1
2//==- MachineBranchProbabilityInfo.h - Machine Branch Probability Analysis -==//
3//
4//                     The LLVM Compiler Infrastructure
5//
6// This file is distributed under the University of Illinois Open Source
7// License. See LICENSE.TXT for details.
8//
9//===----------------------------------------------------------------------===//
10//
11// This pass is used to evaluate branch probabilties on machine basic blocks.
12//
13//===----------------------------------------------------------------------===//
14
15#ifndef LLVM_CODEGEN_MACHINEBRANCHPROBABILITYINFO_H
16#define LLVM_CODEGEN_MACHINEBRANCHPROBABILITYINFO_H
17
18#include "llvm/Pass.h"
19#include "llvm/CodeGen/MachineBasicBlock.h"
20#include "llvm/Support/BranchProbability.h"
21#include <climits>
22
23namespace llvm {
24
25class MachineBranchProbabilityInfo : public ImmutablePass {
26  virtual void anchor();
27
28  // Default weight value. Used when we don't have information about the edge.
29  // TODO: DEFAULT_WEIGHT makes sense during static predication, when none of
30  // the successors have a weight yet. But it doesn't make sense when providing
31  // weight to an edge that may have siblings with non-zero weights. This can
32  // be handled various ways, but it's probably fine for an edge with unknown
33  // weight to just "inherit" the non-zero weight of an adjacent successor.
34  static const uint32_t DEFAULT_WEIGHT = 16;
35
36public:
37  static char ID;
38
39  MachineBranchProbabilityInfo() : ImmutablePass(ID) {
40    PassRegistry &Registry = *PassRegistry::getPassRegistry();
41    initializeMachineBranchProbabilityInfoPass(Registry);
42  }
43
44  void getAnalysisUsage(AnalysisUsage &AU) const {
45    AU.setPreservesAll();
46  }
47
48  // Return edge weight. If we don't have any informations about it - return
49  // DEFAULT_WEIGHT.
50  uint32_t getEdgeWeight(const MachineBasicBlock *Src,
51                         const MachineBasicBlock *Dst) const;
52
53  // Same thing, but using a const_succ_iterator from Src. This is faster when
54  // the iterator is already available.
55  uint32_t getEdgeWeight(const MachineBasicBlock *Src,
56                         MachineBasicBlock::const_succ_iterator Dst) const;
57
58  // Get sum of the block successors' weights, potentially scaling them to fit
59  // within 32-bits. If scaling is required, sets Scale based on the necessary
60  // adjustment. Any edge weights used with the sum should be divided by Scale.
61  uint32_t getSumForBlock(const MachineBasicBlock *MBB, uint32_t &Scale) const;
62
63  // A 'Hot' edge is an edge which probability is >= 80%.
64  bool isEdgeHot(MachineBasicBlock *Src, 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(MachineBasicBlock *Src,
77                                       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, MachineBasicBlock *Src,
83                                    MachineBasicBlock *Dst) const;
84};
85
86}
87
88
89#endif
90