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/CodeGen/MachineBranchProbabilityInfo.h"
15 #include "llvm/CodeGen/MachineBasicBlock.h"
16 #include "llvm/IR/Instructions.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 cl::opt<unsigned> StaticLikelyProb(
28     "static-likely-prob",
29     cl::desc("branch probability threshold to be considered very likely"),
30     cl::init(80), cl::Hidden);
31 
32 cl::opt<unsigned> ProfileLikelyProb(
33     "profile-likely-prob",
34     cl::desc("branch probability threshold to be considered very likely "
35              "when profile is available"),
36     cl::init(51), cl::Hidden);
37 
38 char MachineBranchProbabilityInfo::ID = 0;
39 
40 void MachineBranchProbabilityInfo::anchor() {}
41 
42 BranchProbability MachineBranchProbabilityInfo::getEdgeProbability(
43     const MachineBasicBlock *Src,
44     MachineBasicBlock::const_succ_iterator Dst) const {
45   return Src->getSuccProbability(Dst);
46 }
47 
48 BranchProbability MachineBranchProbabilityInfo::getEdgeProbability(
49     const MachineBasicBlock *Src, const MachineBasicBlock *Dst) const {
50   // This is a linear search. Try to use the const_succ_iterator version when
51   // possible.
52   return getEdgeProbability(Src,
53                             std::find(Src->succ_begin(), Src->succ_end(), Dst));
54 }
55 
56 bool MachineBranchProbabilityInfo::isEdgeHot(
57     const MachineBasicBlock *Src, const MachineBasicBlock *Dst) const {
58   BranchProbability HotProb(StaticLikelyProb, 100);
59   return getEdgeProbability(Src, Dst) > HotProb;
60 }
61 
62 MachineBasicBlock *
63 MachineBranchProbabilityInfo::getHotSucc(MachineBasicBlock *MBB) const {
64   auto MaxProb = BranchProbability::getZero();
65   MachineBasicBlock *MaxSucc = nullptr;
66   for (MachineBasicBlock::const_succ_iterator I = MBB->succ_begin(),
67        E = MBB->succ_end(); I != E; ++I) {
68     auto Prob = getEdgeProbability(MBB, I);
69     if (Prob > MaxProb) {
70       MaxProb = Prob;
71       MaxSucc = *I;
72     }
73   }
74 
75   BranchProbability HotProb(StaticLikelyProb, 100);
76   if (getEdgeProbability(MBB, MaxSucc) >= HotProb)
77     return MaxSucc;
78 
79   return nullptr;
80 }
81 
82 raw_ostream &MachineBranchProbabilityInfo::printEdgeProbability(
83     raw_ostream &OS, const MachineBasicBlock *Src,
84     const MachineBasicBlock *Dst) const {
85 
86   const BranchProbability Prob = getEdgeProbability(Src, Dst);
87   OS << "edge MBB#" << Src->getNumber() << " -> MBB#" << Dst->getNumber()
88      << " probability is " << Prob
89      << (isEdgeHot(Src, Dst) ? " [HOT edge]\n" : "\n");
90 
91   return OS;
92 }
93