1//===--------- LoopIterator.h - Iterate over loop blocks --------*- C++ -*-===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8// This file defines iterators to visit the basic blocks within a loop.
9//
10// These iterators currently visit blocks within subloops as well.
11// Unfortunately we have no efficient way of summarizing loop exits which would
12// allow skipping subloops during traversal.
13//
14// If you want to visit all blocks in a loop and don't need an ordered traveral,
15// use Loop::block_begin() instead.
16//
17// This is intentionally designed to work with ill-formed loops in which the
18// backedge has been deleted. The only prerequisite is that all blocks
19// contained within the loop according to the most recent LoopInfo analysis are
20// reachable from the loop header.
21//===----------------------------------------------------------------------===//
22
23#ifndef LLVM_ANALYSIS_LOOPITERATOR_H
24#define LLVM_ANALYSIS_LOOPITERATOR_H
25
26#include "llvm/ADT/PostOrderIterator.h"
27#include "llvm/Analysis/LoopInfo.h"
28
29namespace llvm {
30
31class LoopBlocksTraversal;
32
33// A traits type that is intended to be used in graph algorithms. The graph
34// traits starts at the loop header, and traverses the BasicBlocks that are in
35// the loop body, but not the loop header. Since the loop header is skipped,
36// the back edges are excluded.
37//
38// TODO: Explore the possibility to implement LoopBlocksTraversal in terms of
39//       LoopBodyTraits, so that insertEdge doesn't have to be specialized.
40struct LoopBodyTraits {
41  using NodeRef = std::pair<const Loop *, BasicBlock *>;
42
43  // This wraps a const Loop * into the iterator, so we know which edges to
44  // filter out.
45  class WrappedSuccIterator
46      : public iterator_adaptor_base<
47            WrappedSuccIterator, succ_iterator,
48            typename std::iterator_traits<succ_iterator>::iterator_category,
49            NodeRef, std::ptrdiff_t, NodeRef *, NodeRef> {
50    using BaseT = iterator_adaptor_base<
51        WrappedSuccIterator, succ_iterator,
52        typename std::iterator_traits<succ_iterator>::iterator_category,
53        NodeRef, std::ptrdiff_t, NodeRef *, NodeRef>;
54
55    const Loop *L;
56
57  public:
58    WrappedSuccIterator(succ_iterator Begin, const Loop *L)
59        : BaseT(Begin), L(L) {}
60
61    NodeRef operator*() const { return {L, *I}; }
62  };
63
64  struct LoopBodyFilter {
65    bool operator()(NodeRef N) const {
66      const Loop *L = N.first;
67      return N.second != L->getHeader() && L->contains(N.second);
68    }
69  };
70
71  using ChildIteratorType =
72      filter_iterator<WrappedSuccIterator, LoopBodyFilter>;
73
74  static NodeRef getEntryNode(const Loop &G) { return {&G, G.getHeader()}; }
75
76  static ChildIteratorType child_begin(NodeRef Node) {
77    return make_filter_range(make_range<WrappedSuccIterator>(
78                                 {succ_begin(Node.second), Node.first},
79                                 {succ_end(Node.second), Node.first}),
80                             LoopBodyFilter{})
81        .begin();
82  }
83
84  static ChildIteratorType child_end(NodeRef Node) {
85    return make_filter_range(make_range<WrappedSuccIterator>(
86                                 {succ_begin(Node.second), Node.first},
87                                 {succ_end(Node.second), Node.first}),
88                             LoopBodyFilter{})
89        .end();
90  }
91};
92
93/// Store the result of a depth first search within basic blocks contained by a
94/// single loop.
95///
96/// TODO: This could be generalized for any CFG region, or the entire CFG.
97class LoopBlocksDFS {
98public:
99  /// Postorder list iterators.
100  typedef std::vector<BasicBlock*>::const_iterator POIterator;
101  typedef std::vector<BasicBlock*>::const_reverse_iterator RPOIterator;
102
103  friend class LoopBlocksTraversal;
104
105private:
106  Loop *L;
107
108  /// Map each block to its postorder number. A block is only mapped after it is
109  /// preorder visited by DFS. It's postorder number is initially zero and set
110  /// to nonzero after it is finished by postorder traversal.
111  DenseMap<BasicBlock*, unsigned> PostNumbers;
112  std::vector<BasicBlock*> PostBlocks;
113
114public:
115  LoopBlocksDFS(Loop *Container) :
116    L(Container), PostNumbers(NextPowerOf2(Container->getNumBlocks())) {
117    PostBlocks.reserve(Container->getNumBlocks());
118  }
119
120  Loop *getLoop() const { return L; }
121
122  /// Traverse the loop blocks and store the DFS result.
123  void perform(LoopInfo *LI);
124
125  /// Return true if postorder numbers are assigned to all loop blocks.
126  bool isComplete() const { return PostBlocks.size() == L->getNumBlocks(); }
127
128  /// Iterate over the cached postorder blocks.
129  POIterator beginPostorder() const {
130    assert(isComplete() && "bad loop DFS");
131    return PostBlocks.begin();
132  }
133  POIterator endPostorder() const { return PostBlocks.end(); }
134
135  /// Reverse iterate over the cached postorder blocks.
136  RPOIterator beginRPO() const {
137    assert(isComplete() && "bad loop DFS");
138    return PostBlocks.rbegin();
139  }
140  RPOIterator endRPO() const { return PostBlocks.rend(); }
141
142  /// Return true if this block has been preorder visited.
143  bool hasPreorder(BasicBlock *BB) const { return PostNumbers.count(BB); }
144
145  /// Return true if this block has a postorder number.
146  bool hasPostorder(BasicBlock *BB) const {
147    DenseMap<BasicBlock*, unsigned>::const_iterator I = PostNumbers.find(BB);
148    return I != PostNumbers.end() && I->second;
149  }
150
151  /// Get a block's postorder number.
152  unsigned getPostorder(BasicBlock *BB) const {
153    DenseMap<BasicBlock*, unsigned>::const_iterator I = PostNumbers.find(BB);
154    assert(I != PostNumbers.end() && "block not visited by DFS");
155    assert(I->second && "block not finished by DFS");
156    return I->second;
157  }
158
159  /// Get a block's reverse postorder number.
160  unsigned getRPO(BasicBlock *BB) const {
161    return 1 + PostBlocks.size() - getPostorder(BB);
162  }
163
164  void clear() {
165    PostNumbers.clear();
166    PostBlocks.clear();
167  }
168};
169
170/// Wrapper class to LoopBlocksDFS that provides a standard begin()/end()
171/// interface for the DFS reverse post-order traversal of blocks in a loop body.
172class LoopBlocksRPO {
173private:
174  LoopBlocksDFS DFS;
175
176public:
177  LoopBlocksRPO(Loop *Container) : DFS(Container) {}
178
179  /// Traverse the loop blocks and store the DFS result.
180  void perform(LoopInfo *LI) {
181    DFS.perform(LI);
182  }
183
184  /// Reverse iterate over the cached postorder blocks.
185  LoopBlocksDFS::RPOIterator begin() const { return DFS.beginRPO(); }
186  LoopBlocksDFS::RPOIterator end() const { return DFS.endRPO(); }
187};
188
189/// Specialize po_iterator_storage to record postorder numbers.
190template<> class po_iterator_storage<LoopBlocksTraversal, true> {
191  LoopBlocksTraversal &LBT;
192public:
193  po_iterator_storage(LoopBlocksTraversal &lbs) : LBT(lbs) {}
194  // These functions are defined below.
195  bool insertEdge(Optional<BasicBlock *> From, BasicBlock *To);
196  void finishPostorder(BasicBlock *BB);
197};
198
199/// Traverse the blocks in a loop using a depth-first search.
200class LoopBlocksTraversal {
201public:
202  /// Graph traversal iterator.
203  typedef po_iterator<BasicBlock*, LoopBlocksTraversal, true> POTIterator;
204
205private:
206  LoopBlocksDFS &DFS;
207  LoopInfo *LI;
208
209public:
210  LoopBlocksTraversal(LoopBlocksDFS &Storage, LoopInfo *LInfo) :
211    DFS(Storage), LI(LInfo) {}
212
213  /// Postorder traversal over the graph. This only needs to be done once.
214  /// po_iterator "automatically" calls back to visitPreorder and
215  /// finishPostorder to record the DFS result.
216  POTIterator begin() {
217    assert(DFS.PostBlocks.empty() && "Need clear DFS result before traversing");
218    assert(DFS.L->getNumBlocks() && "po_iterator cannot handle an empty graph");
219    return po_ext_begin(DFS.L->getHeader(), *this);
220  }
221  POTIterator end() {
222    // po_ext_end interface requires a basic block, but ignores its value.
223    return po_ext_end(DFS.L->getHeader(), *this);
224  }
225
226  /// Called by po_iterator upon reaching a block via a CFG edge. If this block
227  /// is contained in the loop and has not been visited, then mark it preorder
228  /// visited and return true.
229  ///
230  /// TODO: If anyone is interested, we could record preorder numbers here.
231  bool visitPreorder(BasicBlock *BB) {
232    if (!DFS.L->contains(LI->getLoopFor(BB)))
233      return false;
234
235    return DFS.PostNumbers.insert(std::make_pair(BB, 0)).second;
236  }
237
238  /// Called by po_iterator each time it advances, indicating a block's
239  /// postorder.
240  void finishPostorder(BasicBlock *BB) {
241    assert(DFS.PostNumbers.count(BB) && "Loop DFS skipped preorder");
242    DFS.PostBlocks.push_back(BB);
243    DFS.PostNumbers[BB] = DFS.PostBlocks.size();
244  }
245};
246
247inline bool po_iterator_storage<LoopBlocksTraversal, true>::insertEdge(
248    Optional<BasicBlock *> From, BasicBlock *To) {
249  return LBT.visitPreorder(To);
250}
251
252inline void po_iterator_storage<LoopBlocksTraversal, true>::
253finishPostorder(BasicBlock *BB) {
254  LBT.finishPostorder(BB);
255}
256
257} // End namespace llvm
258
259#endif
260