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, uint32_t &Scale) const {
31   // First we compute the sum with 64-bits of precision, ensuring that cannot
32   // overflow by bounding the number of weights considered. Hopefully no one
33   // actually needs 2^32 successors.
34   assert(MBB->succ_size() < UINT32_MAX);
35   uint64_t Sum = 0;
36   Scale = 1;
37   for (MachineBasicBlock::const_succ_iterator I = MBB->succ_begin(),
38        E = MBB->succ_end(); I != E; ++I) {
39     uint32_t Weight = getEdgeWeight(MBB, *I);
40     Sum += Weight;
41   }
42 
43   // If the computed sum fits in 32-bits, we're done.
44   if (Sum <= UINT32_MAX)
45     return Sum;
46 
47   // Otherwise, compute the scale necessary to cause the weights to fit, and
48   // re-sum with that scale applied.
49   assert((Sum / UINT32_MAX) < UINT32_MAX);
50   Scale = (Sum / UINT32_MAX) + 1;
51   Sum = 0;
52   for (MachineBasicBlock::const_succ_iterator I = MBB->succ_begin(),
53        E = MBB->succ_end(); I != E; ++I) {
54     uint32_t Weight = getEdgeWeight(MBB, *I);
55     Sum += Weight / Scale;
56   }
57   assert(Sum <= UINT32_MAX);
58   return Sum;
59 }
60 
61 uint32_t
62 MachineBranchProbabilityInfo::getEdgeWeight(MachineBasicBlock *Src,
63                                             MachineBasicBlock *Dst) const {
64   uint32_t Weight = Src->getSuccWeight(Dst);
65   if (!Weight)
66     return DEFAULT_WEIGHT;
67   return Weight;
68 }
69 
70 bool MachineBranchProbabilityInfo::isEdgeHot(MachineBasicBlock *Src,
71                                              MachineBasicBlock *Dst) const {
72   // Hot probability is at least 4/5 = 80%
73   // FIXME: Compare against a static "hot" BranchProbability.
74   return getEdgeProbability(Src, Dst) > BranchProbability(4, 5);
75 }
76 
77 MachineBasicBlock *
78 MachineBranchProbabilityInfo::getHotSucc(MachineBasicBlock *MBB) const {
79   uint32_t MaxWeight = 0;
80   MachineBasicBlock *MaxSucc = 0;
81   for (MachineBasicBlock::const_succ_iterator I = MBB->succ_begin(),
82        E = MBB->succ_end(); I != E; ++I) {
83     uint32_t Weight = getEdgeWeight(MBB, *I);
84     if (Weight > MaxWeight) {
85       MaxWeight = Weight;
86       MaxSucc = *I;
87     }
88   }
89 
90   if (getEdgeProbability(MBB, MaxSucc) >= BranchProbability(4, 5))
91     return MaxSucc;
92 
93   return 0;
94 }
95 
96 BranchProbability
97 MachineBranchProbabilityInfo::getEdgeProbability(MachineBasicBlock *Src,
98                                                  MachineBasicBlock *Dst) const {
99   uint32_t Scale = 1;
100   uint32_t D = getSumForBlock(Src, Scale);
101   uint32_t N = getEdgeWeight(Src, Dst) / Scale;
102 
103   return BranchProbability(N, D);
104 }
105 
106 raw_ostream &MachineBranchProbabilityInfo::
107 printEdgeProbability(raw_ostream &OS, MachineBasicBlock *Src,
108                      MachineBasicBlock *Dst) const {
109 
110   const BranchProbability Prob = getEdgeProbability(Src, Dst);
111   OS << "edge MBB#" << Src->getNumber() << " -> MBB#" << Dst->getNumber()
112      << " probability is "  << Prob
113      << (isEdgeHot(Src, Dst) ? " [HOT edge]\n" : "\n");
114 
115   return OS;
116 }
117