LLVM  9.0.0svn
CFG.h
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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.
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 
50 /// Determine whether instruction 'To' is reachable from 'From',
51 /// returning true if uncertain.
52 ///
53 /// Determine whether there is a path from From to To within a single function.
54 /// Returns false only if we can prove that once 'From' has been executed then
55 /// 'To' can not be executed. Conservatively returns true.
56 ///
57 /// This function is linear with respect to the number of blocks in the CFG,
58 /// walking down successors from From to reach To, with a fixed threshold.
59 /// Using DT or LI allows us to answer more quickly. LI reduces the cost of
60 /// an entire loop of any number of blocks to be the same as the cost of a
61 /// single block. DT reduces the cost by allowing the search to terminate when
62 /// we find a block that dominates the block containing 'To'. DT is most useful
63 /// on branchy code but not loops, and LI is most useful on code with loops but
64 /// does not help on branchy code outside loops.
65 bool isPotentiallyReachable(const Instruction *From, const Instruction *To,
66  const DominatorTree *DT = nullptr,
67  const LoopInfo *LI = nullptr);
68 
69 /// Determine whether block 'To' is reachable from 'From', returning
70 /// true if uncertain.
71 ///
72 /// Determine whether there is a path from From to To within a single function.
73 /// Returns false only if we can prove that once 'From' has been reached then
74 /// 'To' can not be executed. Conservatively returns true.
75 bool isPotentiallyReachable(const BasicBlock *From, const BasicBlock *To,
76  const DominatorTree *DT = nullptr,
77  const LoopInfo *LI = nullptr);
78 
79 /// Determine whether there is at least one path from a block in
80 /// 'Worklist' to 'StopBB', returning true if uncertain.
81 ///
82 /// Determine whether there is a path from at least one block in Worklist to
83 /// StopBB within a single function. Returns false only if we can prove that
84 /// once any block in 'Worklist' has been reached then 'StopBB' can not be
85 /// executed. Conservatively returns true.
86 bool isPotentiallyReachableFromMany(SmallVectorImpl<BasicBlock *> &Worklist,
87  BasicBlock *StopBB,
88  const DominatorTree *DT = nullptr,
89  const LoopInfo *LI = nullptr);
90 
91 /// Return true if the control flow in \p RPOTraversal is irreducible.
92 ///
93 /// This is a generic implementation to detect CFG irreducibility based on loop
94 /// info analysis. It can be used for any kind of CFG (Loop, MachineLoop,
95 /// Function, MachineFunction, etc.) by providing an RPO traversal (\p
96 /// RPOTraversal) and the loop info analysis (\p LI) of the CFG. This utility
97 /// function is only recommended when loop info analysis is available. If loop
98 /// info analysis isn't available, please, don't compute it explicitly for this
99 /// purpose. There are more efficient ways to detect CFG irreducibility that
100 /// don't require recomputing loop info analysis (e.g., T1/T2 or Tarjan's
101 /// algorithm).
102 ///
103 /// Requirements:
104 /// 1) GraphTraits must be implemented for NodeT type. It is used to access
105 /// NodeT successors.
106 // 2) \p RPOTraversal must be a valid reverse post-order traversal of the
107 /// target CFG with begin()/end() iterator interfaces.
108 /// 3) \p LI must be a valid LoopInfoBase that contains up-to-date loop
109 /// analysis information of the CFG.
110 ///
111 /// This algorithm uses the information about reducible loop back-edges already
112 /// computed in \p LI. When a back-edge is found during the RPO traversal, the
113 /// algorithm checks whether the back-edge is one of the reducible back-edges in
114 /// loop info. If it isn't, the CFG is irreducible. For example, for the CFG
115 /// below (canonical irreducible graph) loop info won't contain any loop, so the
116 /// algorithm will return that the CFG is irreducible when checking the B <-
117 /// -> C back-edge.
118 ///
119 /// (A->B, A->C, B->C, C->B, C->D)
120 /// A
121 /// / \
122 /// B<- ->C
123 /// |
124 /// D
125 ///
126 template <class NodeT, class RPOTraversalT, class LoopInfoT,
127  class GT = GraphTraits<NodeT>>
128 bool containsIrreducibleCFG(RPOTraversalT &RPOTraversal, const LoopInfoT &LI) {
129  /// Check whether the edge (\p Src, \p Dst) is a reducible loop backedge
130  /// according to LI. I.e., check if there exists a loop that contains Src and
131  /// where Dst is the loop header.
132  auto isProperBackedge = [&](NodeT Src, NodeT Dst) {
133  for (const auto *Lp = LI.getLoopFor(Src); Lp; Lp = Lp->getParentLoop()) {
134  if (Lp->getHeader() == Dst)
135  return true;
136  }
137  return false;
138  };
139 
140  SmallPtrSet<NodeT, 32> Visited;
141  for (NodeT Node : RPOTraversal) {
142  Visited.insert(Node);
143  for (NodeT Succ : make_range(GT::child_begin(Node), GT::child_end(Node))) {
144  // Succ hasn't been visited yet
145  if (!Visited.count(Succ))
146  continue;
147  // We already visited Succ, thus Node->Succ must be a backedge. Check that
148  // the head matches what we have in the loop information. Otherwise, we
149  // have an irreducible graph.
150  if (!isProperBackedge(Node, Succ))
151  return true;
152  }
153  }
154 
155  return false;
156 }
157 } // End llvm namespace
158 
159 #endif
This class represents lattice values for constants.
Definition: AllocatorList.h:23
Various leaf nodes.
Definition: ISDOpcodes.h:59
F(f)
bool containsIrreducibleCFG(RPOTraversalT &RPOTraversal, const LoopInfoT &LI)
Return true if the control flow in RPOTraversal is irreducible.
Definition: CFG.h:128
std::pair< iterator, bool > insert(PtrType Ptr)
Inserts Ptr if and only if there is no element in the container equal to Ptr.
Definition: SmallPtrSet.h:370
bool isPotentiallyReachable(const Instruction *From, const Instruction *To, const DominatorTree *DT=nullptr, const LoopInfo *LI=nullptr)
Determine whether instruction &#39;To&#39; is reachable from &#39;From&#39;, returning true if uncertain.
Definition: CFG.cpp:186
size_type count(ConstPtrType Ptr) const
count - Return 1 if the specified pointer is in the set, 0 otherwise.
Definition: SmallPtrSet.h:381
bool isCriticalEdge(const Instruction *TI, unsigned SuccNum, bool AllowIdenticalEdges=false)
Return true if the specified edge is a critical edge.
Definition: CFG.cpp:87
iterator_range< T > make_range(T x, T y)
Convenience function for iterating over sub-ranges.
SmallPtrSet - This class implements a set which is optimized for holding SmallSize or less elements...
Definition: SmallPtrSet.h:417
BlockVerifier::State From
This file provides various utilities for inspecting and working with the control flow graph in LLVM I...
unsigned GetSuccessorNumber(const BasicBlock *BB, const BasicBlock *Succ)
Search for the specified successor of basic block BB and return its position in the terminator instru...
Definition: CFG.cpp:71
void FindFunctionBackedges(const Function &F, SmallVectorImpl< std::pair< const BasicBlock *, const BasicBlock *> > &Result)
Analyze the specified function to find all of the loop backedges in the function and return them...
Definition: CFG.cpp:26
bool isPotentiallyReachableFromMany(SmallVectorImpl< BasicBlock *> &Worklist, BasicBlock *StopBB, const DominatorTree *DT=nullptr, const LoopInfo *LI=nullptr)
Determine whether there is at least one path from a block in &#39;Worklist&#39; to &#39;StopBB&#39;, returning true if uncertain.
Definition: CFG.cpp:130