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(UnknownN) {}
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); }
56 // Create a BranchProbability object from 64-bit integers.
57 static BranchProbability getBranchProbability(uint64_t Numerator,
58 uint64_t Denominator);
60 // Normalize given probabilties so that the sum of them becomes approximate
62 template <class ProbabilityIter>
63 static void normalizeProbabilities(ProbabilityIter Begin,
66 uint32_t getNumerator() const { return N; }
67 static uint32_t getDenominator() { return D; }
69 // Return (1 - Probability).
70 BranchProbability getCompl() const { return BranchProbability(D - N); }
72 raw_ostream &print(raw_ostream &OS) const;
76 /// \brief Scale a large integer.
78 /// Scales \c Num. Guarantees full precision. Returns the floor of the
81 /// \return \c Num times \c this.
82 uint64_t scale(uint64_t Num) const;
84 /// \brief Scale a large integer by the inverse.
86 /// Scales \c Num by the inverse of \c this. Guarantees full precision.
87 /// Returns the floor of the result.
89 /// \return \c Num divided by \c this.
90 uint64_t scaleByInverse(uint64_t Num) const;
92 BranchProbability &operator+=(BranchProbability RHS) {
93 assert(N != UnknownN && RHS.N != UnknownN &&
94 "Unknown probability cannot participate in arithmetics.");
95 // Saturate the result in case of overflow.
96 N = (uint64_t(N) + RHS.N > D) ? D : N + RHS.N;
100 BranchProbability &operator-=(BranchProbability RHS) {
101 assert(N != UnknownN && RHS.N != UnknownN &&
102 "Unknown probability cannot participate in arithmetics.");
103 // Saturate the result in case of underflow.
104 N = N < RHS.N ? 0 : N - RHS.N;
108 BranchProbability &operator*=(BranchProbability RHS) {
109 assert(N != UnknownN && RHS.N != UnknownN &&
110 "Unknown probability cannot participate in arithmetics.");
111 N = (static_cast<uint64_t>(N) * RHS.N + D / 2) / D;
115 BranchProbability &operator/=(uint32_t RHS) {
116 assert(N != UnknownN &&
117 "Unknown probability cannot participate in arithmetics.");
118 assert(RHS > 0 && "The divider cannot be zero.");
123 BranchProbability operator+(BranchProbability RHS) const {
124 BranchProbability Prob(*this);
128 BranchProbability operator-(BranchProbability RHS) const {
129 BranchProbability Prob(*this);
133 BranchProbability operator*(BranchProbability RHS) const {
134 BranchProbability Prob(*this);
138 BranchProbability operator/(uint32_t RHS) const {
139 BranchProbability Prob(*this);
143 bool operator==(BranchProbability RHS) const { return N == RHS.N; }
144 bool operator!=(BranchProbability RHS) const { return !(*this == RHS); }
146 bool operator<(BranchProbability RHS) const {
147 assert(N != UnknownN && RHS.N != UnknownN &&
148 "Unknown probability cannot participate in comparisons.");
152 bool operator>(BranchProbability RHS) const {
153 assert(N != UnknownN && RHS.N != UnknownN &&
154 "Unknown probability cannot participate in comparisons.");
158 bool operator<=(BranchProbability RHS) const {
159 assert(N != UnknownN && RHS.N != UnknownN &&
160 "Unknown probability cannot participate in comparisons.");
161 return !(RHS < *this);
164 bool operator>=(BranchProbability RHS) const {
165 assert(N != UnknownN && RHS.N != UnknownN &&
166 "Unknown probability cannot participate in comparisons.");
167 return !(*this < RHS);
171 inline raw_ostream &operator<<(raw_ostream &OS, BranchProbability Prob) {
172 return Prob.print(OS);
175 template <class ProbabilityIter>
176 void BranchProbability::normalizeProbabilities(ProbabilityIter Begin,
177 ProbabilityIter End) {
181 unsigned UnknownProbCount = 0;
182 uint64_t Sum = std::accumulate(Begin, End, uint64_t(0),
183 [&](uint64_t S, const BranchProbability &BP) {
190 if (UnknownProbCount > 0) {
191 BranchProbability ProbForUnknown = BranchProbability::getZero();
192 // If the sum of all known probabilities is less than one, evenly distribute
193 // the complement of sum to unknown probabilities. Otherwise, set unknown
194 // probabilities to zeros and continue to normalize known probabilities.
195 if (Sum < BranchProbability::getDenominator())
196 ProbForUnknown = BranchProbability::getRaw(
197 (BranchProbability::getDenominator() - Sum) / UnknownProbCount);
199 std::replace_if(Begin, End,
200 [](const BranchProbability &BP) { return BP.isUnknown(); },
203 if (Sum <= BranchProbability::getDenominator())
208 BranchProbability BP(1, std::distance(Begin, End));
209 std::fill(Begin, End, BP);
213 for (auto I = Begin; I != End; ++I)
214 I->N = (I->N * uint64_t(D) + Sum / 2) / Sum;