src/llvm/include/llvm/Analysis/BranchProbabilityInfo.h

   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   static uint32_t getBranchWeightStackProtector(bool IsLikely) {
 115     return IsLikely ? (1u << 20) - 1 : 1;
 116   }
 117
 118 private:
 119   // Since we allow duplicate edges from one basic block to another, we use
 120   // a pair (PredBlock and an index in the successors) to specify an edge.
 121   typedef std::pair<const BasicBlock *, unsigned> Edge;
 122
 123   // Default weight value. Used when we don't have information about the edge.
 124   // TODO: DEFAULT_WEIGHT makes sense during static predication, when none of
 125   // the successors have a weight yet. But it doesn't make sense when providing
 126   // weight to an edge that may have siblings with non-zero weights. This can
 127   // be handled various ways, but it's probably fine for an edge with unknown
 128   // weight to just "inherit" the non-zero weight of an adjacent successor.
 129   static const uint32_t DEFAULT_WEIGHT = 16;
 130
 131   DenseMap<Edge, uint32_t> Weights;
 132
 133   /// \brief Handle to the LoopInfo analysis.
 134   LoopInfo *LI;
 135
 136   /// \brief Track the last function we run over for printing.
 137   Function *LastF;
 138
 139   /// \brief Track the set of blocks directly succeeded by a returning block.
 140   SmallPtrSet<BasicBlock *, 16> PostDominatedByUnreachable;
 141
 142   /// \brief Track the set of blocks that always lead to a cold call.
 143   SmallPtrSet<BasicBlock *, 16> PostDominatedByColdCall;
 144
 145   /// \brief Get sum of the block successors' weights.
 146   uint32_t getSumForBlock(const BasicBlock *BB) const;
 147
 148   bool calcUnreachableHeuristics(BasicBlock *BB);
 149   bool calcMetadataWeights(BasicBlock *BB);
 150   bool calcColdCallHeuristics(BasicBlock *BB);
 151   bool calcPointerHeuristics(BasicBlock *BB);
 152   bool calcLoopBranchHeuristics(BasicBlock *BB);
 153   bool calcZeroHeuristics(BasicBlock *BB);
 154   bool calcFloatingPointHeuristics(BasicBlock *BB);
 155   bool calcInvokeHeuristics(BasicBlock *BB);
 156 };
 157
 158 }
 159
 160 #endif