1 //===--- BranchProbabilityInfo.h - Branch Probability Analysis --*- C++ -*-===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This pass is used to evaluate branch probabilties.
12 //===----------------------------------------------------------------------===//
14 #ifndef LLVM_ANALYSIS_BRANCHPROBABILITYINFO_H
15 #define LLVM_ANALYSIS_BRANCHPROBABILITYINFO_H
17 #include "llvm/InitializePasses.h"
18 #include "llvm/Pass.h"
19 #include "llvm/ADT/DenseMap.h"
20 #include "llvm/ADT/SmallPtrSet.h"
21 #include "llvm/Support/BranchProbability.h"
27 /// \brief Analysis pass providing branch probability information.
29 /// This is a function analysis pass which provides information on the relative
30 /// probabilities of each "edge" in the function's CFG where such an edge is
31 /// defined by a pair of basic blocks. The probability for a given block and
32 /// a successor block are always relative to the probabilities of the other
33 /// successor blocks. Another way of looking at it is that the probabilities
34 /// for a given block B and each of its successors should sum to exactly
36 class BranchProbabilityInfo : public FunctionPass {
40 BranchProbabilityInfo() : FunctionPass(ID) {
41 initializeBranchProbabilityInfoPass(*PassRegistry::getPassRegistry());
44 void getAnalysisUsage(AnalysisUsage &AU) const;
45 bool runOnFunction(Function &F);
46 void print(raw_ostream &OS, const Module *M = 0) const;
48 /// \brief Get an edge's probability, relative to other out-edges of the Src.
50 /// This routine provides access to the fractional probability between zero
51 /// (0%) and one (100%) of this edge executing, relative to other edges
52 /// leaving the 'Src' block. The returned probability is never zero, and can
53 /// only be one if the source block has only one successor.
54 BranchProbability getEdgeProbability(const BasicBlock *Src,
55 const BasicBlock *Dst) const;
57 /// \brief Test if an edge is hot relative to other out-edges of the Src.
59 /// Check whether this edge out of the source block is 'hot'. We define hot
60 /// as having a relative probability >= 80%.
61 bool isEdgeHot(const BasicBlock *Src, const BasicBlock *Dst) const;
63 /// \brief Retrieve the hot successor of a block if one exists.
65 /// Given a basic block, look through its successors and if one exists for
66 /// which \see isEdgeHot would return true, return that successor block.
67 BasicBlock *getHotSucc(BasicBlock *BB) const;
69 /// \brief Print an edge's probability.
71 /// Retrieves an edge's probability similarly to \see getEdgeProbability, but
72 /// then prints that probability to the provided stream. That stream is then
74 raw_ostream &printEdgeProbability(raw_ostream &OS, const BasicBlock *Src,
75 const BasicBlock *Dst) const;
77 /// \brief Get the raw edge weight calculated for the block pair.
79 /// This returns the raw edge weight. It is guaranteed to fall between 1 and
80 /// UINT32_MAX. Note that the raw edge weight is not meaningful in isolation.
81 /// This interface should be very carefully, and primarily by routines that
82 /// are updating the analysis by later calling setEdgeWeight.
83 uint32_t getEdgeWeight(const BasicBlock *Src, const BasicBlock *Dst) const;
85 /// \brief Set the raw edge weight for the block pair.
87 /// This allows a pass to explicitly set the edge weight for a block. It can
88 /// be used when updating the CFG to update and preserve the branch
89 /// probability information. Read the implementation of how these edge
90 /// weights are calculated carefully before using!
91 void setEdgeWeight(const BasicBlock *Src, const BasicBlock *Dst,
95 typedef std::pair<const BasicBlock *, const BasicBlock *> Edge;
97 // Default weight value. Used when we don't have information about the edge.
98 // TODO: DEFAULT_WEIGHT makes sense during static predication, when none of
99 // the successors have a weight yet. But it doesn't make sense when providing
100 // weight to an edge that may have siblings with non-zero weights. This can
101 // be handled various ways, but it's probably fine for an edge with unknown
102 // weight to just "inherit" the non-zero weight of an adjacent successor.
103 static const uint32_t DEFAULT_WEIGHT = 16;
105 DenseMap<Edge, uint32_t> Weights;
107 /// \brief Handle to the LoopInfo analysis.
110 /// \brief Track the last function we run over for printing.
113 /// \brief Track the set of blocks directly succeeded by a returning block.
114 SmallPtrSet<BasicBlock *, 16> PostDominatedByUnreachable;
116 /// \brief Get sum of the block successors' weights.
117 uint32_t getSumForBlock(const BasicBlock *BB) const;
119 bool calcUnreachableHeuristics(BasicBlock *BB);
120 bool calcMetadataWeights(BasicBlock *BB);
121 bool calcPointerHeuristics(BasicBlock *BB);
122 bool calcLoopBranchHeuristics(BasicBlock *BB);
123 bool calcZeroHeuristics(BasicBlock *BB);
124 bool calcFloatingPointHeuristics(BasicBlock *BB);