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
anchor()29 void MachineBranchProbabilityInfo::anchor() { }
30
31 uint32_t MachineBranchProbabilityInfo::
getSumForBlock(const MachineBasicBlock * MBB,uint32_t & Scale) const32 getSumForBlock(const MachineBasicBlock *MBB, uint32_t &Scale) const {
33 // First we compute the sum with 64-bits of precision, ensuring that cannot
34 // overflow by bounding the number of weights considered. Hopefully no one
35 // actually needs 2^32 successors.
36 assert(MBB->succ_size() < UINT32_MAX);
37 uint64_t Sum = 0;
38 Scale = 1;
39 for (MachineBasicBlock::const_succ_iterator I = MBB->succ_begin(),
40 E = MBB->succ_end(); I != E; ++I) {
41 uint32_t Weight = getEdgeWeight(MBB, I);
42 Sum += Weight;
43 }
44
45 // If the computed sum fits in 32-bits, we're done.
46 if (Sum <= UINT32_MAX)
47 return Sum;
48
49 // Otherwise, compute the scale necessary to cause the weights to fit, and
50 // re-sum with that scale applied.
51 assert((Sum / UINT32_MAX) < UINT32_MAX);
52 Scale = (Sum / UINT32_MAX) + 1;
53 Sum = 0;
54 for (MachineBasicBlock::const_succ_iterator I = MBB->succ_begin(),
55 E = MBB->succ_end(); I != E; ++I) {
56 uint32_t Weight = getEdgeWeight(MBB, I);
57 Sum += Weight / Scale;
58 }
59 assert(Sum <= UINT32_MAX);
60 return Sum;
61 }
62
63 uint32_t MachineBranchProbabilityInfo::
getEdgeWeight(const MachineBasicBlock * Src,MachineBasicBlock::const_succ_iterator Dst) const64 getEdgeWeight(const MachineBasicBlock *Src,
65 MachineBasicBlock::const_succ_iterator Dst) const {
66 uint32_t Weight = Src->getSuccWeight(Dst);
67 if (!Weight)
68 return DEFAULT_WEIGHT;
69 return Weight;
70 }
71
72 uint32_t MachineBranchProbabilityInfo::
getEdgeWeight(const MachineBasicBlock * Src,const MachineBasicBlock * Dst) const73 getEdgeWeight(const MachineBasicBlock *Src,
74 const MachineBasicBlock *Dst) const {
75 // This is a linear search. Try to use the const_succ_iterator version when
76 // possible.
77 return getEdgeWeight(Src, std::find(Src->succ_begin(), Src->succ_end(), Dst));
78 }
79
isEdgeHot(MachineBasicBlock * Src,MachineBasicBlock * Dst) const80 bool MachineBranchProbabilityInfo::isEdgeHot(MachineBasicBlock *Src,
81 MachineBasicBlock *Dst) const {
82 // Hot probability is at least 4/5 = 80%
83 // FIXME: Compare against a static "hot" BranchProbability.
84 return getEdgeProbability(Src, Dst) > BranchProbability(4, 5);
85 }
86
87 MachineBasicBlock *
getHotSucc(MachineBasicBlock * MBB) const88 MachineBranchProbabilityInfo::getHotSucc(MachineBasicBlock *MBB) const {
89 uint32_t MaxWeight = 0;
90 MachineBasicBlock *MaxSucc = 0;
91 for (MachineBasicBlock::const_succ_iterator I = MBB->succ_begin(),
92 E = MBB->succ_end(); I != E; ++I) {
93 uint32_t Weight = getEdgeWeight(MBB, I);
94 if (Weight > MaxWeight) {
95 MaxWeight = Weight;
96 MaxSucc = *I;
97 }
98 }
99
100 if (getEdgeProbability(MBB, MaxSucc) >= BranchProbability(4, 5))
101 return MaxSucc;
102
103 return 0;
104 }
105
106 BranchProbability
getEdgeProbability(MachineBasicBlock * Src,MachineBasicBlock * Dst) const107 MachineBranchProbabilityInfo::getEdgeProbability(MachineBasicBlock *Src,
108 MachineBasicBlock *Dst) const {
109 uint32_t Scale = 1;
110 uint32_t D = getSumForBlock(Src, Scale);
111 uint32_t N = getEdgeWeight(Src, Dst) / Scale;
112
113 return BranchProbability(N, D);
114 }
115
116 raw_ostream &MachineBranchProbabilityInfo::
printEdgeProbability(raw_ostream & OS,MachineBasicBlock * Src,MachineBasicBlock * Dst) const117 printEdgeProbability(raw_ostream &OS, MachineBasicBlock *Src,
118 MachineBasicBlock *Dst) const {
119
120 const BranchProbability Prob = getEdgeProbability(Src, Dst);
121 OS << "edge MBB#" << Src->getNumber() << " -> MBB#" << Dst->getNumber()
122 << " probability is " << Prob
123 << (isEdgeHot(Src, Dst) ? " [HOT edge]\n" : "\n");
124
125 return OS;
126 }
127