include/llvm/Support/BranchProbability.h

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