1 //===--- BranchProbabilityInfo.h - Branch Probability Analysis --*- C++ -*-===// 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 pass is used to evaluate branch probabilties. 11 // 12 //===----------------------------------------------------------------------===// 13 14 #ifndef LLVM_ANALYSIS_BRANCHPROBABILITYINFO_H 15 #define LLVM_ANALYSIS_BRANCHPROBABILITYINFO_H 16 17 #include "llvm/ADT/DenseMap.h" 18 #include "llvm/ADT/SmallPtrSet.h" 19 #include "llvm/IR/CFG.h" 20 #include "llvm/IR/PassManager.h" 21 #include "llvm/InitializePasses.h" 22 #include "llvm/Pass.h" 23 #include "llvm/Support/BranchProbability.h" 24 25 namespace llvm { 26 class LoopInfo; 27 class raw_ostream; 28 29 /// \brief Analysis providing branch probability information. 30 /// 31 /// This is a function analysis which provides information on the relative 32 /// probabilities of each "edge" in the function's CFG where such an edge is 33 /// defined by a pair (PredBlock and an index in the successors). The 34 /// probability of an edge from one block is always relative to the 35 /// probabilities of other edges from the block. The probabilites of all edges 36 /// from a block sum to exactly one (100%). 37 /// We use a pair (PredBlock and an index in the successors) to uniquely 38 /// identify an edge, since we can have multiple edges from Src to Dst. 39 /// As an example, we can have a switch which jumps to Dst with value 0 and 40 /// value 10. 41 class BranchProbabilityInfo { 42 public: BranchProbabilityInfo()43 BranchProbabilityInfo() {} BranchProbabilityInfo(const Function & F,const LoopInfo & LI)44 BranchProbabilityInfo(const Function &F, const LoopInfo &LI) { 45 calculate(F, LI); 46 } 47 BranchProbabilityInfo(BranchProbabilityInfo && Arg)48 BranchProbabilityInfo(BranchProbabilityInfo &&Arg) 49 : Probs(std::move(Arg.Probs)), LastF(Arg.LastF), 50 PostDominatedByUnreachable(std::move(Arg.PostDominatedByUnreachable)), 51 PostDominatedByColdCall(std::move(Arg.PostDominatedByColdCall)) {} 52 53 BranchProbabilityInfo &operator=(BranchProbabilityInfo &&RHS) { 54 releaseMemory(); 55 Probs = std::move(RHS.Probs); 56 PostDominatedByColdCall = std::move(RHS.PostDominatedByColdCall); 57 PostDominatedByUnreachable = std::move(RHS.PostDominatedByUnreachable); 58 return *this; 59 } 60 61 void releaseMemory(); 62 63 void print(raw_ostream &OS) const; 64 65 /// \brief Get an edge's probability, relative to other out-edges of the Src. 66 /// 67 /// This routine provides access to the fractional probability between zero 68 /// (0%) and one (100%) of this edge executing, relative to other edges 69 /// leaving the 'Src' block. The returned probability is never zero, and can 70 /// only be one if the source block has only one successor. 71 BranchProbability getEdgeProbability(const BasicBlock *Src, 72 unsigned IndexInSuccessors) const; 73 74 /// \brief Get the probability of going from Src to Dst. 75 /// 76 /// It returns the sum of all probabilities for edges from Src to Dst. 77 BranchProbability getEdgeProbability(const BasicBlock *Src, 78 const BasicBlock *Dst) const; 79 80 BranchProbability getEdgeProbability(const BasicBlock *Src, 81 succ_const_iterator Dst) const; 82 83 /// \brief Test if an edge is hot relative to other out-edges of the Src. 84 /// 85 /// Check whether this edge out of the source block is 'hot'. We define hot 86 /// as having a relative probability >= 80%. 87 bool isEdgeHot(const BasicBlock *Src, const BasicBlock *Dst) const; 88 89 /// \brief Retrieve the hot successor of a block if one exists. 90 /// 91 /// Given a basic block, look through its successors and if one exists for 92 /// which \see isEdgeHot would return true, return that successor block. 93 const BasicBlock *getHotSucc(const BasicBlock *BB) const; 94 95 /// \brief Print an edge's probability. 96 /// 97 /// Retrieves an edge's probability similarly to \see getEdgeProbability, but 98 /// then prints that probability to the provided stream. That stream is then 99 /// returned. 100 raw_ostream &printEdgeProbability(raw_ostream &OS, const BasicBlock *Src, 101 const BasicBlock *Dst) const; 102 103 /// \brief Set the raw edge probability for the given edge. 104 /// 105 /// This allows a pass to explicitly set the edge probability for an edge. It 106 /// can be used when updating the CFG to update and preserve the branch 107 /// probability information. Read the implementation of how these edge 108 /// probabilities are calculated carefully before using! 109 void setEdgeProbability(const BasicBlock *Src, unsigned IndexInSuccessors, 110 BranchProbability Prob); 111 getBranchProbStackProtector(bool IsLikely)112 static BranchProbability getBranchProbStackProtector(bool IsLikely) { 113 static const BranchProbability LikelyProb((1u << 20) - 1, 1u << 20); 114 return IsLikely ? LikelyProb : LikelyProb.getCompl(); 115 } 116 117 void calculate(const Function &F, const LoopInfo &LI); 118 119 private: 120 void operator=(const BranchProbabilityInfo &) = delete; 121 BranchProbabilityInfo(const BranchProbabilityInfo &) = delete; 122 123 // Since we allow duplicate edges from one basic block to another, we use 124 // a pair (PredBlock and an index in the successors) to specify an edge. 125 typedef std::pair<const BasicBlock *, unsigned> Edge; 126 127 // Default weight value. Used when we don't have information about the edge. 128 // TODO: DEFAULT_WEIGHT makes sense during static predication, when none of 129 // the successors have a weight yet. But it doesn't make sense when providing 130 // weight to an edge that may have siblings with non-zero weights. This can 131 // be handled various ways, but it's probably fine for an edge with unknown 132 // weight to just "inherit" the non-zero weight of an adjacent successor. 133 static const uint32_t DEFAULT_WEIGHT = 16; 134 135 DenseMap<Edge, BranchProbability> Probs; 136 137 /// \brief Track the last function we run over for printing. 138 const Function *LastF; 139 140 /// \brief Track the set of blocks directly succeeded by a returning block. 141 SmallPtrSet<const BasicBlock *, 16> PostDominatedByUnreachable; 142 143 /// \brief Track the set of blocks that always lead to a cold call. 144 SmallPtrSet<const BasicBlock *, 16> PostDominatedByColdCall; 145 146 bool calcUnreachableHeuristics(const BasicBlock *BB); 147 bool calcMetadataWeights(const BasicBlock *BB); 148 bool calcColdCallHeuristics(const BasicBlock *BB); 149 bool calcPointerHeuristics(const BasicBlock *BB); 150 bool calcLoopBranchHeuristics(const BasicBlock *BB, const LoopInfo &LI); 151 bool calcZeroHeuristics(const BasicBlock *BB); 152 bool calcFloatingPointHeuristics(const BasicBlock *BB); 153 bool calcInvokeHeuristics(const BasicBlock *BB); 154 }; 155 156 /// \brief Analysis pass which computes \c BranchProbabilityInfo. 157 class BranchProbabilityAnalysis 158 : public AnalysisInfoMixin<BranchProbabilityAnalysis> { 159 friend AnalysisInfoMixin<BranchProbabilityAnalysis>; 160 static char PassID; 161 162 public: 163 /// \brief Provide the result typedef for this analysis pass. 164 typedef BranchProbabilityInfo Result; 165 166 /// \brief Run the analysis pass over a function and produce BPI. 167 BranchProbabilityInfo run(Function &F, AnalysisManager<Function> &AM); 168 }; 169 170 /// \brief Printer pass for the \c BranchProbabilityAnalysis results. 171 class BranchProbabilityPrinterPass 172 : public PassInfoMixin<BranchProbabilityPrinterPass> { 173 raw_ostream &OS; 174 175 public: BranchProbabilityPrinterPass(raw_ostream & OS)176 explicit BranchProbabilityPrinterPass(raw_ostream &OS) : OS(OS) {} 177 PreservedAnalyses run(Function &F, AnalysisManager<Function> &AM); 178 }; 179 180 /// \brief Legacy analysis pass which computes \c BranchProbabilityInfo. 181 class BranchProbabilityInfoWrapperPass : public FunctionPass { 182 BranchProbabilityInfo BPI; 183 184 public: 185 static char ID; 186 BranchProbabilityInfoWrapperPass()187 BranchProbabilityInfoWrapperPass() : FunctionPass(ID) { 188 initializeBranchProbabilityInfoWrapperPassPass( 189 *PassRegistry::getPassRegistry()); 190 } 191 getBPI()192 BranchProbabilityInfo &getBPI() { return BPI; } getBPI()193 const BranchProbabilityInfo &getBPI() const { return BPI; } 194 195 void getAnalysisUsage(AnalysisUsage &AU) const override; 196 bool runOnFunction(Function &F) override; 197 void releaseMemory() override; 198 void print(raw_ostream &OS, const Module *M = nullptr) const override; 199 }; 200 201 } 202 203 #endif 204