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 
20 using namespace llvm;
21 
22 INITIALIZE_PASS_BEGIN(MachineBranchProbabilityInfo, "machine-branch-prob",
23                       "Machine Branch Probability Analysis", false, true)
24 INITIALIZE_PASS_END(MachineBranchProbabilityInfo, "machine-branch-prob",
25                     "Machine Branch Probability Analysis", false, true)
26 
27 char MachineBranchProbabilityInfo::ID = 0;
28 
29 uint32_t MachineBranchProbabilityInfo::
30 getSumForBlock(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 
46 uint32_t
47 MachineBranchProbabilityInfo::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 
55 bool 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 
62 MachineBasicBlock *
63 MachineBranchProbabilityInfo::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 
89 BranchProbability
90 MachineBranchProbabilityInfo::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 
98 raw_ostream &MachineBranchProbabilityInfo::
99 printEdgeProbability(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