1 //===-- Analysis/CFG.h - BasicBlock Analyses --------------------*- 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 //===----------------------------------------------------------------------===//
10 // This family of functions performs analyses on basic blocks, and instructions
11 // contained within basic blocks.
13 //===----------------------------------------------------------------------===//
15 #ifndef LLVM_ANALYSIS_CFG_H
16 #define LLVM_ANALYSIS_CFG_H
18 #include "llvm/IR/BasicBlock.h"
19 #include "llvm/IR/CFG.h"
29 /// Analyze the specified function to find all of the loop backedges in the
30 /// function and return them. This is a relatively cheap (compared to
31 /// computing dominators and loop info) analysis.
33 /// The output is added to Result, as pairs of <from,to> edge info.
34 void FindFunctionBackedges(
36 SmallVectorImpl<std::pair<const BasicBlock *, const BasicBlock *> > &
39 /// Search for the specified successor of basic block BB and return its position
40 /// in the terminator instruction's list of successors. It is an error to call
41 /// this with a block that is not a successor.
42 unsigned GetSuccessorNumber(const BasicBlock *BB, const BasicBlock *Succ);
44 /// Return true if the specified edge is a critical edge. Critical edges are
45 /// edges from a block with multiple successors to a block with multiple
48 bool isCriticalEdge(const Instruction *TI, unsigned SuccNum,
49 bool AllowIdenticalEdges = false);
51 /// Determine whether instruction 'To' is reachable from 'From',
52 /// returning true if uncertain.
54 /// Determine whether there is a path from From to To within a single function.
55 /// Returns false only if we can prove that once 'From' has been executed then
56 /// 'To' can not be executed. Conservatively returns true.
58 /// This function is linear with respect to the number of blocks in the CFG,
59 /// walking down successors from From to reach To, with a fixed threshold.
60 /// Using DT or LI allows us to answer more quickly. LI reduces the cost of
61 /// an entire loop of any number of blocks to be the same as the cost of a
62 /// single block. DT reduces the cost by allowing the search to terminate when
63 /// we find a block that dominates the block containing 'To'. DT is most useful
64 /// on branchy code but not loops, and LI is most useful on code with loops but
65 /// does not help on branchy code outside loops.
66 bool isPotentiallyReachable(const Instruction *From, const Instruction *To,
67 const DominatorTree *DT = nullptr,
68 const LoopInfo *LI = nullptr);
70 /// Determine whether block 'To' is reachable from 'From', returning
71 /// true if uncertain.
73 /// Determine whether there is a path from From to To within a single function.
74 /// Returns false only if we can prove that once 'From' has been reached then
75 /// 'To' can not be executed. Conservatively returns true.
76 bool isPotentiallyReachable(const BasicBlock *From, const BasicBlock *To,
77 const DominatorTree *DT = nullptr,
78 const LoopInfo *LI = nullptr);
80 /// Determine whether there is at least one path from a block in
81 /// 'Worklist' to 'StopBB', returning true if uncertain.
83 /// Determine whether there is a path from at least one block in Worklist to
84 /// StopBB within a single function. Returns false only if we can prove that
85 /// once any block in 'Worklist' has been reached then 'StopBB' can not be
86 /// executed. Conservatively returns true.
87 bool isPotentiallyReachableFromMany(SmallVectorImpl<BasicBlock *> &Worklist,
89 const DominatorTree *DT = nullptr,
90 const LoopInfo *LI = nullptr);
92 /// Return true if the control flow in \p RPOTraversal is irreducible.
94 /// This is a generic implementation to detect CFG irreducibility based on loop
95 /// info analysis. It can be used for any kind of CFG (Loop, MachineLoop,
96 /// Function, MachineFunction, etc.) by providing an RPO traversal (\p
97 /// RPOTraversal) and the loop info analysis (\p LI) of the CFG. This utility
98 /// function is only recommended when loop info analysis is available. If loop
99 /// info analysis isn't available, please, don't compute it explicitly for this
100 /// purpose. There are more efficient ways to detect CFG irreducibility that
101 /// don't require recomputing loop info analysis (e.g., T1/T2 or Tarjan's
105 /// 1) GraphTraits must be implemented for NodeT type. It is used to access
106 /// NodeT successors.
107 // 2) \p RPOTraversal must be a valid reverse post-order traversal of the
108 /// target CFG with begin()/end() iterator interfaces.
109 /// 3) \p LI must be a valid LoopInfoBase that contains up-to-date loop
110 /// analysis information of the CFG.
112 /// This algorithm uses the information about reducible loop back-edges already
113 /// computed in \p LI. When a back-edge is found during the RPO traversal, the
114 /// algorithm checks whether the back-edge is one of the reducible back-edges in
115 /// loop info. If it isn't, the CFG is irreducible. For example, for the CFG
116 /// below (canonical irreducible graph) loop info won't contain any loop, so the
117 /// algorithm will return that the CFG is irreducible when checking the B <-
120 /// (A->B, A->C, B->C, C->B, C->D)
127 template <class NodeT, class RPOTraversalT, class LoopInfoT,
128 class GT = GraphTraits<NodeT>>
129 bool containsIrreducibleCFG(RPOTraversalT &RPOTraversal, const LoopInfoT &LI) {
130 /// Check whether the edge (\p Src, \p Dst) is a reducible loop backedge
131 /// according to LI. I.e., check if there exists a loop that contains Src and
132 /// where Dst is the loop header.
133 auto isProperBackedge = [&](NodeT Src, NodeT Dst) {
134 for (const auto *Lp = LI.getLoopFor(Src); Lp; Lp = Lp->getParentLoop()) {
135 if (Lp->getHeader() == Dst)
141 SmallPtrSet<NodeT, 32> Visited;
142 for (NodeT Node : RPOTraversal) {
143 Visited.insert(Node);
144 for (NodeT Succ : make_range(GT::child_begin(Node), GT::child_end(Node))) {
145 // Succ hasn't been visited yet
146 if (!Visited.count(Succ))
148 // We already visited Succ, thus Node->Succ must be a backedge. Check that
149 // the head matches what we have in the loop information. Otherwise, we
150 // have an irreducible graph.
151 if (!isProperBackedge(Node, Succ))
158 } // End llvm namespace