BranchProbabilityInfo.h revision 36b56886974eae4f9c5ebc96befd3e7bfe5de338 
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/IR/CFG.h"
20#include "llvm/InitializePasses.h"
21#include "llvm/Pass.h"
22#include "llvm/Support/BranchProbability.h"
23
24namespace llvm {
25class LoopInfo;
26class raw_ostream;
27
28/// \brief Analysis pass providing branch probability information.
29///
30/// This is a function analysis pass which provides information on the relative
31/// probabilities of each "edge" in the function's CFG where such an edge is
32/// defined by a pair (PredBlock and an index in the successors). The
33/// probability of an edge from one block is always relative to the
34/// probabilities of other edges from the block. The probabilites of all edges
35/// from a block sum to exactly one (100%).
36/// We use a pair (PredBlock and an index in the successors) to uniquely
37/// identify an edge, since we can have multiple edges from Src to Dst.
38/// As an example, we can have a switch which jumps to Dst with value 0 and
39/// value 10.
40class BranchProbabilityInfo : public FunctionPass {
41public:
42  static char ID;
43
44  BranchProbabilityInfo() : FunctionPass(ID) {
45    initializeBranchProbabilityInfoPass(*PassRegistry::getPassRegistry());
46  }
47
48  void getAnalysisUsage(AnalysisUsage &AU) const override;
49  bool runOnFunction(Function &F) override;
50  void print(raw_ostream &OS, const Module *M = 0) const override;
51
52  /// \brief Get an edge's probability, relative to other out-edges of the Src.
53  ///
54  /// This routine provides access to the fractional probability between zero
55  /// (0%) and one (100%) of this edge executing, relative to other edges
56  /// leaving the 'Src' block. The returned probability is never zero, and can
57  /// only be one if the source block has only one successor.
58  BranchProbability getEdgeProbability(const BasicBlock *Src,
59                                       unsigned IndexInSuccessors) const;
60
61  /// \brief Get the probability of going from Src to Dst.
62  ///
63  /// It returns the sum of all probabilities for edges from Src to Dst.
64  BranchProbability getEdgeProbability(const BasicBlock *Src,
65                                       const BasicBlock *Dst) const;
66
67  /// \brief Test if an edge is hot relative to other out-edges of the Src.
68  ///
69  /// Check whether this edge out of the source block is 'hot'. We define hot
70  /// as having a relative probability >= 80%.
71  bool isEdgeHot(const BasicBlock *Src, const BasicBlock *Dst) const;
72
73  /// \brief Retrieve the hot successor of a block if one exists.
74  ///
75  /// Given a basic block, look through its successors and if one exists for
76  /// which \see isEdgeHot would return true, return that successor block.
77  BasicBlock *getHotSucc(BasicBlock *BB) const;
78
79  /// \brief Print an edge's probability.
80  ///
81  /// Retrieves an edge's probability similarly to \see getEdgeProbability, but
82  /// then prints that probability to the provided stream. That stream is then
83  /// returned.
84  raw_ostream &printEdgeProbability(raw_ostream &OS, const BasicBlock *Src,
85                                    const BasicBlock *Dst) const;
86
87  /// \brief Get the raw edge weight calculated for the edge.
88  ///
89  /// This returns the raw edge weight. It is guaranteed to fall between 1 and
90  /// UINT32_MAX. Note that the raw edge weight is not meaningful in isolation.
91  /// This interface should be very carefully, and primarily by routines that
92  /// are updating the analysis by later calling setEdgeWeight.
93  uint32_t getEdgeWeight(const BasicBlock *Src,
94                         unsigned IndexInSuccessors) const;
95
96  /// \brief Get the raw edge weight calculated for the block pair.
97  ///
98  /// This returns the sum of all raw edge weights from Src to Dst.
99  /// It is guaranteed to fall between 1 and UINT32_MAX.
100  uint32_t getEdgeWeight(const BasicBlock *Src, const BasicBlock *Dst) const;
101
102  uint32_t getEdgeWeight(const BasicBlock *Src,
103                         succ_const_iterator Dst) const;
104
105  /// \brief Set the raw edge weight for a given edge.
106  ///
107  /// This allows a pass to explicitly set the edge weight for an edge. It can
108  /// be used when updating the CFG to update and preserve the branch
109  /// probability information. Read the implementation of how these edge
110  /// weights are calculated carefully before using!
111  void setEdgeWeight(const BasicBlock *Src, unsigned IndexInSuccessors,
112                     uint32_t Weight);
113
114private:
115  // Since we allow duplicate edges from one basic block to another, we use
116  // a pair (PredBlock and an index in the successors) to specify an edge.
117  typedef std::pair<const BasicBlock *, unsigned> Edge;
118
119  // Default weight value. Used when we don't have information about the edge.
120  // TODO: DEFAULT_WEIGHT makes sense during static predication, when none of
121  // the successors have a weight yet. But it doesn't make sense when providing
122  // weight to an edge that may have siblings with non-zero weights. This can
123  // be handled various ways, but it's probably fine for an edge with unknown
124  // weight to just "inherit" the non-zero weight of an adjacent successor.
125  static const uint32_t DEFAULT_WEIGHT = 16;
126
127  DenseMap<Edge, uint32_t> Weights;
128
129  /// \brief Handle to the LoopInfo analysis.
130  LoopInfo *LI;
131
132  /// \brief Track the last function we run over for printing.
133  Function *LastF;
134
135  /// \brief Track the set of blocks directly succeeded by a returning block.
136  SmallPtrSet<BasicBlock *, 16> PostDominatedByUnreachable;
137
138  /// \brief Track the set of blocks that always lead to a cold call.
139  SmallPtrSet<BasicBlock *, 16> PostDominatedByColdCall;
140
141  /// \brief Get sum of the block successors' weights.
142  uint32_t getSumForBlock(const BasicBlock *BB) const;
143
144  bool calcUnreachableHeuristics(BasicBlock *BB);
145  bool calcMetadataWeights(BasicBlock *BB);
146  bool calcColdCallHeuristics(BasicBlock *BB);
147  bool calcPointerHeuristics(BasicBlock *BB);
148  bool calcLoopBranchHeuristics(BasicBlock *BB);
149  bool calcZeroHeuristics(BasicBlock *BB);
150  bool calcFloatingPointHeuristics(BasicBlock *BB);
151  bool calcInvokeHeuristics(BasicBlock *BB);
152};
153
154}
155
156#endif
157