1 //===- MachineBranchProbabilityInfo.cpp - Machine Branch Probability Info -===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 // This analysis uses probability info stored in Machine Basic Blocks.
10 //
11 //===----------------------------------------------------------------------===//
12
13 #include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
14 #include "llvm/CodeGen/MachineBasicBlock.h"
15 #include "llvm/IR/Instructions.h"
16 #include "llvm/InitializePasses.h"
17 #include "llvm/Support/CommandLine.h"
18 #include "llvm/Support/Debug.h"
19 #include "llvm/Support/raw_ostream.h"
20
21 using namespace llvm;
22
23 INITIALIZE_PASS_BEGIN(MachineBranchProbabilityInfo, "machine-branch-prob",
24 "Machine Branch Probability Analysis", false, true)
25 INITIALIZE_PASS_END(MachineBranchProbabilityInfo, "machine-branch-prob",
26 "Machine Branch Probability Analysis", false, true)
27
28 cl::opt<unsigned>
29 StaticLikelyProb("static-likely-prob",
30 cl::desc("branch probability threshold in percentage"
31 "to be considered very likely"),
32 cl::init(80), cl::Hidden);
33
34 cl::opt<unsigned> ProfileLikelyProb(
35 "profile-likely-prob",
36 cl::desc("branch probability threshold in percentage to be considered"
37 " very likely when profile is available"),
38 cl::init(51), cl::Hidden);
39
40 char MachineBranchProbabilityInfo::ID = 0;
41
MachineBranchProbabilityInfo()42 MachineBranchProbabilityInfo::MachineBranchProbabilityInfo()
43 : ImmutablePass(ID) {
44 PassRegistry &Registry = *PassRegistry::getPassRegistry();
45 initializeMachineBranchProbabilityInfoPass(Registry);
46 }
47
anchor()48 void MachineBranchProbabilityInfo::anchor() {}
49
getEdgeProbability(const MachineBasicBlock * Src,MachineBasicBlock::const_succ_iterator Dst) const50 BranchProbability MachineBranchProbabilityInfo::getEdgeProbability(
51 const MachineBasicBlock *Src,
52 MachineBasicBlock::const_succ_iterator Dst) const {
53 return Src->getSuccProbability(Dst);
54 }
55
getEdgeProbability(const MachineBasicBlock * Src,const MachineBasicBlock * Dst) const56 BranchProbability MachineBranchProbabilityInfo::getEdgeProbability(
57 const MachineBasicBlock *Src, const MachineBasicBlock *Dst) const {
58 // This is a linear search. Try to use the const_succ_iterator version when
59 // possible.
60 return getEdgeProbability(Src, find(Src->successors(), Dst));
61 }
62
isEdgeHot(const MachineBasicBlock * Src,const MachineBasicBlock * Dst) const63 bool MachineBranchProbabilityInfo::isEdgeHot(
64 const MachineBasicBlock *Src, const MachineBasicBlock *Dst) const {
65 BranchProbability HotProb(StaticLikelyProb, 100);
66 return getEdgeProbability(Src, Dst) > HotProb;
67 }
68
69 MachineBasicBlock *
getHotSucc(MachineBasicBlock * MBB) const70 MachineBranchProbabilityInfo::getHotSucc(MachineBasicBlock *MBB) const {
71 auto MaxProb = BranchProbability::getZero();
72 MachineBasicBlock *MaxSucc = nullptr;
73 for (MachineBasicBlock::const_succ_iterator I = MBB->succ_begin(),
74 E = MBB->succ_end(); I != E; ++I) {
75 auto Prob = getEdgeProbability(MBB, I);
76 if (Prob > MaxProb) {
77 MaxProb = Prob;
78 MaxSucc = *I;
79 }
80 }
81
82 BranchProbability HotProb(StaticLikelyProb, 100);
83 if (getEdgeProbability(MBB, MaxSucc) >= HotProb)
84 return MaxSucc;
85
86 return nullptr;
87 }
88
printEdgeProbability(raw_ostream & OS,const MachineBasicBlock * Src,const MachineBasicBlock * Dst) const89 raw_ostream &MachineBranchProbabilityInfo::printEdgeProbability(
90 raw_ostream &OS, const MachineBasicBlock *Src,
91 const MachineBasicBlock *Dst) const {
92
93 const BranchProbability Prob = getEdgeProbability(Src, Dst);
94 OS << "edge " << printMBBReference(*Src) << " -> " << printMBBReference(*Dst)
95 << " probability is " << Prob
96 << (isEdgeHot(Src, Dst) ? " [HOT edge]\n" : "\n");
97
98 return OS;
99 }
100