LLVM  7.0.0svn
CFG.h
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1 //===-- Analysis/CFG.h - BasicBlock Analyses --------------------*- C++ -*-===//
2 //
3 // The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // This family of functions performs analyses on basic blocks, and instructions
11 // contained within basic blocks.
12 //
13 //===----------------------------------------------------------------------===//
14 
15 #ifndef LLVM_ANALYSIS_CFG_H
16 #define LLVM_ANALYSIS_CFG_H
17 
18 #include "llvm/IR/BasicBlock.h"
19 #include "llvm/IR/CFG.h"
20 
21 namespace llvm {
22 
23 class BasicBlock;
24 class DominatorTree;
25 class Function;
26 class Instruction;
27 class LoopInfo;
28 class TerminatorInst;
29 
30 /// Analyze the specified function to find all of the loop backedges in the
31 /// function and return them. This is a relatively cheap (compared to
32 /// computing dominators and loop info) analysis.
33 ///
34 /// The output is added to Result, as pairs of <from,to> edge info.
36  const Function &F,
37  SmallVectorImpl<std::pair<const BasicBlock *, const BasicBlock *> > &
38  Result);
39 
40 /// Search for the specified successor of basic block BB and return its position
41 /// in the terminator instruction's list of successors. It is an error to call
42 /// this with a block that is not a successor.
43 unsigned GetSuccessorNumber(const BasicBlock *BB, const BasicBlock *Succ);
44 
45 /// Return true if the specified edge is a critical edge. Critical edges are
46 /// edges from a block with multiple successors to a block with multiple
47 /// predecessors.
48 ///
49 bool isCriticalEdge(const TerminatorInst *TI, unsigned SuccNum,
50  bool AllowIdenticalEdges = false);
51 
52 /// Determine whether instruction 'To' is reachable from 'From',
53 /// 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(const Instruction *From, const Instruction *To,
68  const DominatorTree *DT = nullptr,
69  const LoopInfo *LI = nullptr);
70 
71 /// Determine whether block 'To' is reachable from 'From', returning
72 /// true if uncertain.
73 ///
74 /// Determine whether there is a path from From to To within a single function.
75 /// Returns false only if we can prove that once 'From' has been reached then
76 /// 'To' can not be executed. Conservatively returns true.
77 bool isPotentiallyReachable(const BasicBlock *From, const BasicBlock *To,
78  const DominatorTree *DT = nullptr,
79  const LoopInfo *LI = nullptr);
80 
81 /// Determine whether there is at least one path from a block in
82 /// 'Worklist' to 'StopBB', returning true if uncertain.
83 ///
84 /// Determine whether there is a path from at least one block in Worklist to
85 /// StopBB within a single function. Returns false only if we can prove that
86 /// once any block in 'Worklist' has been reached then 'StopBB' can not be
87 /// executed. Conservatively returns true.
88 bool isPotentiallyReachableFromMany(SmallVectorImpl<BasicBlock *> &Worklist,
89  BasicBlock *StopBB,
90  const DominatorTree *DT = nullptr,
91  const LoopInfo *LI = nullptr);
92 
93 /// Return true if the control flow in \p RPOTraversal is irreducible.
94 ///
95 /// This is a generic implementation to detect CFG irreducibility based on loop
96 /// info analysis. It can be used for any kind of CFG (Loop, MachineLoop,
97 /// Function, MachineFunction, etc.) by providing an RPO traversal (\p
98 /// RPOTraversal) and the loop info analysis (\p LI) of the CFG. This utility
99 /// function is only recommended when loop info analysis is available. If loop
100 /// info analysis isn't available, please, don't compute it explicitly for this
101 /// purpose. There are more efficient ways to detect CFG irreducibility that
102 /// don't require recomputing loop info analysis (e.g., T1/T2 or Tarjan's
103 /// algorithm).
104 ///
105 /// Requirements:
106 /// 1) GraphTraits must be implemented for NodeT type. It is used to access
107 /// NodeT successors.
108 // 2) \p RPOTraversal must be a valid reverse post-order traversal of the
109 /// target CFG with begin()/end() iterator interfaces.
110 /// 3) \p LI must be a valid LoopInfoBase that contains up-to-date loop
111 /// analysis information of the CFG.
112 ///
113 /// This algorithm uses the information about reducible loop back-edges already
114 /// computed in \p LI. When a back-edge is found during the RPO traversal, the
115 /// algorithm checks whether the back-edge is one of the reducible back-edges in
116 /// loop info. If it isn't, the CFG is irreducible. For example, for the CFG
117 /// below (canonical irreducible graph) loop info won't contain any loop, so the
118 /// algorithm will return that the CFG is irreducible when checking the B <-
119 /// -> C back-edge.
120 ///
121 /// (A->B, A->C, B->C, C->B, C->D)
122 /// A
123 /// / \
124 /// B<- ->C
125 /// |
126 /// D
127 ///
128 template <class NodeT, class RPOTraversalT, class LoopInfoT,
129  class GT = GraphTraits<NodeT>>
130 bool containsIrreducibleCFG(RPOTraversalT &RPOTraversal, const LoopInfoT &LI) {
131  /// Check whether the edge (\p Src, \p Dst) is a reducible loop backedge
132  /// according to LI. I.e., check if there exists a loop that contains Src and
133  /// where Dst is the loop header.
134  auto isProperBackedge = [&](NodeT Src, NodeT Dst) {
135  for (const auto *Lp = LI.getLoopFor(Src); Lp; Lp = Lp->getParentLoop()) {
136  if (Lp->getHeader() == Dst)
137  return true;
138  }
139  return false;
140  };
141 
142  SmallPtrSet<NodeT, 32> Visited;
143  for (NodeT Node : RPOTraversal) {
144  Visited.insert(Node);
145  for (NodeT Succ : make_range(GT::child_begin(Node), GT::child_end(Node))) {
146  // Succ hasn't been visited yet
147  if (!Visited.count(Succ))
148  continue;
149  // We already visited Succ, thus Node->Succ must be a backedge. Check that
150  // the head matches what we have in the loop information. Otherwise, we
151  // have an irreducible graph.
152  if (!isProperBackedge(Node, Succ))
153  return true;
154  }
155  }
156 
157  return false;
158 }
159 } // End llvm namespace
160 
161 #endif
Compute iterated dominance frontiers using a linear time algorithm.
Definition: AllocatorList.h:24
Various leaf nodes.
Definition: ISDOpcodes.h:60
F(f)
bool containsIrreducibleCFG(RPOTraversalT &RPOTraversal, const LoopInfoT &LI)
Return true if the control flow in RPOTraversal is irreducible.
Definition: CFG.h:130
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:371
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:382
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:418
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:72
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:27
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
bool isCriticalEdge(const TerminatorInst *TI, unsigned SuccNum, bool AllowIdenticalEdges=false)
Return true if the specified edge is a critical edge.
Definition: CFG.cpp:88