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 TargetLibraryInfo; 30 class raw_ostream; 31 32 /// \brief Analysis providing branch probability information. 33 /// 34 /// This is a function analysis which provides information on the relative 35 /// probabilities of each "edge" in the function's CFG where such an edge is 36 /// defined by a pair (PredBlock and an index in the successors). The 37 /// probability of an edge from one block is always relative to the 38 /// probabilities of other edges from the block. The probabilites of all edges 39 /// from a block sum to exactly one (100%). 40 /// We use a pair (PredBlock and an index in the successors) to uniquely 41 /// identify an edge, since we can have multiple edges from Src to Dst. 42 /// As an example, we can have a switch which jumps to Dst with value 0 and 43 /// value 10. 44 class BranchProbabilityInfo { 45 public: 46 BranchProbabilityInfo() {} 47 BranchProbabilityInfo(const Function &F, const LoopInfo &LI, 48 const TargetLibraryInfo *TLI = nullptr) { 49 calculate(F, LI, TLI); 50 } 51 52 BranchProbabilityInfo(BranchProbabilityInfo &&Arg) 53 : Probs(std::move(Arg.Probs)), LastF(Arg.LastF), 54 PostDominatedByUnreachable(std::move(Arg.PostDominatedByUnreachable)), 55 PostDominatedByColdCall(std::move(Arg.PostDominatedByColdCall)) {} 56 57 BranchProbabilityInfo &operator=(BranchProbabilityInfo &&RHS) { 58 releaseMemory(); 59 Probs = std::move(RHS.Probs); 60 PostDominatedByColdCall = std::move(RHS.PostDominatedByColdCall); 61 PostDominatedByUnreachable = std::move(RHS.PostDominatedByUnreachable); 62 return *this; 63 } 64 65 void releaseMemory(); 66 67 void print(raw_ostream &OS) const; 68 69 /// \brief Get an edge's probability, relative to other out-edges of the Src. 70 /// 71 /// This routine provides access to the fractional probability between zero 72 /// (0%) and one (100%) of this edge executing, relative to other edges 73 /// leaving the 'Src' block. The returned probability is never zero, and can 74 /// only be one if the source block has only one successor. 75 BranchProbability getEdgeProbability(const BasicBlock *Src, 76 unsigned IndexInSuccessors) const; 77 78 /// \brief Get the probability of going from Src to Dst. 79 /// 80 /// It returns the sum of all probabilities for edges from Src to Dst. 81 BranchProbability getEdgeProbability(const BasicBlock *Src, 82 const BasicBlock *Dst) const; 83 84 BranchProbability getEdgeProbability(const BasicBlock *Src, 85 succ_const_iterator Dst) const; 86 87 /// \brief Test if an edge is hot relative to other out-edges of the Src. 88 /// 89 /// Check whether this edge out of the source block is 'hot'. We define hot 90 /// as having a relative probability >= 80%. 91 bool isEdgeHot(const BasicBlock *Src, const BasicBlock *Dst) const; 92 93 /// \brief Retrieve the hot successor of a block if one exists. 94 /// 95 /// Given a basic block, look through its successors and if one exists for 96 /// which \see isEdgeHot would return true, return that successor block. 97 const BasicBlock *getHotSucc(const BasicBlock *BB) const; 98 99 /// \brief Print an edge's probability. 100 /// 101 /// Retrieves an edge's probability similarly to \see getEdgeProbability, but 102 /// then prints that probability to the provided stream. That stream is then 103 /// returned. 104 raw_ostream &printEdgeProbability(raw_ostream &OS, const BasicBlock *Src, 105 const BasicBlock *Dst) const; 106 107 /// \brief Set the raw edge probability for the given edge. 108 /// 109 /// This allows a pass to explicitly set the edge probability for an edge. It 110 /// can be used when updating the CFG to update and preserve the branch 111 /// probability information. Read the implementation of how these edge 112 /// probabilities are calculated carefully before using! 113 void setEdgeProbability(const BasicBlock *Src, unsigned IndexInSuccessors, 114 BranchProbability Prob); 115 116 static BranchProbability getBranchProbStackProtector(bool IsLikely) { 117 static const BranchProbability LikelyProb((1u << 20) - 1, 1u << 20); 118 return IsLikely ? LikelyProb : LikelyProb.getCompl(); 119 } 120 121 void calculate(const Function &F, const LoopInfo &LI, 122 const TargetLibraryInfo *TLI = nullptr); 123 124 /// Forget analysis results for the given basic block. 125 void eraseBlock(const BasicBlock *BB); 126 127 private: 128 void operator=(const BranchProbabilityInfo &) = delete; 129 BranchProbabilityInfo(const BranchProbabilityInfo &) = delete; 130 131 // We need to store CallbackVH's in order to correctly handle basic block 132 // removal. 133 class BasicBlockCallbackVH final : public CallbackVH { 134 BranchProbabilityInfo *BPI; 135 void deleted() override { 136 assert(BPI != nullptr); 137 BPI->eraseBlock(cast<BasicBlock>(getValPtr())); 138 BPI->Handles.erase(*this); 139 } 140 141 public: 142 BasicBlockCallbackVH(const Value *V, BranchProbabilityInfo *BPI=nullptr) 143 : CallbackVH(const_cast<Value *>(V)), BPI(BPI) {} 144 }; 145 DenseSet<BasicBlockCallbackVH, DenseMapInfo<Value*>> Handles; 146 147 // Since we allow duplicate edges from one basic block to another, we use 148 // a pair (PredBlock and an index in the successors) to specify an edge. 149 typedef std::pair<const BasicBlock *, unsigned> Edge; 150 151 // Default weight value. Used when we don't have information about the edge. 152 // TODO: DEFAULT_WEIGHT makes sense during static predication, when none of 153 // the successors have a weight yet. But it doesn't make sense when providing 154 // weight to an edge that may have siblings with non-zero weights. This can 155 // be handled various ways, but it's probably fine for an edge with unknown 156 // weight to just "inherit" the non-zero weight of an adjacent successor. 157 static const uint32_t DEFAULT_WEIGHT = 16; 158 159 DenseMap<Edge, BranchProbability> Probs; 160 161 /// \brief Track the last function we run over for printing. 162 const Function *LastF; 163 164 /// \brief Track the set of blocks directly succeeded by a returning block. 165 SmallPtrSet<const BasicBlock *, 16> PostDominatedByUnreachable; 166 167 /// \brief Track the set of blocks that always lead to a cold call. 168 SmallPtrSet<const BasicBlock *, 16> PostDominatedByColdCall; 169 170 void updatePostDominatedByUnreachable(const BasicBlock *BB); 171 void updatePostDominatedByColdCall(const BasicBlock *BB); 172 bool calcUnreachableHeuristics(const BasicBlock *BB); 173 bool calcMetadataWeights(const BasicBlock *BB); 174 bool calcColdCallHeuristics(const BasicBlock *BB); 175 bool calcPointerHeuristics(const BasicBlock *BB); 176 bool calcLoopBranchHeuristics(const BasicBlock *BB, const LoopInfo &LI); 177 bool calcZeroHeuristics(const BasicBlock *BB, const TargetLibraryInfo *TLI); 178 bool calcFloatingPointHeuristics(const BasicBlock *BB); 179 bool calcInvokeHeuristics(const BasicBlock *BB); 180 }; 181 182 /// \brief Analysis pass which computes \c BranchProbabilityInfo. 183 class BranchProbabilityAnalysis 184 : public AnalysisInfoMixin<BranchProbabilityAnalysis> { 185 friend AnalysisInfoMixin<BranchProbabilityAnalysis>; 186 static AnalysisKey Key; 187 188 public: 189 /// \brief Provide the result typedef for this analysis pass. 190 typedef BranchProbabilityInfo Result; 191 192 /// \brief Run the analysis pass over a function and produce BPI. 193 BranchProbabilityInfo run(Function &F, FunctionAnalysisManager &AM); 194 }; 195 196 /// \brief Printer pass for the \c BranchProbabilityAnalysis results. 197 class BranchProbabilityPrinterPass 198 : public PassInfoMixin<BranchProbabilityPrinterPass> { 199 raw_ostream &OS; 200 201 public: 202 explicit BranchProbabilityPrinterPass(raw_ostream &OS) : OS(OS) {} 203 PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM); 204 }; 205 206 /// \brief Legacy analysis pass which computes \c BranchProbabilityInfo. 207 class BranchProbabilityInfoWrapperPass : public FunctionPass { 208 BranchProbabilityInfo BPI; 209 210 public: 211 static char ID; 212 213 BranchProbabilityInfoWrapperPass() : FunctionPass(ID) { 214 initializeBranchProbabilityInfoWrapperPassPass( 215 *PassRegistry::getPassRegistry()); 216 } 217 218 BranchProbabilityInfo &getBPI() { return BPI; } 219 const BranchProbabilityInfo &getBPI() const { return BPI; } 220 221 void getAnalysisUsage(AnalysisUsage &AU) const override; 222 bool runOnFunction(Function &F) override; 223 void releaseMemory() override; 224 void print(raw_ostream &OS, const Module *M = nullptr) const override; 225 }; 226 227 } 228 229 #endif 230
