1 //===-- Analysis/CFG.h - BasicBlock Analyses --------------------*- 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 //
9 // This family of functions performs analyses on basic blocks, and instructions
10 // contained within basic blocks.
11 //
12 //===----------------------------------------------------------------------===//
13
14 #ifndef LLVM_ANALYSIS_CFG_H
15 #define LLVM_ANALYSIS_CFG_H
16
17 #include "llvm/IR/BasicBlock.h"
18 #include "llvm/IR/CFG.h"
19
20 namespace llvm {
21
22 class BasicBlock;
23 class DominatorTree;
24 class Function;
25 class Instruction;
26 class LoopInfo;
27
28 /// Analyze the specified function to find all of the loop backedges in the
29 /// function and return them. This is a relatively cheap (compared to
30 /// computing dominators and loop info) analysis.
31 ///
32 /// The output is added to Result, as pairs of <from,to> edge info.
33 void FindFunctionBackedges(
34 const Function &F,
35 SmallVectorImpl<std::pair<const BasicBlock *, const BasicBlock *> > &
36 Result);
37
38 /// Search for the specified successor of basic block BB and return its position
39 /// in the terminator instruction's list of successors. It is an error to call
40 /// this with a block that is not a successor.
41 unsigned GetSuccessorNumber(const BasicBlock *BB, const BasicBlock *Succ);
42
43 /// Return true if the specified edge is a critical edge. Critical edges are
44 /// edges from a block with multiple successors to a block with multiple
45 /// predecessors.
46 ///
47 bool isCriticalEdge(const Instruction *TI, unsigned SuccNum,
48 bool AllowIdenticalEdges = false);
49 bool isCriticalEdge(const Instruction *TI, const BasicBlock *Succ,
50 bool AllowIdenticalEdges = false);
51
52 /// Determine whether instruction 'To' is reachable from 'From', without passing
53 /// through any blocks in ExclusionSet, returning true if uncertain.
54 ///
55 /// Determine whether there is a path from From to To within a single function.
56 /// Returns false only if we can prove that once 'From' has been executed then
57 /// 'To' can not be executed. Conservatively returns true.
58 ///
59 /// This function is linear with respect to the number of blocks in the CFG,
60 /// walking down successors from From to reach To, with a fixed threshold.
61 /// Using DT or LI allows us to answer more quickly. LI reduces the cost of
62 /// an entire loop of any number of blocks to be the same as the cost of a
63 /// single block. DT reduces the cost by allowing the search to terminate when
64 /// we find a block that dominates the block containing 'To'. DT is most useful
65 /// on branchy code but not loops, and LI is most useful on code with loops but
66 /// does not help on branchy code outside loops.
67 bool isPotentiallyReachable(
68 const Instruction *From, const Instruction *To,
69 const SmallPtrSetImpl<BasicBlock *> *ExclusionSet = nullptr,
70 const DominatorTree *DT = nullptr, const LoopInfo *LI = nullptr);
71
72 /// Determine whether block 'To' is reachable from 'From', returning
73 /// true if uncertain.
74 ///
75 /// Determine whether there is a path from From to To within a single function.
76 /// Returns false only if we can prove that once 'From' has been reached then
77 /// 'To' can not be executed. Conservatively returns true.
78 bool isPotentiallyReachable(const BasicBlock *From, const BasicBlock *To,
79 const DominatorTree *DT = nullptr,
80 const LoopInfo *LI = nullptr);
81
82 /// Determine whether there is at least one path from a block in
83 /// 'Worklist' to 'StopBB', returning true if uncertain.
84 ///
85 /// Determine whether there is a path from at least one block in Worklist to
86 /// StopBB within a single function. Returns false only if we can prove that
87 /// once any block in 'Worklist' has been reached then 'StopBB' can not be
88 /// executed. Conservatively returns true.
89 bool isPotentiallyReachableFromMany(SmallVectorImpl<BasicBlock *> &Worklist,
90 BasicBlock *StopBB,
91 const DominatorTree *DT = nullptr,
92 const LoopInfo *LI = nullptr);
93
94 /// Determine whether there is at least one path from a block in
95 /// 'Worklist' to 'StopBB' without passing through any blocks in
96 /// 'ExclusionSet', returning true if uncertain.
97 ///
98 /// Determine whether there is a path from at least one block in Worklist to
99 /// StopBB within a single function without passing through any of the blocks
100 /// in 'ExclusionSet'. Returns false only if we can prove that once any block
101 /// in 'Worklist' has been reached then 'StopBB' can not be executed.
102 /// Conservatively returns true.
103 bool isPotentiallyReachableFromMany(
104 SmallVectorImpl<BasicBlock *> &Worklist, BasicBlock *StopBB,
105 const SmallPtrSetImpl<BasicBlock *> *ExclusionSet,
106 const DominatorTree *DT = nullptr, const LoopInfo *LI = nullptr);
107
108 /// Return true if the control flow in \p RPOTraversal is irreducible.
109 ///
110 /// This is a generic implementation to detect CFG irreducibility based on loop
111 /// info analysis. It can be used for any kind of CFG (Loop, MachineLoop,
112 /// Function, MachineFunction, etc.) by providing an RPO traversal (\p
113 /// RPOTraversal) and the loop info analysis (\p LI) of the CFG. This utility
114 /// function is only recommended when loop info analysis is available. If loop
115 /// info analysis isn't available, please, don't compute it explicitly for this
116 /// purpose. There are more efficient ways to detect CFG irreducibility that
117 /// don't require recomputing loop info analysis (e.g., T1/T2 or Tarjan's
118 /// algorithm).
119 ///
120 /// Requirements:
121 /// 1) GraphTraits must be implemented for NodeT type. It is used to access
122 /// NodeT successors.
123 // 2) \p RPOTraversal must be a valid reverse post-order traversal of the
124 /// target CFG with begin()/end() iterator interfaces.
125 /// 3) \p LI must be a valid LoopInfoBase that contains up-to-date loop
126 /// analysis information of the CFG.
127 ///
128 /// This algorithm uses the information about reducible loop back-edges already
129 /// computed in \p LI. When a back-edge is found during the RPO traversal, the
130 /// algorithm checks whether the back-edge is one of the reducible back-edges in
131 /// loop info. If it isn't, the CFG is irreducible. For example, for the CFG
132 /// below (canonical irreducible graph) loop info won't contain any loop, so the
133 /// algorithm will return that the CFG is irreducible when checking the B <-
134 /// -> C back-edge.
135 ///
136 /// (A->B, A->C, B->C, C->B, C->D)
137 /// A
138 /// / \
139 /// B<- ->C
140 /// |
141 /// D
142 ///
143 template <class NodeT, class RPOTraversalT, class LoopInfoT,
144 class GT = GraphTraits<NodeT>>
containsIrreducibleCFG(RPOTraversalT & RPOTraversal,const LoopInfoT & LI)145 bool containsIrreducibleCFG(RPOTraversalT &RPOTraversal, const LoopInfoT &LI) {
146 /// Check whether the edge (\p Src, \p Dst) is a reducible loop backedge
147 /// according to LI. I.e., check if there exists a loop that contains Src and
148 /// where Dst is the loop header.
149 auto isProperBackedge = [&](NodeT Src, NodeT Dst) {
150 for (const auto *Lp = LI.getLoopFor(Src); Lp; Lp = Lp->getParentLoop()) {
151 if (Lp->getHeader() == Dst)
152 return true;
153 }
154 return false;
155 };
156
157 SmallPtrSet<NodeT, 32> Visited;
158 for (NodeT Node : RPOTraversal) {
159 Visited.insert(Node);
160 for (NodeT Succ : make_range(GT::child_begin(Node), GT::child_end(Node))) {
161 // Succ hasn't been visited yet
162 if (!Visited.count(Succ))
163 continue;
164 // We already visited Succ, thus Node->Succ must be a backedge. Check that
165 // the head matches what we have in the loop information. Otherwise, we
166 // have an irreducible graph.
167 if (!isProperBackedge(Node, Succ))
168 return true;
169 }
170 }
171
172 return false;
173 }
174 } // End llvm namespace
175
176 #endif
177