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1 //===- BranchProbability.h - Branch Probability Wrapper ---------*- C++ -*-===//
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2 //
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3 // The LLVM Compiler Infrastructure
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4 //
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5 // This file is distributed under the University of Illinois Open Source
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6 // License. See LICENSE.TXT for details.
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7 //
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8 //===----------------------------------------------------------------------===//
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9 //
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10 // Definition of BranchProbability shared by IR and Machine Instructions.
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11 //
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12 //===----------------------------------------------------------------------===//
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13
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14 #ifndef LLVM_SUPPORT_BRANCHPROBABILITY_H
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15 #define LLVM_SUPPORT_BRANCHPROBABILITY_H
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16
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17 #include "llvm/Support/DataTypes.h"
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18 #include <algorithm>
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19 #include <cassert>
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100
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20 #include <climits>
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21 #include <numeric>
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22
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23 namespace llvm {
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24
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25 class raw_ostream;
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26
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95
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27 // This class represents Branch Probability as a non-negative fraction that is
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28 // no greater than 1. It uses a fixed-point-like implementation, in which the
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29 // denominator is always a constant value (here we use 1<<31 for maximum
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30 // precision).
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31 class BranchProbability {
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32 // Numerator
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33 uint32_t N;
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34
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35 // Denominator, which is a constant value.
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36 static const uint32_t D = 1u << 31;
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37 static const uint32_t UnknownN = UINT32_MAX;
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38
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39 // Construct a BranchProbability with only numerator assuming the denominator
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40 // is 1<<31. For internal use only.
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41 explicit BranchProbability(uint32_t n) : N(n) {}
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42
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43 public:
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44 BranchProbability() : N(UnknownN) {}
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45 BranchProbability(uint32_t Numerator, uint32_t Denominator);
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46
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47 bool isZero() const { return N == 0; }
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48 bool isUnknown() const { return N == UnknownN; }
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49
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50 static BranchProbability getZero() { return BranchProbability(0); }
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51 static BranchProbability getOne() { return BranchProbability(D); }
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52 static BranchProbability getUnknown() { return BranchProbability(UnknownN); }
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53 // Create a BranchProbability object with the given numerator and 1<<31
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54 // as denominator.
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55 static BranchProbability getRaw(uint32_t N) { return BranchProbability(N); }
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56 // Create a BranchProbability object from 64-bit integers.
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57 static BranchProbability getBranchProbability(uint64_t Numerator,
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58 uint64_t Denominator);
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59
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60 // Normalize given probabilties so that the sum of them becomes approximate
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61 // one.
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62 template <class ProbabilityIter>
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63 static void normalizeProbabilities(ProbabilityIter Begin,
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64 ProbabilityIter End);
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65
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66 uint32_t getNumerator() const { return N; }
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67 static uint32_t getDenominator() { return D; }
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68
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69 // Return (1 - Probability).
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70 BranchProbability getCompl() const { return BranchProbability(D - N); }
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71
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72 raw_ostream &print(raw_ostream &OS) const;
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73
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74 void dump() const;
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75
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77
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76 /// \brief Scale a large integer.
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77 ///
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78 /// Scales \c Num. Guarantees full precision. Returns the floor of the
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79 /// result.
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80 ///
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81 /// \return \c Num times \c this.
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82 uint64_t scale(uint64_t Num) const;
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83
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84 /// \brief Scale a large integer by the inverse.
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85 ///
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86 /// Scales \c Num by the inverse of \c this. Guarantees full precision.
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87 /// Returns the floor of the result.
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88 ///
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89 /// \return \c Num divided by \c this.
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90 uint64_t scaleByInverse(uint64_t Num) const;
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91
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92 BranchProbability &operator+=(BranchProbability RHS) {
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93 assert(N != UnknownN && RHS.N != UnknownN &&
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94 "Unknown probability cannot participate in arithmetics.");
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95 // Saturate the result in case of overflow.
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96 N = (uint64_t(N) + RHS.N > D) ? D : N + RHS.N;
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95
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97 return *this;
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98 }
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99
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100 BranchProbability &operator-=(BranchProbability RHS) {
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100
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101 assert(N != UnknownN && RHS.N != UnknownN &&
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102 "Unknown probability cannot participate in arithmetics.");
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103 // Saturate the result in case of underflow.
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104 N = N < RHS.N ? 0 : N - RHS.N;
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105 return *this;
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106 }
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107
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108 BranchProbability &operator*=(BranchProbability RHS) {
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109 assert(N != UnknownN && RHS.N != UnknownN &&
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110 "Unknown probability cannot participate in arithmetics.");
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111 N = (static_cast<uint64_t>(N) * RHS.N + D / 2) / D;
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112 return *this;
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113 }
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114
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115 BranchProbability &operator*=(uint32_t RHS) {
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116 assert(N != UnknownN &&
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117 "Unknown probability cannot participate in arithmetics.");
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118 N = (uint64_t(N) * RHS > D) ? D : N * RHS;
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119 return *this;
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120 }
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121
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122 BranchProbability &operator/=(uint32_t RHS) {
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123 assert(N != UnknownN &&
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124 "Unknown probability cannot participate in arithmetics.");
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125 assert(RHS > 0 && "The divider cannot be zero.");
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126 N /= RHS;
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127 return *this;
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128 }
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129
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130 BranchProbability operator+(BranchProbability RHS) const {
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131 BranchProbability Prob(*this);
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132 return Prob += RHS;
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133 }
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134
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135 BranchProbability operator-(BranchProbability RHS) const {
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136 BranchProbability Prob(*this);
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137 return Prob -= RHS;
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138 }
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139
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140 BranchProbability operator*(BranchProbability RHS) const {
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141 BranchProbability Prob(*this);
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142 return Prob *= RHS;
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143 }
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144
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145 BranchProbability operator*(uint32_t RHS) const {
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146 BranchProbability Prob(*this);
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147 return Prob *= RHS;
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148 }
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149
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150 BranchProbability operator/(uint32_t RHS) const {
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151 BranchProbability Prob(*this);
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152 return Prob /= RHS;
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153 }
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154
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155 bool operator==(BranchProbability RHS) const { return N == RHS.N; }
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156 bool operator!=(BranchProbability RHS) const { return !(*this == RHS); }
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157
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158 bool operator<(BranchProbability RHS) const {
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159 assert(N != UnknownN && RHS.N != UnknownN &&
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160 "Unknown probability cannot participate in comparisons.");
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161 return N < RHS.N;
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162 }
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163
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164 bool operator>(BranchProbability RHS) const {
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165 assert(N != UnknownN && RHS.N != UnknownN &&
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166 "Unknown probability cannot participate in comparisons.");
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167 return RHS < *this;
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168 }
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169
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170 bool operator<=(BranchProbability RHS) const {
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171 assert(N != UnknownN && RHS.N != UnknownN &&
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172 "Unknown probability cannot participate in comparisons.");
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173 return !(RHS < *this);
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174 }
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175
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176 bool operator>=(BranchProbability RHS) const {
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177 assert(N != UnknownN && RHS.N != UnknownN &&
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178 "Unknown probability cannot participate in comparisons.");
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179 return !(*this < RHS);
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180 }
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181 };
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182
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183 inline raw_ostream &operator<<(raw_ostream &OS, BranchProbability Prob) {
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184 return Prob.print(OS);
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185 }
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186
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187 template <class ProbabilityIter>
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188 void BranchProbability::normalizeProbabilities(ProbabilityIter Begin,
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189 ProbabilityIter End) {
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190 if (Begin == End)
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191 return;
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192
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193 unsigned UnknownProbCount = 0;
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194 uint64_t Sum = std::accumulate(Begin, End, uint64_t(0),
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195 [&](uint64_t S, const BranchProbability &BP) {
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196 if (!BP.isUnknown())
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197 return S + BP.N;
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198 UnknownProbCount++;
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199 return S;
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200 });
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201
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202 if (UnknownProbCount > 0) {
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203 BranchProbability ProbForUnknown = BranchProbability::getZero();
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204 // If the sum of all known probabilities is less than one, evenly distribute
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205 // the complement of sum to unknown probabilities. Otherwise, set unknown
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206 // probabilities to zeros and continue to normalize known probabilities.
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207 if (Sum < BranchProbability::getDenominator())
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208 ProbForUnknown = BranchProbability::getRaw(
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209 (BranchProbability::getDenominator() - Sum) / UnknownProbCount);
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210
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211 std::replace_if(Begin, End,
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212 [](const BranchProbability &BP) { return BP.isUnknown(); },
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213 ProbForUnknown);
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214
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215 if (Sum <= BranchProbability::getDenominator())
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216 return;
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217 }
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218
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219 if (Sum == 0) {
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220 BranchProbability BP(1, std::distance(Begin, End));
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221 std::fill(Begin, End, BP);
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222 return;
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223 }
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224
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225 for (auto I = Begin; I != End; ++I)
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226 I->N = (I->N * uint64_t(D) + Sum / 2) / Sum;
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227 }
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228
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229 }
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230
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231 #endif
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