BranchProbabilityInfo.h revision 255f89faee13dc491cb64fbeae3c763e7e2ea4e6 
1//===--- BranchProbabilityInfo.h - Branch Probability Analysis --*- 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 pass is used to evaluate branch probabilties.
11//
12//===----------------------------------------------------------------------===//
13
14#ifndef LLVM_ANALYSIS_BRANCHPROBABILITYINFO_H
15#define LLVM_ANALYSIS_BRANCHPROBABILITYINFO_H
16
17#include "llvm/ADT/DenseMap.h"
18#include "llvm/ADT/SmallPtrSet.h"
19#include "llvm/InitializePasses.h"
20#include "llvm/Pass.h"
21#include "llvm/Support/BranchProbability.h"
22
23namespace llvm {
24class LoopInfo;
25class raw_ostream;
26
27/// \brief Analysis pass providing branch probability information.
28///
29/// This is a function analysis pass which provides information on the relative
30/// probabilities of each "edge" in the function's CFG where such an edge is
31/// defined by a pair (PredBlock and an index in the successors). The
32/// probability of an edge from one block is always relative to the
33/// probabilities of other edges from the block. The probabilites of all edges
34/// from a block sum to exactly one (100%).
35/// We use a pair (PredBlock and an index in the successors) to uniquely
36/// identify an edge, since we can have multiple edges from Src to Dst.
37/// As an example, we can have a switch which jumps to Dst with value 0 and
38/// value 10.
39class BranchProbabilityInfo : public FunctionPass {
40public:
41  static char ID;
42
43  BranchProbabilityInfo() : FunctionPass(ID) {
44    initializeBranchProbabilityInfoPass(*PassRegistry::getPassRegistry());
45  }
46
47  void getAnalysisUsage(AnalysisUsage &AU) const;
48  bool runOnFunction(Function &F);
49  void print(raw_ostream &OS, const Module *M = 0) const;
50
51  /// \brief Get an edge's probability, relative to other out-edges of the Src.
52  ///
53  /// This routine provides access to the fractional probability between zero
54  /// (0%) and one (100%) of this edge executing, relative to other edges
55  /// leaving the 'Src' block. The returned probability is never zero, and can
56  /// only be one if the source block has only one successor.
57  BranchProbability getEdgeProbability(const BasicBlock *Src,
58                                       unsigned IndexInSuccessors) const;
59
60  /// \brief Get the probability of going from Src to Dst.
61  ///
62  /// It returns the sum of all probabilities for edges from Src to Dst.
63  BranchProbability getEdgeProbability(const BasicBlock *Src,
64                                       const BasicBlock *Dst) const;
65
66  /// \brief Test if an edge is hot relative to other out-edges of the Src.
67  ///
68  /// Check whether this edge out of the source block is 'hot'. We define hot
69  /// as having a relative probability >= 80%.
70  bool isEdgeHot(const BasicBlock *Src, const BasicBlock *Dst) const;
71
72  /// \brief Retrieve the hot successor of a block if one exists.
73  ///
74  /// Given a basic block, look through its successors and if one exists for
75  /// which \see isEdgeHot would return true, return that successor block.
76  BasicBlock *getHotSucc(BasicBlock *BB) const;
77
78  /// \brief Print an edge's probability.
79  ///
80  /// Retrieves an edge's probability similarly to \see getEdgeProbability, but
81  /// then prints that probability to the provided stream. That stream is then
82  /// returned.
83  raw_ostream &printEdgeProbability(raw_ostream &OS, const BasicBlock *Src,
84                                    const BasicBlock *Dst) const;
85
86  /// \brief Get the raw edge weight calculated for the edge.
87  ///
88  /// This returns the raw edge weight. It is guaranteed to fall between 1 and
89  /// UINT32_MAX. Note that the raw edge weight is not meaningful in isolation.
90  /// This interface should be very carefully, and primarily by routines that
91  /// are updating the analysis by later calling setEdgeWeight.
92  uint32_t getEdgeWeight(const BasicBlock *Src,
93                         unsigned IndexInSuccessors) const;
94
95  /// \brief Get the raw edge weight calculated for the block pair.
96  ///
97  /// This returns the sum of all raw edge weights from Src to Dst.
98  /// It is guaranteed to fall between 1 and UINT32_MAX.
99  uint32_t getEdgeWeight(const BasicBlock *Src, const BasicBlock *Dst) const;
100
101  /// \brief Set the raw edge weight for a given edge.
102  ///
103  /// This allows a pass to explicitly set the edge weight for an edge. It can
104  /// be used when updating the CFG to update and preserve the branch
105  /// probability information. Read the implementation of how these edge
106  /// weights are calculated carefully before using!
107  void setEdgeWeight(const BasicBlock *Src, unsigned IndexInSuccessors,
108                     uint32_t Weight);
109
110private:
111  // Since we allow duplicate edges from one basic block to another, we use
112  // a pair (PredBlock and an index in the successors) to specify an edge.
113  typedef std::pair<const BasicBlock *, unsigned> Edge;
114
115  // Default weight value. Used when we don't have information about the edge.
116  // TODO: DEFAULT_WEIGHT makes sense during static predication, when none of
117  // the successors have a weight yet. But it doesn't make sense when providing
118  // weight to an edge that may have siblings with non-zero weights. This can
119  // be handled various ways, but it's probably fine for an edge with unknown
120  // weight to just "inherit" the non-zero weight of an adjacent successor.
121  static const uint32_t DEFAULT_WEIGHT = 16;
122
123  DenseMap<Edge, uint32_t> Weights;
124
125  /// \brief Handle to the LoopInfo analysis.
126  LoopInfo *LI;
127
128  /// \brief Track the last function we run over for printing.
129  Function *LastF;
130
131  /// \brief Track the set of blocks directly succeeded by a returning block.
132  SmallPtrSet<BasicBlock *, 16> PostDominatedByUnreachable;
133
134  /// \brief Get sum of the block successors' weights.
135  uint32_t getSumForBlock(const BasicBlock *BB) const;
136
137  bool calcUnreachableHeuristics(BasicBlock *BB);
138  bool calcMetadataWeights(BasicBlock *BB);
139  bool calcPointerHeuristics(BasicBlock *BB);
140  bool calcLoopBranchHeuristics(BasicBlock *BB);
141  bool calcZeroHeuristics(BasicBlock *BB);
142  bool calcFloatingPointHeuristics(BasicBlock *BB);
143  bool calcInvokeHeuristics(BasicBlock *BB);
144};
145
146}
147
148#endif
149