MachineBranchProbabilityInfo.cpp revision 91bbe237167bf84ce41d01eff3c028ff2b10be26 
1//===- MachineBranchProbabilityInfo.cpp - Machine Branch Probability Info -===//
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 analysis uses probability info stored in Machine Basic Blocks.
11//
12//===----------------------------------------------------------------------===//
13
14#include "llvm/Instructions.h"
15#include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
16#include "llvm/CodeGen/MachineBasicBlock.h"
17#include "llvm/Support/Debug.h"
18#include "llvm/Support/raw_ostream.h"
19
20using namespace llvm;
21
22INITIALIZE_PASS_BEGIN(MachineBranchProbabilityInfo, "machine-branch-prob",
23                      "Machine Branch Probability Analysis", false, true)
24INITIALIZE_PASS_END(MachineBranchProbabilityInfo, "machine-branch-prob",
25                    "Machine Branch Probability Analysis", false, true)
26
27char MachineBranchProbabilityInfo::ID = 0;
28
29uint32_t MachineBranchProbabilityInfo::
30getSumForBlock(MachineBasicBlock *MBB) const {
31  uint32_t Sum = 0;
32
33  for (MachineBasicBlock::const_succ_iterator I = MBB->succ_begin(),
34       E = MBB->succ_end(); I != E; ++I) {
35    MachineBasicBlock *Succ = *I;
36    uint32_t Weight = getEdgeWeight(MBB, Succ);
37    uint32_t PrevSum = Sum;
38
39    Sum += Weight;
40    assert(Sum > PrevSum); (void) PrevSum;
41  }
42
43  return Sum;
44}
45
46uint32_t
47MachineBranchProbabilityInfo::getEdgeWeight(MachineBasicBlock *Src,
48                                            MachineBasicBlock *Dst) const {
49  uint32_t Weight = Src->getSuccWeight(Dst);
50  if (!Weight)
51    return DEFAULT_WEIGHT;
52  return Weight;
53}
54
55bool MachineBranchProbabilityInfo::isEdgeHot(MachineBasicBlock *Src,
56                                             MachineBasicBlock *Dst) const {
57  // Hot probability is at least 4/5 = 80%
58  // FIXME: Compare against a static "hot" BranchProbability.
59  return getEdgeProbability(Src, Dst) > BranchProbability(4, 5);
60}
61
62MachineBasicBlock *
63MachineBranchProbabilityInfo::getHotSucc(MachineBasicBlock *MBB) const {
64  uint32_t Sum = 0;
65  uint32_t MaxWeight = 0;
66  MachineBasicBlock *MaxSucc = 0;
67
68  for (MachineBasicBlock::const_succ_iterator I = MBB->succ_begin(),
69       E = MBB->succ_end(); I != E; ++I) {
70    MachineBasicBlock *Succ = *I;
71    uint32_t Weight = getEdgeWeight(MBB, Succ);
72    uint32_t PrevSum = Sum;
73
74    Sum += Weight;
75    assert(Sum > PrevSum); (void) PrevSum;
76
77    if (Weight > MaxWeight) {
78      MaxWeight = Weight;
79      MaxSucc = Succ;
80    }
81  }
82
83  if (BranchProbability(MaxWeight, Sum) >= BranchProbability(4, 5))
84    return MaxSucc;
85
86  return 0;
87}
88
89BranchProbability
90MachineBranchProbabilityInfo::getEdgeProbability(MachineBasicBlock *Src,
91                                                 MachineBasicBlock *Dst) const {
92  uint32_t N = getEdgeWeight(Src, Dst);
93  uint32_t D = getSumForBlock(Src);
94
95  return BranchProbability(N, D);
96}
97
98raw_ostream &MachineBranchProbabilityInfo::
99printEdgeProbability(raw_ostream &OS, MachineBasicBlock *Src,
100                     MachineBasicBlock *Dst) const {
101
102  const BranchProbability Prob = getEdgeProbability(Src, Dst);
103  OS << "edge MBB#" << Src->getNumber() << " -> MBB#" << Dst->getNumber()
104     << " probability is "  << Prob
105     << (isEdgeHot(Src, Dst) ? " [HOT edge]\n" : "\n");
106
107  return OS;
108}
109