1 //===--------- LoopIterator.h - Iterate over loop blocks --------*- C++ -*-===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
9 // This file defines iterators to visit the basic blocks within a loop.
11 // These iterators currently visit blocks within subloops as well.
12 // Unfortunately we have no efficient way of summarizing loop exits which would
13 // allow skipping subloops during traversal.
15 // If you want to visit all blocks in a loop and don't need an ordered traveral,
16 // use Loop::block_begin() instead.
18 // This is intentionally designed to work with ill-formed loops in which the
19 // backedge has been deleted. The only prerequisite is that all blocks
20 // contained within the loop according to the most recent LoopInfo analysis are
21 // reachable from the loop header.
22 //===----------------------------------------------------------------------===//
24 #ifndef LLVM_ANALYSIS_LOOP_ITERATOR_H
25 #define LLVM_ANALYSIS_LOOP_ITERATOR_H
27 #include "llvm/ADT/DepthFirstIterator.h"
28 #include "llvm/ADT/PostOrderIterator.h"
29 #include "llvm/Analysis/LoopInfo.h"
33 class LoopBlocksTraversal;
35 /// Store the result of a depth first search within basic blocks contained by a
38 /// TODO: This could be generalized for any CFG region, or the entire CFG.
41 /// Postorder list iterators.
42 typedef std::vector<BasicBlock*>::const_iterator POIterator;
43 typedef std::vector<BasicBlock*>::const_reverse_iterator RPOIterator;
45 friend class LoopBlocksTraversal;
50 /// Map each block to its postorder number. A block is only mapped after it is
51 /// preorder visited by DFS. It's postorder number is initially zero and set
52 /// to nonzero after it is finished by postorder traversal.
53 DenseMap<BasicBlock*, unsigned> PostNumbers;
54 std::vector<BasicBlock*> PostBlocks;
57 LoopBlocksDFS(Loop *Container) :
58 L(Container), PostNumbers(NextPowerOf2(Container->getNumBlocks())) {
59 PostBlocks.reserve(Container->getNumBlocks());
62 Loop *getLoop() const { return L; }
64 /// Traverse the loop blocks and store the DFS result.
65 void perform(LoopInfo *LI);
67 /// Return true if postorder numbers are assigned to all loop blocks.
68 bool isComplete() const { return PostBlocks.size() == L->getNumBlocks(); }
70 /// Iterate over the cached postorder blocks.
71 POIterator beginPostorder() const {
72 assert(isComplete() && "bad loop DFS");
73 return PostBlocks.begin();
75 POIterator endPostorder() const { return PostBlocks.end(); }
77 /// Reverse iterate over the cached postorder blocks.
78 RPOIterator beginRPO() const {
79 assert(isComplete() && "bad loop DFS");
80 return PostBlocks.rbegin();
82 RPOIterator endRPO() const { return PostBlocks.rend(); }
84 /// Return true if this block has been preorder visited.
85 bool hasPreorder(BasicBlock *BB) const { return PostNumbers.count(BB); }
87 /// Return true if this block has a postorder number.
88 bool hasPostorder(BasicBlock *BB) const {
89 DenseMap<BasicBlock*, unsigned>::const_iterator I = PostNumbers.find(BB);
90 return I != PostNumbers.end() && I->second;
93 /// Get a block's postorder number.
94 unsigned getPostorder(BasicBlock *BB) const {
95 DenseMap<BasicBlock*, unsigned>::const_iterator I = PostNumbers.find(BB);
96 assert(I != PostNumbers.end() && "block not visited by DFS");
97 assert(I->second && "block not finished by DFS");
101 /// Get a block's reverse postorder number.
102 unsigned getRPO(BasicBlock *BB) const {
103 return 1 + PostBlocks.size() - getPostorder(BB);
112 /// Traverse the blocks in a loop using a depth-first search.
113 class LoopBlocksTraversal {
115 /// Graph traversal iterator.
116 typedef po_iterator<BasicBlock*, LoopBlocksTraversal, true> POTIterator;
123 LoopBlocksTraversal(LoopBlocksDFS &Storage, LoopInfo *LInfo) :
124 DFS(Storage), LI(LInfo) {}
126 /// Postorder traversal over the graph. This only needs to be done once.
127 /// po_iterator "automatically" calls back to visitPreorder and
128 /// finishPostorder to record the DFS result.
129 POTIterator begin() {
130 assert(DFS.PostBlocks.empty() && "Need clear DFS result before traversing");
131 assert(DFS.L->getNumBlocks() && "po_iterator cannot handle an empty graph");
132 return po_ext_begin(DFS.L->getHeader(), *this);
135 // po_ext_end interface requires a basic block, but ignores its value.
136 return po_ext_end(DFS.L->getHeader(), *this);
139 /// Called by po_iterator upon reaching a block via a CFG edge. If this block
140 /// is contained in the loop and has not been visited, then mark it preorder
141 /// visited and return true.
143 /// TODO: If anyone is interested, we could record preorder numbers here.
144 bool visitPreorder(BasicBlock *BB) {
145 if (!DFS.L->contains(LI->getLoopFor(BB)))
148 return DFS.PostNumbers.insert(std::make_pair(BB, 0)).second;
151 /// Called by po_iterator each time it advances, indicating a block's
153 void finishPostorder(BasicBlock *BB) {
154 assert(DFS.PostNumbers.count(BB) && "Loop DFS skipped preorder");
155 DFS.PostBlocks.push_back(BB);
156 DFS.PostNumbers[BB] = DFS.PostBlocks.size();
159 //===----------------------------------------------------------------------
160 // Implement part of the std::set interface for the purpose of driving the
161 // generic po_iterator.
163 /// Return true if the block is outside the loop or has already been visited.
164 /// Sorry if this is counterintuitive.
165 bool count(BasicBlock *BB) const {
166 return !DFS.L->contains(LI->getLoopFor(BB)) || DFS.PostNumbers.count(BB);
169 /// If this block is contained in the loop and has not been visited, return
170 /// true and assign a preorder number. This is a proxy for visitPreorder
171 /// called by POIterator.
172 bool insert(BasicBlock *BB) {
173 return visitPreorder(BB);
177 /// Specialize DFSetTraits to record postorder numbers.
178 template<> struct DFSetTraits<LoopBlocksTraversal> {
179 static void finishPostorder(BasicBlock *BB, LoopBlocksTraversal& LBT) {
180 LBT.finishPostorder(BB);
184 } // End namespace llvm