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(BB: 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(Range: make_range<WrappedSuccIterator>(
78 x: {succ_begin(BB: Node.second), Node.first},
79 y: {succ_end(BB: Node.second), Node.first}),
80 Pred: LoopBodyFilter{})
81 .begin();
82 }
83
84 static ChildIteratorType child_end(NodeRef Node) {
85 return make_filter_range(Range: make_range<WrappedSuccIterator>(
86 x: {succ_begin(BB: Node.second), Node.first},
87 y: {succ_end(BB: Node.second), Node.first}),
88 Pred: 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(A: Container->getNumBlocks())) {
117 PostBlocks.reserve(n: Container->getNumBlocks());
118 }
119
120 Loop *getLoop() const { return L; }
121
122 /// Traverse the loop blocks and store the DFS result.
123 void perform(const 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(Val: 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(Val: 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(Val: 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(const 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(std::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 const LoopInfo *LI;
208
209public:
210 LoopBlocksTraversal(LoopBlocksDFS &Storage, const 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(G: DFS.L->getHeader(), S&: *this);
220 }
221 POTIterator end() {
222 // po_ext_end interface requires a basic block, but ignores its value.
223 return po_ext_end(G: DFS.L->getHeader(), S&: *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(L: LI->getLoopFor(BB)))
233 return false;
234
235 return DFS.PostNumbers.insert(KV: std::make_pair(x&: BB, y: 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(x: BB);
243 DFS.PostNumbers[BB] = DFS.PostBlocks.size();
244 }
245};
246
247inline bool po_iterator_storage<LoopBlocksTraversal, true>::insertEdge(
248 std::optional<BasicBlock *> From, BasicBlock *To) {
249 return LBT.visitPreorder(BB: 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

source code of llvm/include/llvm/Analysis/LoopIterator.h