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 {
42 BranchProbabilityInfo() {}
43 BranchProbabilityInfo(Function &F, const LoopInfo &LI) { calculate(F, LI); }
47 void print(raw_ostream &OS) const;
49 /// \brief Get an edge's probability, relative to other out-edges of the Src.
51 /// This routine provides access to the fractional probability between zero
52 /// (0%) and one (100%) of this edge executing, relative to other edges
53 /// leaving the 'Src' block. The returned probability is never zero, and can
54 /// only be one if the source block has only one successor.
55 BranchProbability getEdgeProbability(const BasicBlock *Src,
56 unsigned IndexInSuccessors) const;
58 /// \brief Get the probability of going from Src to Dst.
60 /// It returns the sum of all probabilities for edges from Src to Dst.
61 BranchProbability getEdgeProbability(const BasicBlock *Src,
62 const BasicBlock *Dst) const;
64 BranchProbability getEdgeProbability(const BasicBlock *Src,
65 succ_const_iterator Dst) const;
67 /// \brief Test if an edge is hot relative to other out-edges of the Src.
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;
73 /// \brief Retrieve the hot successor of a block if one exists.
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;
79 /// \brief Print an edge's probability.
81 /// Retrieves an edge's probability similarly to \see getEdgeProbability, but
82 /// then prints that probability to the provided stream. That stream is then
84 raw_ostream &printEdgeProbability(raw_ostream &OS, const BasicBlock *Src,
85 const BasicBlock *Dst) const;
87 /// \brief Set the raw edge probability for the given edge.
89 /// This allows a pass to explicitly set the edge probability for an edge. It
90 /// can be used when updating the CFG to update and preserve the branch
91 /// probability information. Read the implementation of how these edge
92 /// probabilities are calculated carefully before using!
93 void setEdgeProbability(const BasicBlock *Src, unsigned IndexInSuccessors,
94 BranchProbability Prob);
96 static BranchProbability getBranchProbStackProtector(bool IsLikely) {
97 static const BranchProbability LikelyProb((1u << 20) - 1, 1u << 20);
98 return IsLikely ? LikelyProb : LikelyProb.getCompl();
101 void calculate(Function &F, const LoopInfo& LI);
104 // Since we allow duplicate edges from one basic block to another, we use
105 // a pair (PredBlock and an index in the successors) to specify an edge.
106 typedef std::pair<const BasicBlock *, unsigned> Edge;
108 // Default weight value. Used when we don't have information about the edge.
109 // TODO: DEFAULT_WEIGHT makes sense during static predication, when none of
110 // the successors have a weight yet. But it doesn't make sense when providing
111 // weight to an edge that may have siblings with non-zero weights. This can
112 // be handled various ways, but it's probably fine for an edge with unknown
113 // weight to just "inherit" the non-zero weight of an adjacent successor.
114 static const uint32_t DEFAULT_WEIGHT = 16;
116 DenseMap<Edge, BranchProbability> Probs;
118 /// \brief Track the last function we run over for printing.
121 /// \brief Track the set of blocks directly succeeded by a returning block.
122 SmallPtrSet<BasicBlock *, 16> PostDominatedByUnreachable;
124 /// \brief Track the set of blocks that always lead to a cold call.
125 SmallPtrSet<BasicBlock *, 16> PostDominatedByColdCall;
127 bool calcUnreachableHeuristics(BasicBlock *BB);
128 bool calcMetadataWeights(BasicBlock *BB);
129 bool calcColdCallHeuristics(BasicBlock *BB);
130 bool calcPointerHeuristics(BasicBlock *BB);
131 bool calcLoopBranchHeuristics(BasicBlock *BB, const LoopInfo &LI);
132 bool calcZeroHeuristics(BasicBlock *BB);
133 bool calcFloatingPointHeuristics(BasicBlock *BB);
134 bool calcInvokeHeuristics(BasicBlock *BB);
137 /// \brief Legacy analysis pass which computes \c BranchProbabilityInfo.
138 class BranchProbabilityInfoWrapperPass : public FunctionPass {
139 BranchProbabilityInfo BPI;
144 BranchProbabilityInfoWrapperPass() : FunctionPass(ID) {
145 initializeBranchProbabilityInfoWrapperPassPass(
146 *PassRegistry::getPassRegistry());
149 BranchProbabilityInfo &getBPI() { return BPI; }
150 const BranchProbabilityInfo &getBPI() const { return BPI; }
152 void getAnalysisUsage(AnalysisUsage &AU) const override;
153 bool runOnFunction(Function &F) override;
154 void releaseMemory() override;
155 void print(raw_ostream &OS, const Module *M = nullptr) const override;