BranchProbabilityInfo.h revision 0c34ae88bfe6ab40fc30784f131510992438ea43 
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/InitializePasses.h"
18#include "llvm/Pass.h"
19#include "llvm/ADT/DenseMap.h"
20#include "llvm/ADT/SmallPtrSet.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 of basic blocks. The probability for a given block and
32/// a successor block are always relative to the probabilities of the other
33/// successor blocks. Another way of looking at it is that the probabilities
34/// for a given block B and each of its successors should sum to exactly
35/// one (100%).
36class BranchProbabilityInfo : public FunctionPass {
37public:
38  static char ID;
39
40  BranchProbabilityInfo() : FunctionPass(ID) {
41    initializeBranchProbabilityInfoPass(*PassRegistry::getPassRegistry());
42  }
43
44  void getAnalysisUsage(AnalysisUsage &AU) const;
45  bool runOnFunction(Function &F);
46  void print(raw_ostream &OS, const Module *M = 0) const;
47
48  /// \brief Get an edge's probability, relative to other out-edges of the Src.
49  ///
50  /// This routine provides access to the fractional probability between zero
51  /// (0%) and one (100%) of this edge executing, relative to other edges
52  /// leaving the 'Src' block. The returned probability is never zero, and can
53  /// only be one if the source block has only one successor.
54  BranchProbability getEdgeProbability(const BasicBlock *Src,
55                                       const BasicBlock *Dst) const;
56
57  /// \brief Test if an edge is hot relative to other out-edges of the Src.
58  ///
59  /// Check whether this edge out of the source block is 'hot'. We define hot
60  /// as having a relative probability >= 80%.
61  bool isEdgeHot(const BasicBlock *Src, const BasicBlock *Dst) const;
62
63  /// \brief Retrieve the hot successor of a block if one exists.
64  ///
65  /// Given a basic block, look through its successors and if one exists for
66  /// which \see isEdgeHot would return true, return that successor block.
67  BasicBlock *getHotSucc(BasicBlock *BB) const;
68
69  /// \brief Print an edge's probability.
70  ///
71  /// Retrieves an edge's probability similarly to \see getEdgeProbability, but
72  /// then prints that probability to the provided stream. That stream is then
73  /// returned.
74  raw_ostream &printEdgeProbability(raw_ostream &OS, const BasicBlock *Src,
75                                    const BasicBlock *Dst) const;
76
77  /// \brief Get the raw edge weight calculated for the block pair.
78  ///
79  /// This returns the raw edge weight. It is guaranteed to fall between 1 and
80  /// UINT32_MAX. Note that the raw edge weight is not meaningful in isolation.
81  /// This interface should be very carefully, and primarily by routines that
82  /// are updating the analysis by later calling setEdgeWeight.
83  uint32_t getEdgeWeight(const BasicBlock *Src, const BasicBlock *Dst) const;
84
85  /// \brief Set the raw edge weight for the block pair.
86  ///
87  /// This allows a pass to explicitly set the edge weight for a block. It can
88  /// be used when updating the CFG to update and preserve the branch
89  /// probability information. Read the implementation of how these edge
90  /// weights are calculated carefully before using!
91  void setEdgeWeight(const BasicBlock *Src, const BasicBlock *Dst,
92                     uint32_t Weight);
93
94private:
95  typedef std::pair<const BasicBlock *, const BasicBlock *> Edge;
96
97  // Default weight value. Used when we don't have information about the edge.
98  // TODO: DEFAULT_WEIGHT makes sense during static predication, when none of
99  // the successors have a weight yet. But it doesn't make sense when providing
100  // weight to an edge that may have siblings with non-zero weights. This can
101  // be handled various ways, but it's probably fine for an edge with unknown
102  // weight to just "inherit" the non-zero weight of an adjacent successor.
103  static const uint32_t DEFAULT_WEIGHT = 16;
104
105  DenseMap<Edge, uint32_t> Weights;
106
107  /// \brief Handle to the LoopInfo analysis.
108  LoopInfo *LI;
109
110  /// \brief Track the last function we run over for printing.
111  Function *LastF;
112
113  /// \brief Track the set of blocks directly succeeded by a returning block.
114  SmallPtrSet<BasicBlock *, 16> PostDominatedByUnreachable;
115
116  /// \brief Get sum of the block successors' weights.
117  uint32_t getSumForBlock(const BasicBlock *BB) const;
118
119  bool calcUnreachableHeuristics(BasicBlock *BB);
120  bool calcMetadataWeights(BasicBlock *BB);
121  bool calcPointerHeuristics(BasicBlock *BB);
122  bool calcLoopBranchHeuristics(BasicBlock *BB);
123  bool calcZeroHeuristics(BasicBlock *BB);
124  bool calcFloatingPointHeuristics(BasicBlock *BB);
125  bool calcInvokeHeuristics(BasicBlock *BB);
126};
127
128}
129
130#endif
131