1 //===- BranchProbability.h - Branch Probability Wrapper ---------*- C++ -*-===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // Definition of BranchProbability shared by IR and Machine Instructions.
12 //===----------------------------------------------------------------------===//
14 #ifndef LLVM_SUPPORT_BRANCHPROBABILITY_H
15 #define LLVM_SUPPORT_BRANCHPROBABILITY_H
17 #include "llvm/Support/DataTypes.h"
27 // This class represents Branch Probability as a non-negative fraction that is
28 // no greater than 1. It uses a fixed-point-like implementation, in which the
29 // denominator is always a constant value (here we use 1<<31 for maximum
31 class BranchProbability {
35 // Denominator, which is a constant value.
36 static const uint32_t D = 1u << 31;
37 static const uint32_t UnknownN = UINT32_MAX;
39 // Construct a BranchProbability with only numerator assuming the denominator
40 // is 1<<31. For internal use only.
41 explicit BranchProbability(uint32_t n) : N(n) {}
44 BranchProbability() : N(0) {}
45 BranchProbability(uint32_t Numerator, uint32_t Denominator);
47 bool isZero() const { return N == 0; }
48 bool isUnknown() const { return N == UnknownN; }
50 static BranchProbability getZero() { return BranchProbability(0); }
51 static BranchProbability getOne() { return BranchProbability(D); }
52 static BranchProbability getUnknown() { return BranchProbability(UnknownN); }
53 // Create a BranchProbability object with the given numerator and 1<<31
55 static BranchProbability getRaw(uint32_t N) { return BranchProbability(N); }
57 // Normalize given probabilties so that the sum of them becomes approximate
59 template <class ProbabilityList>
60 static void normalizeProbabilities(ProbabilityList &Probs);
62 // Normalize a list of weights by scaling them down so that the sum of them
63 // doesn't exceed UINT32_MAX.
64 template <class WeightListIter>
65 static void normalizeEdgeWeights(WeightListIter Begin, WeightListIter End);
67 uint32_t getNumerator() const { return N; }
68 static uint32_t getDenominator() { return D; }
70 // Return (1 - Probability).
71 BranchProbability getCompl() const { return BranchProbability(D - N); }
73 raw_ostream &print(raw_ostream &OS) const;
77 /// \brief Scale a large integer.
79 /// Scales \c Num. Guarantees full precision. Returns the floor of the
82 /// \return \c Num times \c this.
83 uint64_t scale(uint64_t Num) const;
85 /// \brief Scale a large integer by the inverse.
87 /// Scales \c Num by the inverse of \c this. Guarantees full precision.
88 /// Returns the floor of the result.
90 /// \return \c Num divided by \c this.
91 uint64_t scaleByInverse(uint64_t Num) const;
93 BranchProbability &operator+=(BranchProbability RHS) {
94 assert(N <= D - RHS.N &&
95 "The sum of branch probabilities should not exceed one!");
100 BranchProbability &operator-=(BranchProbability RHS) {
102 "Can only subtract a smaller probability from a larger one!");
107 BranchProbability &operator*=(BranchProbability RHS) {
108 N = (static_cast<uint64_t>(N) * RHS.N + D / 2) / D;
112 BranchProbability operator+(BranchProbability RHS) const {
113 BranchProbability Prob(*this);
117 BranchProbability operator-(BranchProbability RHS) const {
118 BranchProbability Prob(*this);
122 BranchProbability operator*(BranchProbability RHS) const {
123 BranchProbability Prob(*this);
127 bool operator==(BranchProbability RHS) const { return N == RHS.N; }
128 bool operator!=(BranchProbability RHS) const { return !(*this == RHS); }
129 bool operator<(BranchProbability RHS) const { return N < RHS.N; }
130 bool operator>(BranchProbability RHS) const { return RHS < *this; }
131 bool operator<=(BranchProbability RHS) const { return !(RHS < *this); }
132 bool operator>=(BranchProbability RHS) const { return !(*this < RHS); }
135 inline raw_ostream &operator<<(raw_ostream &OS, BranchProbability Prob) {
136 return Prob.print(OS);
139 inline BranchProbability operator/(BranchProbability LHS, uint32_t RHS) {
140 return BranchProbability::getRaw(LHS.getNumerator() / RHS);
143 template <class ProbabilityList>
144 void BranchProbability::normalizeProbabilities(ProbabilityList &Probs) {
146 for (auto Prob : Probs)
149 for (auto &Prob : Probs)
150 Prob.N = (Prob.N * uint64_t(D) + Sum / 2) / Sum;
153 template <class WeightListIter>
154 void BranchProbability::normalizeEdgeWeights(WeightListIter Begin,
155 WeightListIter End) {
156 // First we compute the sum with 64-bits of precision.
157 uint64_t Sum = std::accumulate(Begin, End, uint64_t(0));
159 if (Sum > UINT32_MAX) {
160 // Compute the scale necessary to cause the weights to fit, and re-sum with
161 // that scale applied.
162 assert(Sum / UINT32_MAX < UINT32_MAX &&
163 "The sum of weights exceeds UINT32_MAX^2!");
164 uint32_t Scale = Sum / UINT32_MAX + 1;
165 for (auto I = Begin; I != End; ++I)
167 Sum = std::accumulate(Begin, End, uint64_t(0));
170 // Eliminate zero weights.
171 auto ZeroWeightNum = std::count(Begin, End, 0u);
172 if (ZeroWeightNum > 0) {
173 // If all weights are zeros, replace them by 1.
175 std::fill(Begin, End, 1u);
177 // We are converting zeros into ones, and here we need to make sure that
178 // after this the sum won't exceed UINT32_MAX.
179 if (Sum + ZeroWeightNum > UINT32_MAX) {
180 for (auto I = Begin; I != End; ++I)
182 ZeroWeightNum = std::count(Begin, End, 0u);
183 Sum = std::accumulate(Begin, End, uint64_t(0));
185 // Scale up non-zero weights and turn zero weights into ones.
186 uint64_t ScalingFactor = (UINT32_MAX - ZeroWeightNum) / Sum;
187 assert(ScalingFactor >= 1);
188 if (ScalingFactor > 1)
189 for (auto I = Begin; I != End; ++I)
191 std::replace(Begin, End, 0u, 1u);