BranchProbabilityInfo.h revision 360784 
1//===- BranchProbabilityInfo.h - Branch Probability Analysis ----*- C++ -*-===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8//
9// This pass is used to evaluate branch probabilties.
10//
11//===----------------------------------------------------------------------===//
12
13#ifndef LLVM_ANALYSIS_BRANCHPROBABILITYINFO_H
14#define LLVM_ANALYSIS_BRANCHPROBABILITYINFO_H
15
16#include "llvm/ADT/DenseMap.h"
17#include "llvm/ADT/DenseMapInfo.h"
18#include "llvm/ADT/DenseSet.h"
19#include "llvm/ADT/SmallPtrSet.h"
20#include "llvm/IR/BasicBlock.h"
21#include "llvm/IR/CFG.h"
22#include "llvm/IR/PassManager.h"
23#include "llvm/IR/ValueHandle.h"
24#include "llvm/Pass.h"
25#include "llvm/Support/BranchProbability.h"
26#include "llvm/Support/Casting.h"
27#include <algorithm>
28#include <cassert>
29#include <cstdint>
30#include <utility>
31
32namespace llvm {
33
34class Function;
35class LoopInfo;
36class raw_ostream;
37class PostDominatorTree;
38class TargetLibraryInfo;
39class Value;
40
41/// Analysis providing branch probability information.
42///
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
52/// value 10.
53class BranchProbabilityInfo {
54public:
55  BranchProbabilityInfo() = default;
56
57  BranchProbabilityInfo(const Function &F, const LoopInfo &LI,
58                        const TargetLibraryInfo *TLI = nullptr) {
59    calculate(F, LI, TLI);
60  }
61
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)) {}
66
67  BranchProbabilityInfo(const BranchProbabilityInfo &) = delete;
68  BranchProbabilityInfo &operator=(const BranchProbabilityInfo &) = delete;
69
70  BranchProbabilityInfo &operator=(BranchProbabilityInfo &&RHS) {
71    releaseMemory();
72    Probs = std::move(RHS.Probs);
73    PostDominatedByColdCall = std::move(RHS.PostDominatedByColdCall);
74    PostDominatedByUnreachable = std::move(RHS.PostDominatedByUnreachable);
75    return *this;
76  }
77
78  void releaseMemory();
79
80  void print(raw_ostream &OS) const;
81
82  /// Get an edge's probability, relative to other out-edges of the Src.
83  ///
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;
90
91  /// Get the probability of going from Src to Dst.
92  ///
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;
96
97  BranchProbability getEdgeProbability(const BasicBlock *Src,
98                                       succ_const_iterator Dst) const;
99
100  /// Test if an edge is hot relative to other out-edges of the Src.
101  ///
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;
105
106  /// Retrieve the hot successor of a block if one exists.
107  ///
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;
111
112  /// Print an edge's probability.
113  ///
114  /// Retrieves an edge's probability similarly to \see getEdgeProbability, but
115  /// then prints that probability to the provided stream. That stream is then
116  /// returned.
117  raw_ostream &printEdgeProbability(raw_ostream &OS, const BasicBlock *Src,
118                                    const BasicBlock *Dst) const;
119
120  /// Set the raw edge probability for the given edge.
121  ///
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);
128
129  static BranchProbability getBranchProbStackProtector(bool IsLikely) {
130    static const BranchProbability LikelyProb((1u << 20) - 1, 1u << 20);
131    return IsLikely ? LikelyProb : LikelyProb.getCompl();
132  }
133
134  void calculate(const Function &F, const LoopInfo &LI,
135                 const TargetLibraryInfo *TLI = nullptr);
136
137  /// Forget analysis results for the given basic block.
138  void eraseBlock(const BasicBlock *BB);
139
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>;
144  struct SccInfo {
145    SccMap SccNums;
146    SccHeaderMaps SccHeaders;
147  };
148
149private:
150  // We need to store CallbackVH's in order to correctly handle basic block
151  // removal.
152  class BasicBlockCallbackVH final : public CallbackVH {
153    BranchProbabilityInfo *BPI;
154
155    void deleted() override {
156      assert(BPI != nullptr);
157      BPI->eraseBlock(cast<BasicBlock>(getValPtr()));
158      BPI->Handles.erase(*this);
159    }
160
161  public:
162    BasicBlockCallbackVH(const Value *V, BranchProbabilityInfo *BPI = nullptr)
163        : CallbackVH(const_cast<Value *>(V)), BPI(BPI) {}
164  };
165
166  DenseSet<BasicBlockCallbackVH, DenseMapInfo<Value*>> Handles;
167
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>;
171
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;
179
180  DenseMap<Edge, BranchProbability> Probs;
181
182  /// Track the last function we run over for printing.
183  const Function *LastF = nullptr;
184
185  /// Track the set of blocks directly succeeded by a returning block.
186  SmallPtrSet<const BasicBlock *, 16> PostDominatedByUnreachable;
187
188  /// Track the set of blocks that always lead to a cold call.
189  SmallPtrSet<const BasicBlock *, 16> PostDominatedByColdCall;
190
191  void computePostDominatedByUnreachable(const Function &F,
192                                         PostDominatorTree *PDT);
193  void computePostDominatedByColdCall(const Function &F,
194                                      PostDominatorTree *PDT);
195  bool calcUnreachableHeuristics(const BasicBlock *BB);
196  bool calcMetadataWeights(const BasicBlock *BB);
197  bool calcColdCallHeuristics(const BasicBlock *BB);
198  bool calcPointerHeuristics(const BasicBlock *BB);
199  bool calcLoopBranchHeuristics(const BasicBlock *BB, const LoopInfo &LI,
200                                SccInfo &SccI);
201  bool calcZeroHeuristics(const BasicBlock *BB, const TargetLibraryInfo *TLI);
202  bool calcFloatingPointHeuristics(const BasicBlock *BB);
203  bool calcInvokeHeuristics(const BasicBlock *BB);
204};
205
206/// Analysis pass which computes \c BranchProbabilityInfo.
207class BranchProbabilityAnalysis
208    : public AnalysisInfoMixin<BranchProbabilityAnalysis> {
209  friend AnalysisInfoMixin<BranchProbabilityAnalysis>;
210
211  static AnalysisKey Key;
212
213public:
214  /// Provide the result type for this analysis pass.
215  using Result = BranchProbabilityInfo;
216
217  /// Run the analysis pass over a function and produce BPI.
218  BranchProbabilityInfo run(Function &F, FunctionAnalysisManager &AM);
219};
220
221/// Printer pass for the \c BranchProbabilityAnalysis results.
222class BranchProbabilityPrinterPass
223    : public PassInfoMixin<BranchProbabilityPrinterPass> {
224  raw_ostream &OS;
225
226public:
227  explicit BranchProbabilityPrinterPass(raw_ostream &OS) : OS(OS) {}
228
229  PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM);
230};
231
232/// Legacy analysis pass which computes \c BranchProbabilityInfo.
233class BranchProbabilityInfoWrapperPass : public FunctionPass {
234  BranchProbabilityInfo BPI;
235
236public:
237  static char ID;
238
239  BranchProbabilityInfoWrapperPass();
240
241  BranchProbabilityInfo &getBPI() { return BPI; }
242  const BranchProbabilityInfo &getBPI() const { return BPI; }
243
244  void getAnalysisUsage(AnalysisUsage &AU) const override;
245  bool runOnFunction(Function &F) override;
246  void releaseMemory() override;
247  void print(raw_ostream &OS, const Module *M = nullptr) const override;
248};
249
250} // end namespace llvm
251
252#endif // LLVM_ANALYSIS_BRANCHPROBABILITYINFO_H
253