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/DenseSet.h" 19 #include "llvm/ADT/SmallPtrSet.h" 20 #include "llvm/IR/CFG.h" 21 #include "llvm/IR/PassManager.h" 22 #include "llvm/IR/ValueHandle.h" 23 #include "llvm/InitializePasses.h" 24 #include "llvm/Pass.h" 25 #include "llvm/Support/BranchProbability.h" 26 27 namespace llvm { 28 class LoopInfo; 29 class raw_ostream; 30 31 /// \brief Analysis providing branch probability information. 32 /// 33 /// This is a function analysis which provides information on the relative 34 /// probabilities of each "edge" in the function's CFG where such an edge is 35 /// defined by a pair (PredBlock and an index in the successors). The 36 /// probability of an edge from one block is always relative to the 37 /// probabilities of other edges from the block. The probabilites of all edges 38 /// from a block sum to exactly one (100%). 39 /// We use a pair (PredBlock and an index in the successors) to uniquely 40 /// identify an edge, since we can have multiple edges from Src to Dst. 41 /// As an example, we can have a switch which jumps to Dst with value 0 and 42 /// value 10. 43 class BranchProbabilityInfo { 44 public: 45 BranchProbabilityInfo() {} 46 BranchProbabilityInfo(const Function &F, const LoopInfo &LI) { 47 calculate(F, LI); 48 } 49 50 BranchProbabilityInfo(BranchProbabilityInfo &&Arg) 51 : Probs(std::move(Arg.Probs)), LastF(Arg.LastF), 52 PostDominatedByUnreachable(std::move(Arg.PostDominatedByUnreachable)), 53 PostDominatedByColdCall(std::move(Arg.PostDominatedByColdCall)) {} 54 55 BranchProbabilityInfo &operator=(BranchProbabilityInfo &&RHS) { 56 releaseMemory(); 57 Probs = std::move(RHS.Probs); 58 PostDominatedByColdCall = std::move(RHS.PostDominatedByColdCall); 59 PostDominatedByUnreachable = std::move(RHS.PostDominatedByUnreachable); 60 return *this; 61 } 62 63 void releaseMemory(); 64 65 void print(raw_ostream &OS) const; 66 67 /// \brief Get an edge's probability, relative to other out-edges of the Src. 68 /// 69 /// This routine provides access to the fractional probability between zero 70 /// (0%) and one (100%) of this edge executing, relative to other edges 71 /// leaving the 'Src' block. The returned probability is never zero, and can 72 /// only be one if the source block has only one successor. 73 BranchProbability getEdgeProbability(const BasicBlock *Src, 74 unsigned IndexInSuccessors) const; 75 76 /// \brief Get the probability of going from Src to Dst. 77 /// 78 /// It returns the sum of all probabilities for edges from Src to Dst. 79 BranchProbability getEdgeProbability(const BasicBlock *Src, 80 const BasicBlock *Dst) const; 81 82 BranchProbability getEdgeProbability(const BasicBlock *Src, 83 succ_const_iterator Dst) const; 84 85 /// \brief Test if an edge is hot relative to other out-edges of the Src. 86 /// 87 /// Check whether this edge out of the source block is 'hot'. We define hot 88 /// as having a relative probability >= 80%. 89 bool isEdgeHot(const BasicBlock *Src, const BasicBlock *Dst) const; 90 91 /// \brief Retrieve the hot successor of a block if one exists. 92 /// 93 /// Given a basic block, look through its successors and if one exists for 94 /// which \see isEdgeHot would return true, return that successor block. 95 const BasicBlock *getHotSucc(const BasicBlock *BB) const; 96 97 /// \brief Print an edge's probability. 98 /// 99 /// Retrieves an edge's probability similarly to \see getEdgeProbability, but 100 /// then prints that probability to the provided stream. That stream is then 101 /// returned. 102 raw_ostream &printEdgeProbability(raw_ostream &OS, const BasicBlock *Src, 103 const BasicBlock *Dst) const; 104 105 /// \brief Set the raw edge probability for the given edge. 106 /// 107 /// This allows a pass to explicitly set the edge probability for an edge. It 108 /// can be used when updating the CFG to update and preserve the branch 109 /// probability information. Read the implementation of how these edge 110 /// probabilities are calculated carefully before using! 111 void setEdgeProbability(const BasicBlock *Src, unsigned IndexInSuccessors, 112 BranchProbability Prob); 113 114 static BranchProbability getBranchProbStackProtector(bool IsLikely) { 115 static const BranchProbability LikelyProb((1u << 20) - 1, 1u << 20); 116 return IsLikely ? LikelyProb : LikelyProb.getCompl(); 117 } 118 119 void calculate(const Function &F, const LoopInfo &LI); 120 121 /// Forget analysis results for the given basic block. 122 void eraseBlock(const BasicBlock *BB); 123 124 private: 125 void operator=(const BranchProbabilityInfo &) = delete; 126 BranchProbabilityInfo(const BranchProbabilityInfo &) = delete; 127 128 // We need to store CallbackVH's in order to correctly handle basic block 129 // removal. 130 class BasicBlockCallbackVH final : public CallbackVH { 131 BranchProbabilityInfo *BPI; 132 void deleted() override { 133 assert(BPI != nullptr); 134 BPI->eraseBlock(cast<BasicBlock>(getValPtr())); 135 BPI->Handles.erase(*this); 136 } 137 138 public: 139 BasicBlockCallbackVH(const Value *V, BranchProbabilityInfo *BPI=nullptr) 140 : CallbackVH(const_cast<Value *>(V)), BPI(BPI) {} 141 }; 142 DenseSet<BasicBlockCallbackVH, DenseMapInfo<Value*>> Handles; 143 144 // Since we allow duplicate edges from one basic block to another, we use 145 // a pair (PredBlock and an index in the successors) to specify an edge. 146 typedef std::pair<const BasicBlock *, unsigned> Edge; 147 148 // Default weight value. Used when we don't have information about the edge. 149 // TODO: DEFAULT_WEIGHT makes sense during static predication, when none of 150 // the successors have a weight yet. But it doesn't make sense when providing 151 // weight to an edge that may have siblings with non-zero weights. This can 152 // be handled various ways, but it's probably fine for an edge with unknown 153 // weight to just "inherit" the non-zero weight of an adjacent successor. 154 static const uint32_t DEFAULT_WEIGHT = 16; 155 156 DenseMap<Edge, BranchProbability> Probs; 157 158 /// \brief Track the last function we run over for printing. 159 const Function *LastF; 160 161 /// \brief Track the set of blocks directly succeeded by a returning block. 162 SmallPtrSet<const BasicBlock *, 16> PostDominatedByUnreachable; 163 164 /// \brief Track the set of blocks that always lead to a cold call. 165 SmallPtrSet<const BasicBlock *, 16> PostDominatedByColdCall; 166 167 void updatePostDominatedByUnreachable(const BasicBlock *BB); 168 void updatePostDominatedByColdCall(const BasicBlock *BB); 169 bool calcUnreachableHeuristics(const BasicBlock *BB); 170 bool calcMetadataWeights(const BasicBlock *BB); 171 bool calcColdCallHeuristics(const BasicBlock *BB); 172 bool calcPointerHeuristics(const BasicBlock *BB); 173 bool calcLoopBranchHeuristics(const BasicBlock *BB, const LoopInfo &LI); 174 bool calcZeroHeuristics(const BasicBlock *BB); 175 bool calcFloatingPointHeuristics(const BasicBlock *BB); 176 bool calcInvokeHeuristics(const BasicBlock *BB); 177 }; 178 179 /// \brief Analysis pass which computes \c BranchProbabilityInfo. 180 class BranchProbabilityAnalysis 181 : public AnalysisInfoMixin<BranchProbabilityAnalysis> { 182 friend AnalysisInfoMixin<BranchProbabilityAnalysis>; 183 static AnalysisKey Key; 184 185 public: 186 /// \brief Provide the result typedef for this analysis pass. 187 typedef BranchProbabilityInfo Result; 188 189 /// \brief Run the analysis pass over a function and produce BPI. 190 BranchProbabilityInfo run(Function &F, FunctionAnalysisManager &AM); 191 }; 192 193 /// \brief Printer pass for the \c BranchProbabilityAnalysis results. 194 class BranchProbabilityPrinterPass 195 : public PassInfoMixin<BranchProbabilityPrinterPass> { 196 raw_ostream &OS; 197 198 public: 199 explicit BranchProbabilityPrinterPass(raw_ostream &OS) : OS(OS) {} 200 PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM); 201 }; 202 203 /// \brief Legacy analysis pass which computes \c BranchProbabilityInfo. 204 class BranchProbabilityInfoWrapperPass : public FunctionPass { 205 BranchProbabilityInfo BPI; 206 207 public: 208 static char ID; 209 210 BranchProbabilityInfoWrapperPass() : FunctionPass(ID) { 211 initializeBranchProbabilityInfoWrapperPassPass( 212 *PassRegistry::getPassRegistry()); 213 } 214 215 BranchProbabilityInfo &getBPI() { return BPI; } 216 const BranchProbabilityInfo &getBPI() const { return BPI; } 217 218 void getAnalysisUsage(AnalysisUsage &AU) const override; 219 bool runOnFunction(Function &F) override; 220 void releaseMemory() override; 221 void print(raw_ostream &OS, const Module *M = nullptr) const override; 222 }; 223 224 } 225 226 #endif 227
