1 //===- IteratedDominanceFrontier.h - Calculate IDF --------------*- 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 /// \file
9 /// Compute iterated dominance frontiers using a linear time algorithm.
10 ///
11 /// The algorithm used here is based on:
12 ///
13 /// Sreedhar and Gao. A linear time algorithm for placing phi-nodes.
14 /// In Proceedings of the 22nd ACM SIGPLAN-SIGACT Symposium on Principles of
15 /// Programming Languages
16 /// POPL '95. ACM, New York, NY, 62-73.
17 ///
18 /// It has been modified to not explicitly use the DJ graph data structure and
19 /// to directly compute pruned SSA using per-variable liveness information.
20 //
21 //===----------------------------------------------------------------------===//
22
23 #ifndef LLVM_SUPPORT_GENERIC_IDF_H
24 #define LLVM_SUPPORT_GENERIC_IDF_H
25
26 #include "llvm/ADT/DenseMap.h"
27 #include "llvm/ADT/SmallPtrSet.h"
28 #include "llvm/ADT/SmallVector.h"
29 #include "llvm/Support/GenericDomTree.h"
30 #include <queue>
31
32 namespace llvm {
33
34 namespace IDFCalculatorDetail {
35
36 /// Generic utility class used for getting the children of a basic block.
37 /// May be specialized if, for example, one wouldn't like to return nullpointer
38 /// successors.
39 template <class NodeTy, bool IsPostDom> struct ChildrenGetterTy {
40 using NodeRef = typename GraphTraits<NodeTy>::NodeRef;
41 using ChildrenTy = SmallVector<NodeRef, 8>;
42
43 ChildrenTy get(const NodeRef &N);
44 };
45
46 } // end of namespace IDFCalculatorDetail
47
48 /// Determine the iterated dominance frontier, given a set of defining
49 /// blocks, and optionally, a set of live-in blocks.
50 ///
51 /// In turn, the results can be used to place phi nodes.
52 ///
53 /// This algorithm is a linear time computation of Iterated Dominance Frontiers,
54 /// pruned using the live-in set.
55 /// By default, liveness is not used to prune the IDF computation.
56 /// The template parameters should be of a CFG block type.
57 template <class NodeTy, bool IsPostDom> class IDFCalculatorBase {
58 public:
59 using OrderedNodeTy =
60 std::conditional_t<IsPostDom, Inverse<NodeTy *>, NodeTy *>;
61 using ChildrenGetterTy =
62 IDFCalculatorDetail::ChildrenGetterTy<NodeTy, IsPostDom>;
63
IDFCalculatorBase(DominatorTreeBase<NodeTy,IsPostDom> & DT)64 IDFCalculatorBase(DominatorTreeBase<NodeTy, IsPostDom> &DT) : DT(DT) {}
65
IDFCalculatorBase(DominatorTreeBase<NodeTy,IsPostDom> & DT,const ChildrenGetterTy & C)66 IDFCalculatorBase(DominatorTreeBase<NodeTy, IsPostDom> &DT,
67 const ChildrenGetterTy &C)
68 : DT(DT), ChildrenGetter(C) {}
69
70 /// Give the IDF calculator the set of blocks in which the value is
71 /// defined. This is equivalent to the set of starting blocks it should be
72 /// calculating the IDF for (though later gets pruned based on liveness).
73 ///
74 /// Note: This set *must* live for the entire lifetime of the IDF calculator.
setDefiningBlocks(const SmallPtrSetImpl<NodeTy * > & Blocks)75 void setDefiningBlocks(const SmallPtrSetImpl<NodeTy *> &Blocks) {
76 DefBlocks = &Blocks;
77 }
78
79 /// Give the IDF calculator the set of blocks in which the value is
80 /// live on entry to the block. This is used to prune the IDF calculation to
81 /// not include blocks where any phi insertion would be dead.
82 ///
83 /// Note: This set *must* live for the entire lifetime of the IDF calculator.
setLiveInBlocks(const SmallPtrSetImpl<NodeTy * > & Blocks)84 void setLiveInBlocks(const SmallPtrSetImpl<NodeTy *> &Blocks) {
85 LiveInBlocks = &Blocks;
86 useLiveIn = true;
87 }
88
89 /// Reset the live-in block set to be empty, and tell the IDF
90 /// calculator to not use liveness anymore.
resetLiveInBlocks()91 void resetLiveInBlocks() {
92 LiveInBlocks = nullptr;
93 useLiveIn = false;
94 }
95
96 /// Calculate iterated dominance frontiers
97 ///
98 /// This uses the linear-time phi algorithm based on DJ-graphs mentioned in
99 /// the file-level comment. It performs DF->IDF pruning using the live-in
100 /// set, to avoid computing the IDF for blocks where an inserted PHI node
101 /// would be dead.
102 void calculate(SmallVectorImpl<NodeTy *> &IDFBlocks);
103
104 private:
105 DominatorTreeBase<NodeTy, IsPostDom> &DT;
106 ChildrenGetterTy ChildrenGetter;
107 bool useLiveIn = false;
108 const SmallPtrSetImpl<NodeTy *> *LiveInBlocks;
109 const SmallPtrSetImpl<NodeTy *> *DefBlocks;
110 };
111
112 //===----------------------------------------------------------------------===//
113 // Implementation.
114 //===----------------------------------------------------------------------===//
115
116 namespace IDFCalculatorDetail {
117
118 template <class NodeTy, bool IsPostDom>
119 typename ChildrenGetterTy<NodeTy, IsPostDom>::ChildrenTy
get(const NodeRef & N)120 ChildrenGetterTy<NodeTy, IsPostDom>::get(const NodeRef &N) {
121 using OrderedNodeTy =
122 typename IDFCalculatorBase<NodeTy, IsPostDom>::OrderedNodeTy;
123
124 auto Children = children<OrderedNodeTy>(N);
125 return {Children.begin(), Children.end()};
126 }
127
128 } // end of namespace IDFCalculatorDetail
129
130 template <class NodeTy, bool IsPostDom>
calculate(SmallVectorImpl<NodeTy * > & IDFBlocks)131 void IDFCalculatorBase<NodeTy, IsPostDom>::calculate(
132 SmallVectorImpl<NodeTy *> &IDFBlocks) {
133 // Use a priority queue keyed on dominator tree level so that inserted nodes
134 // are handled from the bottom of the dominator tree upwards. We also augment
135 // the level with a DFS number to ensure that the blocks are ordered in a
136 // deterministic way.
137 using DomTreeNodePair =
138 std::pair<DomTreeNodeBase<NodeTy> *, std::pair<unsigned, unsigned>>;
139 using IDFPriorityQueue =
140 std::priority_queue<DomTreeNodePair, SmallVector<DomTreeNodePair, 32>,
141 less_second>;
142
143 IDFPriorityQueue PQ;
144
145 DT.updateDFSNumbers();
146
147 SmallVector<DomTreeNodeBase<NodeTy> *, 32> Worklist;
148 SmallPtrSet<DomTreeNodeBase<NodeTy> *, 32> VisitedPQ;
149 SmallPtrSet<DomTreeNodeBase<NodeTy> *, 32> VisitedWorklist;
150
151 for (NodeTy *BB : *DefBlocks)
152 if (DomTreeNodeBase<NodeTy> *Node = DT.getNode(BB)) {
153 PQ.push({Node, std::make_pair(Node->getLevel(), Node->getDFSNumIn())});
154 VisitedWorklist.insert(Node);
155 }
156
157 while (!PQ.empty()) {
158 DomTreeNodePair RootPair = PQ.top();
159 PQ.pop();
160 DomTreeNodeBase<NodeTy> *Root = RootPair.first;
161 unsigned RootLevel = RootPair.second.first;
162
163 // Walk all dominator tree children of Root, inspecting their CFG edges with
164 // targets elsewhere on the dominator tree. Only targets whose level is at
165 // most Root's level are added to the iterated dominance frontier of the
166 // definition set.
167
168 assert(Worklist.empty());
169 Worklist.push_back(Root);
170
171 while (!Worklist.empty()) {
172 DomTreeNodeBase<NodeTy> *Node = Worklist.pop_back_val();
173 NodeTy *BB = Node->getBlock();
174 // Succ is the successor in the direction we are calculating IDF, so it is
175 // successor for IDF, and predecessor for Reverse IDF.
176 auto DoWork = [&](NodeTy *Succ) {
177 DomTreeNodeBase<NodeTy> *SuccNode = DT.getNode(Succ);
178
179 const unsigned SuccLevel = SuccNode->getLevel();
180 if (SuccLevel > RootLevel)
181 return;
182
183 if (!VisitedPQ.insert(SuccNode).second)
184 return;
185
186 NodeTy *SuccBB = SuccNode->getBlock();
187 if (useLiveIn && !LiveInBlocks->count(SuccBB))
188 return;
189
190 IDFBlocks.emplace_back(SuccBB);
191 if (!DefBlocks->count(SuccBB))
192 PQ.push(std::make_pair(
193 SuccNode, std::make_pair(SuccLevel, SuccNode->getDFSNumIn())));
194 };
195
196 for (auto Succ : ChildrenGetter.get(BB))
197 DoWork(Succ);
198
199 for (auto DomChild : *Node) {
200 if (VisitedWorklist.insert(DomChild).second)
201 Worklist.push_back(DomChild);
202 }
203 }
204 }
205 }
206
207 } // end of namespace llvm
208
209 #endif
210