1//===- BranchProbabilityInfo.h - Branch Probability Analysis ----*- C++ -*-===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
10// This pass is used to evaluate branch probabilties.
11//
12//===----------------------------------------------------------------------===//
13
14#ifndef LLVM_ANALYSIS_BRANCHPROBABILITYINFO_H
15#define LLVM_ANALYSIS_BRANCHPROBABILITYINFO_H
16
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"
28#include <algorithm>
29#include <cassert>
30#include <cstdint>
31#include <utility>
32
33namespace llvm {
34
35class Function;
36class LoopInfo;
37class raw_ostream;
38class TargetLibraryInfo;
39class Value;
40
41/// \brief 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  /// \brief 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  /// \brief 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  /// \brief 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  /// \brief 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  /// \brief 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  /// \brief 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
140private:
141  // We need to store CallbackVH's in order to correctly handle basic block
142  // removal.
143  class BasicBlockCallbackVH final : public CallbackVH {
144    BranchProbabilityInfo *BPI;
145
146    void deleted() override {
147      assert(BPI != nullptr);
148      BPI->eraseBlock(cast<BasicBlock>(getValPtr()));
149      BPI->Handles.erase(*this);
150    }
151
152  public:
153    BasicBlockCallbackVH(const Value *V, BranchProbabilityInfo *BPI = nullptr)
154        : CallbackVH(const_cast<Value *>(V)), BPI(BPI) {}
155  };
156
157  DenseSet<BasicBlockCallbackVH, DenseMapInfo<Value*>> Handles;
158
159  // Since we allow duplicate edges from one basic block to another, we use
160  // a pair (PredBlock and an index in the successors) to specify an edge.
161  using Edge = std::pair<const BasicBlock *, unsigned>;
162
163  // Default weight value. Used when we don't have information about the edge.
164  // TODO: DEFAULT_WEIGHT makes sense during static predication, when none of
165  // the successors have a weight yet. But it doesn't make sense when providing
166  // weight to an edge that may have siblings with non-zero weights. This can
167  // be handled various ways, but it's probably fine for an edge with unknown
168  // weight to just "inherit" the non-zero weight of an adjacent successor.
169  static const uint32_t DEFAULT_WEIGHT = 16;
170
171  DenseMap<Edge, BranchProbability> Probs;
172
173  /// \brief Track the last function we run over for printing.
174  const Function *LastF;
175
176  /// \brief Track the set of blocks directly succeeded by a returning block.
177  SmallPtrSet<const BasicBlock *, 16> PostDominatedByUnreachable;
178
179  /// \brief Track the set of blocks that always lead to a cold call.
180  SmallPtrSet<const BasicBlock *, 16> PostDominatedByColdCall;
181
182  void updatePostDominatedByUnreachable(const BasicBlock *BB);
183  void updatePostDominatedByColdCall(const BasicBlock *BB);
184  bool calcUnreachableHeuristics(const BasicBlock *BB);
185  bool calcMetadataWeights(const BasicBlock *BB);
186  bool calcColdCallHeuristics(const BasicBlock *BB);
187  bool calcPointerHeuristics(const BasicBlock *BB);
188  bool calcLoopBranchHeuristics(const BasicBlock *BB, const LoopInfo &LI);
189  bool calcZeroHeuristics(const BasicBlock *BB, const TargetLibraryInfo *TLI);
190  bool calcFloatingPointHeuristics(const BasicBlock *BB);
191  bool calcInvokeHeuristics(const BasicBlock *BB);
192};
193
194/// \brief Analysis pass which computes \c BranchProbabilityInfo.
195class BranchProbabilityAnalysis
196    : public AnalysisInfoMixin<BranchProbabilityAnalysis> {
197  friend AnalysisInfoMixin<BranchProbabilityAnalysis>;
198
199  static AnalysisKey Key;
200
201public:
202  /// \brief Provide the result type for this analysis pass.
203  using Result = BranchProbabilityInfo;
204
205  /// \brief Run the analysis pass over a function and produce BPI.
206  BranchProbabilityInfo run(Function &F, FunctionAnalysisManager &AM);
207};
208
209/// \brief Printer pass for the \c BranchProbabilityAnalysis results.
210class BranchProbabilityPrinterPass
211    : public PassInfoMixin<BranchProbabilityPrinterPass> {
212  raw_ostream &OS;
213
214public:
215  explicit BranchProbabilityPrinterPass(raw_ostream &OS) : OS(OS) {}
216
217  PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM);
218};
219
220/// \brief Legacy analysis pass which computes \c BranchProbabilityInfo.
221class BranchProbabilityInfoWrapperPass : public FunctionPass {
222  BranchProbabilityInfo BPI;
223
224public:
225  static char ID;
226
227  BranchProbabilityInfoWrapperPass() : FunctionPass(ID) {
228    initializeBranchProbabilityInfoWrapperPassPass(
229        *PassRegistry::getPassRegistry());
230  }
231
232  BranchProbabilityInfo &getBPI() { return BPI; }
233  const BranchProbabilityInfo &getBPI() const { return BPI; }
234
235  void getAnalysisUsage(AnalysisUsage &AU) const override;
236  bool runOnFunction(Function &F) override;
237  void releaseMemory() override;
238  void print(raw_ostream &OS, const Module *M = nullptr) const override;
239};
240
241} // end namespace llvm
242
243#endif // LLVM_ANALYSIS_BRANCHPROBABILITYINFO_H
244