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/IR/CFG.h"
20 #include "llvm/InitializePasses.h"
21 #include "llvm/Pass.h"
22 #include "llvm/Support/BranchProbability.h"
28 /// \brief Analysis pass providing branch probability information.
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
40 class BranchProbabilityInfo : public FunctionPass {
44 BranchProbabilityInfo() : FunctionPass(ID) {
45 initializeBranchProbabilityInfoPass(*PassRegistry::getPassRegistry());
48 void getAnalysisUsage(AnalysisUsage &AU) const override;
49 bool runOnFunction(Function &F) override;
51 void releaseMemory() override;
53 void print(raw_ostream &OS, const Module *M = nullptr) const override;
55 /// \brief Get an edge's probability, relative to other out-edges of the Src.
57 /// This routine provides access to the fractional probability between zero
58 /// (0%) and one (100%) of this edge executing, relative to other edges
59 /// leaving the 'Src' block. The returned probability is never zero, and can
60 /// only be one if the source block has only one successor.
61 BranchProbability getEdgeProbability(const BasicBlock *Src,
62 unsigned IndexInSuccessors) const;
64 /// \brief Get the probability of going from Src to Dst.
66 /// It returns the sum of all probabilities for edges from Src to Dst.
67 BranchProbability getEdgeProbability(const BasicBlock *Src,
68 const BasicBlock *Dst) const;
70 /// \brief Test if an edge is hot relative to other out-edges of the Src.
72 /// Check whether this edge out of the source block is 'hot'. We define hot
73 /// as having a relative probability >= 80%.
74 bool isEdgeHot(const BasicBlock *Src, const BasicBlock *Dst) const;
76 /// \brief Retrieve the hot successor of a block if one exists.
78 /// Given a basic block, look through its successors and if one exists for
79 /// which \see isEdgeHot would return true, return that successor block.
80 BasicBlock *getHotSucc(BasicBlock *BB) const;
82 /// \brief Print an edge's probability.
84 /// Retrieves an edge's probability similarly to \see getEdgeProbability, but
85 /// then prints that probability to the provided stream. That stream is then
87 raw_ostream &printEdgeProbability(raw_ostream &OS, const BasicBlock *Src,
88 const BasicBlock *Dst) const;
90 /// \brief Get the raw edge weight calculated for the edge.
92 /// This returns the raw edge weight. It is guaranteed to fall between 1 and
93 /// UINT32_MAX. Note that the raw edge weight is not meaningful in isolation.
94 /// This interface should be very carefully, and primarily by routines that
95 /// are updating the analysis by later calling setEdgeWeight.
96 uint32_t getEdgeWeight(const BasicBlock *Src,
97 unsigned IndexInSuccessors) const;
99 /// \brief Get the raw edge weight calculated for the block pair.
101 /// This returns the sum of all raw edge weights from Src to Dst.
102 /// It is guaranteed to fall between 1 and UINT32_MAX.
103 uint32_t getEdgeWeight(const BasicBlock *Src, const BasicBlock *Dst) const;
105 uint32_t getEdgeWeight(const BasicBlock *Src,
106 succ_const_iterator Dst) const;
108 /// \brief Set the raw edge weight for a given edge.
110 /// This allows a pass to explicitly set the edge weight for an edge. It can
111 /// be used when updating the CFG to update and preserve the branch
112 /// probability information. Read the implementation of how these edge
113 /// weights are calculated carefully before using!
114 void setEdgeWeight(const BasicBlock *Src, unsigned IndexInSuccessors,
117 static uint32_t getBranchWeightStackProtector(bool IsLikely) {
118 return IsLikely ? (1u << 20) - 1 : 1;
122 // Since we allow duplicate edges from one basic block to another, we use
123 // a pair (PredBlock and an index in the successors) to specify an edge.
124 typedef std::pair<const BasicBlock *, unsigned> Edge;
126 // Default weight value. Used when we don't have information about the edge.
127 // TODO: DEFAULT_WEIGHT makes sense during static predication, when none of
128 // the successors have a weight yet. But it doesn't make sense when providing
129 // weight to an edge that may have siblings with non-zero weights. This can
130 // be handled various ways, but it's probably fine for an edge with unknown
131 // weight to just "inherit" the non-zero weight of an adjacent successor.
132 static const uint32_t DEFAULT_WEIGHT = 16;
134 DenseMap<Edge, uint32_t> Weights;
136 /// \brief Handle to the LoopInfo analysis.
139 /// \brief Track the last function we run over for printing.
142 /// \brief Track the set of blocks directly succeeded by a returning block.
143 SmallPtrSet<BasicBlock *, 16> PostDominatedByUnreachable;
145 /// \brief Track the set of blocks that always lead to a cold call.
146 SmallPtrSet<BasicBlock *, 16> PostDominatedByColdCall;
148 /// \brief Get sum of the block successors' weights.
149 uint32_t getSumForBlock(const BasicBlock *BB) const;
151 bool calcUnreachableHeuristics(BasicBlock *BB);
152 bool calcMetadataWeights(BasicBlock *BB);
153 bool calcColdCallHeuristics(BasicBlock *BB);
154 bool calcPointerHeuristics(BasicBlock *BB);
155 bool calcLoopBranchHeuristics(BasicBlock *BB);
156 bool calcZeroHeuristics(BasicBlock *BB);
157 bool calcFloatingPointHeuristics(BasicBlock *BB);
158 bool calcInvokeHeuristics(BasicBlock *BB);