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/InitializePasses.h" 21 #include "llvm/Pass.h" 22 #include "llvm/Support/BranchProbability.h" 23 24 namespace llvm { 25 class LoopInfo; 26 class raw_ostream; 27 28 /// \brief Analysis pass providing branch probability information. 29 /// 30 /// This is a function analysis pass which provides information on the relative 31 /// probabilities of each "edge" in the function's CFG where such an edge is 32 /// defined by a pair (PredBlock and an index in the successors). The 33 /// probability of an edge from one block is always relative to the 34 /// probabilities of other edges from the block. The probabilites of all edges 35 /// from a block sum to exactly one (100%). 36 /// We use a pair (PredBlock and an index in the successors) to uniquely 37 /// identify an edge, since we can have multiple edges from Src to Dst. 38 /// As an example, we can have a switch which jumps to Dst with value 0 and 39 /// value 10. 40 class BranchProbabilityInfo : public FunctionPass { 41 public: 42 static char ID; 43 44 BranchProbabilityInfo() : FunctionPass(ID) { 45 initializeBranchProbabilityInfoPass(*PassRegistry::getPassRegistry()); 46 } 47 48 void getAnalysisUsage(AnalysisUsage &AU) const override; 49 bool runOnFunction(Function &F) override; 50 void print(raw_ostream &OS, const Module *M = nullptr) const override; 51 52 /// \brief Get an edge's probability, relative to other out-edges of the Src. 53 /// 54 /// This routine provides access to the fractional probability between zero 55 /// (0%) and one (100%) of this edge executing, relative to other edges 56 /// leaving the 'Src' block. The returned probability is never zero, and can 57 /// only be one if the source block has only one successor. 58 BranchProbability getEdgeProbability(const BasicBlock *Src, 59 unsigned IndexInSuccessors) const; 60 61 /// \brief Get the probability of going from Src to Dst. 62 /// 63 /// It returns the sum of all probabilities for edges from Src to Dst. 64 BranchProbability getEdgeProbability(const BasicBlock *Src, 65 const BasicBlock *Dst) const; 66 67 /// \brief Test if an edge is hot relative to other out-edges of the Src. 68 /// 69 /// Check whether this edge out of the source block is 'hot'. We define hot 70 /// as having a relative probability >= 80%. 71 bool isEdgeHot(const BasicBlock *Src, const BasicBlock *Dst) const; 72 73 /// \brief Retrieve the hot successor of a block if one exists. 74 /// 75 /// Given a basic block, look through its successors and if one exists for 76 /// which \see isEdgeHot would return true, return that successor block. 77 BasicBlock *getHotSucc(BasicBlock *BB) const; 78 79 /// \brief Print an edge's probability. 80 /// 81 /// Retrieves an edge's probability similarly to \see getEdgeProbability, but 82 /// then prints that probability to the provided stream. That stream is then 83 /// returned. 84 raw_ostream &printEdgeProbability(raw_ostream &OS, const BasicBlock *Src, 85 const BasicBlock *Dst) const; 86 87 /// \brief Get the raw edge weight calculated for the edge. 88 /// 89 /// This returns the raw edge weight. It is guaranteed to fall between 1 and 90 /// UINT32_MAX. Note that the raw edge weight is not meaningful in isolation. 91 /// This interface should be very carefully, and primarily by routines that 92 /// are updating the analysis by later calling setEdgeWeight. 93 uint32_t getEdgeWeight(const BasicBlock *Src, 94 unsigned IndexInSuccessors) const; 95 96 /// \brief Get the raw edge weight calculated for the block pair. 97 /// 98 /// This returns the sum of all raw edge weights from Src to Dst. 99 /// It is guaranteed to fall between 1 and UINT32_MAX. 100 uint32_t getEdgeWeight(const BasicBlock *Src, const BasicBlock *Dst) const; 101 102 uint32_t getEdgeWeight(const BasicBlock *Src, 103 succ_const_iterator Dst) const; 104 105 /// \brief Set the raw edge weight for a given edge. 106 /// 107 /// This allows a pass to explicitly set the edge weight for an edge. It can 108 /// be used when updating the CFG to update and preserve the branch 109 /// probability information. Read the implementation of how these edge 110 /// weights are calculated carefully before using! 111 void setEdgeWeight(const BasicBlock *Src, unsigned IndexInSuccessors, 112 uint32_t Weight); 113 114 private: 115 // Since we allow duplicate edges from one basic block to another, we use 116 // a pair (PredBlock and an index in the successors) to specify an edge. 117 typedef std::pair<const BasicBlock *, unsigned> Edge; 118 119 // Default weight value. Used when we don't have information about the edge. 120 // TODO: DEFAULT_WEIGHT makes sense during static predication, when none of 121 // the successors have a weight yet. But it doesn't make sense when providing 122 // weight to an edge that may have siblings with non-zero weights. This can 123 // be handled various ways, but it's probably fine for an edge with unknown 124 // weight to just "inherit" the non-zero weight of an adjacent successor. 125 static const uint32_t DEFAULT_WEIGHT = 16; 126 127 DenseMap<Edge, uint32_t> Weights; 128 129 /// \brief Handle to the LoopInfo analysis. 130 LoopInfo *LI; 131 132 /// \brief Track the last function we run over for printing. 133 Function *LastF; 134 135 /// \brief Track the set of blocks directly succeeded by a returning block. 136 SmallPtrSet<BasicBlock *, 16> PostDominatedByUnreachable; 137 138 /// \brief Track the set of blocks that always lead to a cold call. 139 SmallPtrSet<BasicBlock *, 16> PostDominatedByColdCall; 140 141 /// \brief Get sum of the block successors' weights. 142 uint32_t getSumForBlock(const BasicBlock *BB) const; 143 144 bool calcUnreachableHeuristics(BasicBlock *BB); 145 bool calcMetadataWeights(BasicBlock *BB); 146 bool calcColdCallHeuristics(BasicBlock *BB); 147 bool calcPointerHeuristics(BasicBlock *BB); 148 bool calcLoopBranchHeuristics(BasicBlock *BB); 149 bool calcZeroHeuristics(BasicBlock *BB); 150 bool calcFloatingPointHeuristics(BasicBlock *BB); 151 bool calcInvokeHeuristics(BasicBlock *BB); 152 }; 153 154 } 155 156 #endif 157
