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   uint32_t Weight = getEdgeWeight(Src, Dst);
59   uint32_t Sum = getSumForBlock(Src);
60 
61   // FIXME: Implement BranchProbability::compare then change this code to
62   // compare this BranchProbability against a static "hot" BranchProbability.
63   return (uint64_t)Weight * 5 > (uint64_t)Sum * 4;
64 }
65 
66 MachineBasicBlock *
67 MachineBranchProbabilityInfo::getHotSucc(MachineBasicBlock *MBB) const {
68   uint32_t Sum = 0;
69   uint32_t MaxWeight = 0;
70   MachineBasicBlock *MaxSucc = 0;
71 
72   for (MachineBasicBlock::const_succ_iterator I = MBB->succ_begin(),
73        E = MBB->succ_end(); I != E; ++I) {
74     MachineBasicBlock *Succ = *I;
75     uint32_t Weight = getEdgeWeight(MBB, Succ);
76     uint32_t PrevSum = Sum;
77 
78     Sum += Weight;
79     assert(Sum > PrevSum); (void) PrevSum;
80 
81     if (Weight > MaxWeight) {
82       MaxWeight = Weight;
83       MaxSucc = Succ;
84     }
85   }
86 
87   // FIXME: Use BranchProbability::compare.
88   if ((uint64_t)MaxWeight * 5 >= (uint64_t)Sum * 4)
89     return MaxSucc;
90 
91   return 0;
92 }
93 
94 BranchProbability
95 MachineBranchProbabilityInfo::getEdgeProbability(MachineBasicBlock *Src,
96                                                  MachineBasicBlock *Dst) const {
97   uint32_t N = getEdgeWeight(Src, Dst);
98   uint32_t D = getSumForBlock(Src);
99 
100   return BranchProbability(N, D);
101 }
102 
103 raw_ostream &MachineBranchProbabilityInfo::
104 printEdgeProbability(raw_ostream &OS, MachineBasicBlock *Src,
105                      MachineBasicBlock *Dst) const {
106 
107   const BranchProbability Prob = getEdgeProbability(Src, Dst);
108   OS << "edge MBB#" << Src->getNumber() << " -> MBB#" << Dst->getNumber()
109      << " probability is "  << Prob
110      << (isEdgeHot(Src, Dst) ? " [HOT edge]\n" : "\n");
111 
112   return OS;
113 }
114