1//=- MachineBranchProbabilityInfo.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 on machine basic blocks.
11//
12//===----------------------------------------------------------------------===//
13
14#ifndef LLVM_CODEGEN_MACHINEBRANCHPROBABILITYINFO_H
15#define LLVM_CODEGEN_MACHINEBRANCHPROBABILITYINFO_H
16
17#include "llvm/CodeGen/MachineBasicBlock.h"
18#include "llvm/Pass.h"
19#include "llvm/Support/BranchProbability.h"
20#include <climits>
21
22namespace llvm {
23
24class MachineBranchProbabilityInfo : public ImmutablePass {
25  virtual void anchor();
26
27  // Default weight value. Used when we don't have information about the edge.
28  // TODO: DEFAULT_WEIGHT makes sense during static predication, when none of
29  // the successors have a weight yet. But it doesn't make sense when providing
30  // weight to an edge that may have siblings with non-zero weights. This can
31  // be handled various ways, but it's probably fine for an edge with unknown
32  // weight to just "inherit" the non-zero weight of an adjacent successor.
33  static const uint32_t DEFAULT_WEIGHT = 16;
34
35public:
36  static char ID;
37
38  MachineBranchProbabilityInfo() : ImmutablePass(ID) {
39    PassRegistry &Registry = *PassRegistry::getPassRegistry();
40    initializeMachineBranchProbabilityInfoPass(Registry);
41  }
42
43  void getAnalysisUsage(AnalysisUsage &AU) const override {
44    AU.setPreservesAll();
45  }
46
47  // Return edge weight. If we don't have any informations about it - return
48  // DEFAULT_WEIGHT.
49  uint32_t getEdgeWeight(const MachineBasicBlock *Src,
50                         const MachineBasicBlock *Dst) const;
51
52  // Same thing, but using a const_succ_iterator from Src. This is faster when
53  // the iterator is already available.
54  uint32_t getEdgeWeight(const MachineBasicBlock *Src,
55                         MachineBasicBlock::const_succ_iterator Dst) const;
56
57  // Get sum of the block successors' weights, potentially scaling them to fit
58  // within 32-bits. If scaling is required, sets Scale based on the necessary
59  // adjustment. Any edge weights used with the sum should be divided by Scale.
60  uint32_t getSumForBlock(const MachineBasicBlock *MBB, uint32_t &Scale) const;
61
62  // A 'Hot' edge is an edge which probability is >= 80%.
63  bool isEdgeHot(const MachineBasicBlock *Src,
64                 const MachineBasicBlock *Dst) const;
65
66  // Return a hot successor for the block BB or null if there isn't one.
67  // NB: This routine's complexity is linear on the number of successors.
68  MachineBasicBlock *getHotSucc(MachineBasicBlock *MBB) const;
69
70  // Return a probability as a fraction between 0 (0% probability) and
71  // 1 (100% probability), however the value is never equal to 0, and can be 1
72  // only iff SRC block has only one successor.
73  // NB: This routine's complexity is linear on the number of successors of
74  // Src. Querying sequentially for each successor's probability is a quadratic
75  // query pattern.
76  BranchProbability getEdgeProbability(const MachineBasicBlock *Src,
77                                       const MachineBasicBlock *Dst) const;
78
79  // Print value between 0 (0% probability) and 1 (100% probability),
80  // however the value is never equal to 0, and can be 1 only iff SRC block
81  // has only one successor.
82  raw_ostream &printEdgeProbability(raw_ostream &OS,
83                                    const MachineBasicBlock *Src,
84                                    const MachineBasicBlock *Dst) const;
85};
86
87}
88
89
90#endif
91