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 providing branch probability information.
30 /// This is a function analysis 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 {
44 void print(raw_ostream &OS) const;
46 /// \brief Get an edge's probability, relative to other out-edges of the Src.
48 /// This routine provides access to the fractional probability between zero
49 /// (0%) and one (100%) of this edge executing, relative to other edges
50 /// leaving the 'Src' block. The returned probability is never zero, and can
51 /// only be one if the source block has only one successor.
52 BranchProbability getEdgeProbability(const BasicBlock *Src,
53 unsigned IndexInSuccessors) const;
55 /// \brief Get the probability of going from Src to Dst.
57 /// It returns the sum of all probabilities for edges from Src to Dst.
58 BranchProbability getEdgeProbability(const BasicBlock *Src,
59 const BasicBlock *Dst) const;
61 /// \brief Test if an edge is hot relative to other out-edges of the Src.
63 /// Check whether this edge out of the source block is 'hot'. We define hot
64 /// as having a relative probability >= 80%.
65 bool isEdgeHot(const BasicBlock *Src, const BasicBlock *Dst) const;
67 /// \brief Retrieve the hot successor of a block if one exists.
69 /// Given a basic block, look through its successors and if one exists for
70 /// which \see isEdgeHot would return true, return that successor block.
71 BasicBlock *getHotSucc(BasicBlock *BB) const;
73 /// \brief Print an edge's probability.
75 /// Retrieves an edge's probability similarly to \see getEdgeProbability, but
76 /// then prints that probability to the provided stream. That stream is then
78 raw_ostream &printEdgeProbability(raw_ostream &OS, const BasicBlock *Src,
79 const BasicBlock *Dst) const;
81 /// \brief Get the raw edge weight calculated for the edge.
83 /// This returns the raw edge weight. It is guaranteed to fall between 1 and
84 /// UINT32_MAX. Note that the raw edge weight is not meaningful in isolation.
85 /// This interface should be very carefully, and primarily by routines that
86 /// are updating the analysis by later calling setEdgeWeight.
87 uint32_t getEdgeWeight(const BasicBlock *Src,
88 unsigned IndexInSuccessors) const;
90 /// \brief Get the raw edge weight calculated for the block pair.
92 /// This returns the sum of all raw edge weights from Src to Dst.
93 /// It is guaranteed to fall between 1 and UINT32_MAX.
94 uint32_t getEdgeWeight(const BasicBlock *Src, const BasicBlock *Dst) const;
96 uint32_t getEdgeWeight(const BasicBlock *Src,
97 succ_const_iterator Dst) const;
99 /// \brief Set the raw edge weight for a given edge.
101 /// This allows a pass to explicitly set the edge weight for an edge. It can
102 /// be used when updating the CFG to update and preserve the branch
103 /// probability information. Read the implementation of how these edge
104 /// weights are calculated carefully before using!
105 void setEdgeWeight(const BasicBlock *Src, unsigned IndexInSuccessors,
108 static uint32_t getBranchWeightStackProtector(bool IsLikely) {
109 return IsLikely ? (1u << 20) - 1 : 1;
112 void calculate(Function &F, const LoopInfo& LI);
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;
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;
127 DenseMap<Edge, uint32_t> Weights;
129 /// \brief Track the last function we run over for printing.
132 /// \brief Track the set of blocks directly succeeded by a returning block.
133 SmallPtrSet<BasicBlock *, 16> PostDominatedByUnreachable;
135 /// \brief Track the set of blocks that always lead to a cold call.
136 SmallPtrSet<BasicBlock *, 16> PostDominatedByColdCall;
138 /// \brief Get sum of the block successors' weights.
139 uint32_t getSumForBlock(const BasicBlock *BB) const;
141 bool calcUnreachableHeuristics(BasicBlock *BB);
142 bool calcMetadataWeights(BasicBlock *BB);
143 bool calcColdCallHeuristics(BasicBlock *BB);
144 bool calcPointerHeuristics(BasicBlock *BB);
145 bool calcLoopBranchHeuristics(BasicBlock *BB, const LoopInfo &LI);
146 bool calcZeroHeuristics(BasicBlock *BB);
147 bool calcFloatingPointHeuristics(BasicBlock *BB);
148 bool calcInvokeHeuristics(BasicBlock *BB);
151 /// \brief Legacy analysis pass which computes \c BranchProbabilityInfo.
152 class BranchProbabilityInfoWrapperPass : public FunctionPass {
153 BranchProbabilityInfo BPI;
158 BranchProbabilityInfoWrapperPass() : FunctionPass(ID) {
159 initializeBranchProbabilityInfoWrapperPassPass(
160 *PassRegistry::getPassRegistry());
163 BranchProbabilityInfo &getBPI() { return BPI; }
164 const BranchProbabilityInfo &getBPI() const { return BPI; }
166 void getAnalysisUsage(AnalysisUsage &AU) const override;
167 bool runOnFunction(Function &F) override;
168 void releaseMemory() override;
169 void print(raw_ostream &OS, const Module *M = nullptr) const override;