An interface for iterating over a loop's blocks in DFS order.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@136838 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 207 insertions, 0 deletions

diff --git a/include/llvm/ADT/PostOrderIterator.h b/include/llvm/ADT/PostOrderIterator.h
index e3b4994..63a2b52 100644
--- a/include/llvm/ADT/PostOrderIterator.h
+++ b/include/llvm/ADT/PostOrderIterator.h
@@ -29,6 +29,14 @@ public:
   SetType Visited;
 };
 
+/// DFSetTraits - Allow the SetType used to record depth-first search results to
+/// optionally record node postorder.
+template<class SetType>
+struct DFSetTraits {
+  static void finishPostorder(
+    typename SetType::iterator::value_type, SetType &) {}
+};
+
 template<class SetType>
 class po_iterator_storage<SetType, true> {
 public:
@@ -109,6 +117,8 @@ public:
   inline NodeType *operator->() const { return operator*(); }
 
   inline _Self& operator++() {   // Preincrement
+    DFSetTraits<SetType>::finishPostorder(VisitStack.back().first,
+                                          this->Visited);
     VisitStack.pop_back();
     if (!VisitStack.empty())
       traverseChild();
diff --git a/include/llvm/Analysis/LoopIterator.h b/include/llvm/Analysis/LoopIterator.h
new file mode 100644
index 0000000..2797e67
--- /dev/null
+++ b/include/llvm/Analysis/LoopIterator.h
@@ -0,0 +1,197 @@
+//===--------- LoopIterator.h - Iterate over loop blocks --------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+// This file defines iterators to visit the basic blocks within a loop.
+//
+// These iterators currently visit blocks within subloops as well.
+// Unfortunately we have no efficient way of summarizing loop exits which would
+// allow skipping subloops during traversal.
+//
+// If you want to visit all blocks in a loop and don't need an ordered traveral,
+// use Loop::block_begin() instead.
+//
+// This is intentionally designed to work with ill-formed loops in which the
+// backedge has been deleted. The only prerequisite is that all blocks
+// contained within the loop according to the most recent LoopInfo analysis are
+// reachable from the loop header.
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_ANALYSIS_LOOP_ITERATOR_H
+#define LLVM_ANALYSIS_LOOP_ITERATOR_H
+
+#include "llvm/ADT/DepthFirstIterator.h"
+#include "llvm/ADT/PostOrderIterator.h"
+#include "llvm/Analysis/LoopInfo.h"
+
+namespace llvm {
+
+class LoopBlocksTraversal;
+
+/// Store the result of a depth first search within basic blocks contained by a
+/// single loop.
+///
+/// TODO: This could be generalized for any CFG region, or the entire CFG.
+class LoopBlocksDFS {
+public:
+  /// Postorder list iterators.
+  typedef std::vector<BasicBlock*>::const_iterator POIterator;
+  typedef std::vector<BasicBlock*>::const_reverse_iterator RPOIterator;
+
+  friend class LoopBlocksTraversal;
+
+private:
+  Loop *L;
+
+  /// Map each block to its postorder number. A block is only mapped after it is
+  /// preorder visited by DFS. It's postorder number is initially zero and set
+  /// to nonzero after it is finished by postorder traversal.
+  DenseMap<BasicBlock*, unsigned> PostNumbers;
+  std::vector<BasicBlock*> PostBlocks;
+
+public:
+  LoopBlocksDFS(Loop *Container) :
+    L(Container), PostNumbers(NextPowerOf2(Container->getNumBlocks())) {
+    PostBlocks.reserve(Container->getNumBlocks());
+  }
+
+  Loop *getLoop() const { return L; }
+
+  /// Return true if postorder numbers are assigned to all loop blocks.
+  bool isComplete() const { return PostBlocks.size() == L->getNumBlocks(); }
+
+  /// Iterate over the cached postorder blocks.
+  POIterator beginPostorder() const {
+    assert(isComplete() && "bad loop DFS");
+    return PostBlocks.begin();
+  }
+  POIterator endPostorder() const { return PostBlocks.end(); }
+
+  /// Reverse iterate over the cached postorder blocks.
+  RPOIterator beginRPO() const {
+    assert(isComplete() && "bad loop DFS");
+    return PostBlocks.rbegin();
+  }
+  RPOIterator endRPO() const { return PostBlocks.rend(); }
+
+  /// Return true if this block has been preorder visited.
+  bool hasPreorder(BasicBlock *BB) const { return PostNumbers.count(BB); }
+
+  /// Return true if this block has a postorder number.
+  bool hasPostorder(BasicBlock *BB) const {
+    DenseMap<BasicBlock*, unsigned>::const_iterator I = PostNumbers.find(BB);
+    return I != PostNumbers.end() && I->second;
+  }
+
+  /// Get a block's postorder number.
+  unsigned getPostorder(BasicBlock *BB) const {
+    DenseMap<BasicBlock*, unsigned>::const_iterator I = PostNumbers.find(BB);
+    assert(I != PostNumbers.end() && "block not visited by DFS");
+    assert(I->second && "block not finished by DFS");
+    return I->second;
+  }
+
+  /// Get a block's reverse postorder number.
+  unsigned getRPO(BasicBlock *BB) const {
+    return 1 + PostBlocks.size() - getPostorder(BB);
+  }
+
+  void clear() {
+    PostNumbers.clear();
+    PostBlocks.clear();
+  }
+};
+
+/// Define a graph of blocks within a loop. Allows LoopBlocksTraversal to
+/// use the generic po_iterator with specialized GraphTraits.
+struct LoopBlocksGraph {
+  Loop *L;
+
+  LoopBlocksGraph(Loop *Container) : L(Container) {}
+};
+
+template<> struct GraphTraits<LoopBlocksGraph> :
+    public GraphTraits<BasicBlock*> {
+
+  static BasicBlock *getEntryNode(LoopBlocksGraph G) {
+    return G.L->getHeader();
+  }
+};
+
+/// Traverse the blocks in a loop using a depth-first search.
+class LoopBlocksTraversal {
+public:
+  /// Graph traversal iterator.
+  typedef po_iterator<LoopBlocksGraph, LoopBlocksTraversal, true> POTIterator;
+
+private:
+  LoopBlocksDFS &DFS;
+  LoopInfo *LI;
+
+public:
+  LoopBlocksTraversal(LoopBlocksDFS &Storage, LoopInfo *LInfo) :
+    DFS(Storage), LI(LInfo) {}
+
+  /// Postorder traversal over the graph. This only needs to be done once.
+  /// po_iterator "automatically" calls back to visitPreorder and
+  /// finishPostorder to record the DFS result.
+  POTIterator begin() {
+    assert(DFS.PostBlocks.empty() && "Need clear DFS result before traversing");
+    return po_ext_begin(LoopBlocksGraph(DFS.L), *this);
+  }
+  POTIterator end() {
+    return po_ext_end(LoopBlocksGraph(DFS.L), *this);
+  }
+
+  /// Called by po_iterator upon reaching a block via a CFG edge. If this block
+  /// is contained in the loop and has not been visited, then mark it preorder
+  /// visited and return true.
+  ///
+  /// TODO: If anyone is interested, we could record preorder numbers here.
+  bool visitPreorder(BasicBlock *BB) {
+    if (!DFS.L->contains(LI->getLoopFor(BB)))
+      return false;
+
+    return DFS.PostNumbers.insert(std::make_pair(BB, 0)).second;
+  }
+
+  /// Called by po_iterator each time it advances, indicating a block's
+  /// postorder.
+  void finishPostorder(BasicBlock *BB) {
+    assert(DFS.PostNumbers.count(BB) && "Loop DFS skipped preorder");
+    DFS.PostBlocks.push_back(BB);
+    DFS.PostNumbers[BB] = DFS.PostBlocks.size();
+  }
+
+  //===----------------------------------------------------------------------
+  // Implement part of the std::set interface for the purpose of driving the
+  // generic po_iterator.
+
+  /// Return true if the block is outside the loop or has already been visited.
+  /// Sorry if this is counterintuitive.
+  bool count(BasicBlock *BB) const {
+    return !DFS.L->contains(LI->getLoopFor(BB)) || DFS.PostNumbers.count(BB);
+  }
+
+  /// If this block is contained in the loop and has not been visited, return
+  /// true and assign a preorder number. This is a proxy for visitPreorder
+  /// called by POIterator.
+  bool insert(BasicBlock *BB) {
+    return visitPreorder(BB);
+  }
+};
+
+/// Specialize DFSetTraits to record postorder numbers.
+template<> struct DFSetTraits<LoopBlocksTraversal> {
+  static void finishPostorder(BasicBlock *BB, LoopBlocksTraversal& LBT) {
+    LBT.finishPostorder(BB);
+  }
+};
+
+} // End namespace llvm
+
+#endif