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/DenseMapInfo.h"
19 #include "llvm/ADT/DenseSet.h"
20 #include "llvm/ADT/SmallPtrSet.h"
21 #include "llvm/IR/BasicBlock.h"
22 #include "llvm/IR/CFG.h"
23 #include "llvm/IR/PassManager.h"
24 #include "llvm/IR/ValueHandle.h"
25 #include "llvm/Pass.h"
26 #include "llvm/Support/BranchProbability.h"
27 #include "llvm/Support/Casting.h"
38 class TargetLibraryInfo;
41 /// \brief Analysis providing branch probability information.
43 /// This is a function analysis which provides information on the relative
44 /// probabilities of each "edge" in the function's CFG where such an edge is
45 /// defined by a pair (PredBlock and an index in the successors). The
46 /// probability of an edge from one block is always relative to the
47 /// probabilities of other edges from the block. The probabilites of all edges
48 /// from a block sum to exactly one (100%).
49 /// We use a pair (PredBlock and an index in the successors) to uniquely
50 /// identify an edge, since we can have multiple edges from Src to Dst.
51 /// As an example, we can have a switch which jumps to Dst with value 0 and
53 class BranchProbabilityInfo {
55 BranchProbabilityInfo() = default;
57 BranchProbabilityInfo(const Function &F, const LoopInfo &LI,
58 const TargetLibraryInfo *TLI = nullptr) {
59 calculate(F, LI, TLI);
62 BranchProbabilityInfo(BranchProbabilityInfo &&Arg)
63 : Probs(std::move(Arg.Probs)), LastF(Arg.LastF),
64 PostDominatedByUnreachable(std::move(Arg.PostDominatedByUnreachable)),
65 PostDominatedByColdCall(std::move(Arg.PostDominatedByColdCall)) {}
67 BranchProbabilityInfo(const BranchProbabilityInfo &) = delete;
68 BranchProbabilityInfo &operator=(const BranchProbabilityInfo &) = delete;
70 BranchProbabilityInfo &operator=(BranchProbabilityInfo &&RHS) {
72 Probs = std::move(RHS.Probs);
73 PostDominatedByColdCall = std::move(RHS.PostDominatedByColdCall);
74 PostDominatedByUnreachable = std::move(RHS.PostDominatedByUnreachable);
80 void print(raw_ostream &OS) const;
82 /// \brief Get an edge's probability, relative to other out-edges of the Src.
84 /// This routine provides access to the fractional probability between zero
85 /// (0%) and one (100%) of this edge executing, relative to other edges
86 /// leaving the 'Src' block. The returned probability is never zero, and can
87 /// only be one if the source block has only one successor.
88 BranchProbability getEdgeProbability(const BasicBlock *Src,
89 unsigned IndexInSuccessors) const;
91 /// \brief Get the probability of going from Src to Dst.
93 /// It returns the sum of all probabilities for edges from Src to Dst.
94 BranchProbability getEdgeProbability(const BasicBlock *Src,
95 const BasicBlock *Dst) const;
97 BranchProbability getEdgeProbability(const BasicBlock *Src,
98 succ_const_iterator Dst) const;
100 /// \brief Test if an edge is hot relative to other out-edges of the Src.
102 /// Check whether this edge out of the source block is 'hot'. We define hot
103 /// as having a relative probability >= 80%.
104 bool isEdgeHot(const BasicBlock *Src, const BasicBlock *Dst) const;
106 /// \brief Retrieve the hot successor of a block if one exists.
108 /// Given a basic block, look through its successors and if one exists for
109 /// which \see isEdgeHot would return true, return that successor block.
110 const BasicBlock *getHotSucc(const BasicBlock *BB) const;
112 /// \brief Print an edge's probability.
114 /// Retrieves an edge's probability similarly to \see getEdgeProbability, but
115 /// then prints that probability to the provided stream. That stream is then
117 raw_ostream &printEdgeProbability(raw_ostream &OS, const BasicBlock *Src,
118 const BasicBlock *Dst) const;
120 /// \brief Set the raw edge probability for the given edge.
122 /// This allows a pass to explicitly set the edge probability for an edge. It
123 /// can be used when updating the CFG to update and preserve the branch
124 /// probability information. Read the implementation of how these edge
125 /// probabilities are calculated carefully before using!
126 void setEdgeProbability(const BasicBlock *Src, unsigned IndexInSuccessors,
127 BranchProbability Prob);
129 static BranchProbability getBranchProbStackProtector(bool IsLikely) {
130 static const BranchProbability LikelyProb((1u << 20) - 1, 1u << 20);
131 return IsLikely ? LikelyProb : LikelyProb.getCompl();
134 void calculate(const Function &F, const LoopInfo &LI,
135 const TargetLibraryInfo *TLI = nullptr);
137 /// Forget analysis results for the given basic block.
138 void eraseBlock(const BasicBlock *BB);
140 // Use to track SCCs for handling irreducible loops.
141 using SccMap = DenseMap<const BasicBlock *, int>;
142 using SccHeaderMap = DenseMap<const BasicBlock *, bool>;
143 using SccHeaderMaps = std::vector<SccHeaderMap>;
146 SccHeaderMaps SccHeaders;
150 // We need to store CallbackVH's in order to correctly handle basic block
152 class BasicBlockCallbackVH final : public CallbackVH {
153 BranchProbabilityInfo *BPI;
155 void deleted() override {
156 assert(BPI != nullptr);
157 BPI->eraseBlock(cast<BasicBlock>(getValPtr()));
158 BPI->Handles.erase(*this);
162 BasicBlockCallbackVH(const Value *V, BranchProbabilityInfo *BPI = nullptr)
163 : CallbackVH(const_cast<Value *>(V)), BPI(BPI) {}
166 DenseSet<BasicBlockCallbackVH, DenseMapInfo<Value*>> Handles;
168 // Since we allow duplicate edges from one basic block to another, we use
169 // a pair (PredBlock and an index in the successors) to specify an edge.
170 using Edge = std::pair<const BasicBlock *, unsigned>;
172 // Default weight value. Used when we don't have information about the edge.
173 // TODO: DEFAULT_WEIGHT makes sense during static predication, when none of
174 // the successors have a weight yet. But it doesn't make sense when providing
175 // weight to an edge that may have siblings with non-zero weights. This can
176 // be handled various ways, but it's probably fine for an edge with unknown
177 // weight to just "inherit" the non-zero weight of an adjacent successor.
178 static const uint32_t DEFAULT_WEIGHT = 16;
180 DenseMap<Edge, BranchProbability> Probs;
182 /// \brief Track the last function we run over for printing.
183 const Function *LastF;
185 /// \brief Track the set of blocks directly succeeded by a returning block.
186 SmallPtrSet<const BasicBlock *, 16> PostDominatedByUnreachable;
188 /// \brief Track the set of blocks that always lead to a cold call.
189 SmallPtrSet<const BasicBlock *, 16> PostDominatedByColdCall;
191 void updatePostDominatedByUnreachable(const BasicBlock *BB);
192 void updatePostDominatedByColdCall(const BasicBlock *BB);
193 bool calcUnreachableHeuristics(const BasicBlock *BB);
194 bool calcMetadataWeights(const BasicBlock *BB);
195 bool calcColdCallHeuristics(const BasicBlock *BB);
196 bool calcPointerHeuristics(const BasicBlock *BB);
197 bool calcLoopBranchHeuristics(const BasicBlock *BB, const LoopInfo &LI,
199 bool calcZeroHeuristics(const BasicBlock *BB, const TargetLibraryInfo *TLI);
200 bool calcFloatingPointHeuristics(const BasicBlock *BB);
201 bool calcInvokeHeuristics(const BasicBlock *BB);
204 /// \brief Analysis pass which computes \c BranchProbabilityInfo.
205 class BranchProbabilityAnalysis
206 : public AnalysisInfoMixin<BranchProbabilityAnalysis> {
207 friend AnalysisInfoMixin<BranchProbabilityAnalysis>;
209 static AnalysisKey Key;
212 /// \brief Provide the result type for this analysis pass.
213 using Result = BranchProbabilityInfo;
215 /// \brief Run the analysis pass over a function and produce BPI.
216 BranchProbabilityInfo run(Function &F, FunctionAnalysisManager &AM);
219 /// \brief Printer pass for the \c BranchProbabilityAnalysis results.
220 class BranchProbabilityPrinterPass
221 : public PassInfoMixin<BranchProbabilityPrinterPass> {
225 explicit BranchProbabilityPrinterPass(raw_ostream &OS) : OS(OS) {}
227 PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM);
230 /// \brief Legacy analysis pass which computes \c BranchProbabilityInfo.
231 class BranchProbabilityInfoWrapperPass : public FunctionPass {
232 BranchProbabilityInfo BPI;
237 BranchProbabilityInfoWrapperPass() : FunctionPass(ID) {
238 initializeBranchProbabilityInfoWrapperPassPass(
239 *PassRegistry::getPassRegistry());
242 BranchProbabilityInfo &getBPI() { return BPI; }
243 const BranchProbabilityInfo &getBPI() const { return BPI; }
245 void getAnalysisUsage(AnalysisUsage &AU) const override;
246 bool runOnFunction(Function &F) override;
247 void releaseMemory() override;
248 void print(raw_ostream &OS, const Module *M = nullptr) const override;
251 } // end namespace llvm
253 #endif // LLVM_ANALYSIS_BRANCHPROBABILITYINFO_H