBDFSNum;
-      Worklist.push_back(std::make_pair(Succ, GraphT::child_begin(Succ)));
-    }
+    IsChildOfArtificialExit = false;
   }
-#endif
-    return N;
+  return N;
+}
+template <class GraphT>
+unsigned DFSPass(DominatorTreeBaseByGraphTraits<GraphT> &DT,
+                 typename GraphT::NodeRef V, unsigned N) {
+  df_iterator_dom_storage<
+      typename GraphT::NodeRef,
+      typename DominatorTreeBaseByGraphTraits<GraphT>::InfoRec>
+      DFStorage(DT.Info);
+  for (auto I = df_ext_begin(V, DFStorage), E = df_ext_end(V, DFStorage);
+       I != E; ++I) {
+    typename GraphT::NodeRef BB = *I;
+    auto &BBInfo = DT.Info[BB];
+    BBInfo.DFSNum = BBInfo.Semi = ++N;
+    BBInfo.Label = BB;
+    // Set the parent to the top of the visited stack.  The stack includes us,
+    // and is 1 based, so we subtract to account for both of these.
+    if (I.getPathLength() > 1)
+      BBInfo.Parent = DT.Info[I.getPath(I.getPathLength() - 2)].DFSNum;
+    DT.Vertex.push_back(BB); // Vertex[n] = V;
+  }
+  return N;
 }
 
 template <class GraphT>
@@ -163,9 +158,13 @@ void Calculate(DominatorTreeBaseByGraphTraits<GraphTraits<NodeT>> &DT,
 
   // Step #1: Number blocks in depth-first order and initialize variables used
   // in later stages of the algorithm.
-  for (unsigned i = 0, e = static_cast<unsigned>(DT.Roots.size());
-       i != e; ++i)
-    N = DFSPass<GraphT>(DT, DT.Roots[i], N);
+  if (DT.isPostDominator()){
+    for (unsigned i = 0, e = static_cast<unsigned>(DT.Roots.size());
+         i != e; ++i)
+      N = ReverseDFSPass<GraphT>(DT, DT.Roots[i], N);
+  } else {
+    N = DFSPass<GraphT>(DT, DT.Roots[0], N);
+  }
 
   // it might be that some blocks did not get a DFS number (e.g., blocks of
   // infinite loops). In these cases an artificial exit node is required.