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>> 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