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