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BranchProbabilityInfo.h
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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"
26 #include "llvm/Support/Casting.h"
27 #include <algorithm>
28 #include <cassert>
29 #include <cstdint>
30 #include <utility>
31 
32 namespace llvm {
33 
34 class Function;
35 class LoopInfo;
36 class raw_ostream;
37 class TargetLibraryInfo;
38 class Value;
39 
40 /// Analysis providing branch probability information.
41 ///
42 /// This is a function analysis which provides information on the relative
43 /// probabilities of each "edge" in the function's CFG where such an edge is
44 /// defined by a pair (PredBlock and an index in the successors). The
45 /// probability of an edge from one block is always relative to the
46 /// probabilities of other edges from the block. The probabilites of all edges
47 /// from a block sum to exactly one (100%).
48 /// We use a pair (PredBlock and an index in the successors) to uniquely
49 /// identify an edge, since we can have multiple edges from Src to Dst.
50 /// As an example, we can have a switch which jumps to Dst with value 0 and
51 /// value 10.
53 public:
54  BranchProbabilityInfo() = default;
55 
57  const TargetLibraryInfo *TLI = nullptr) {
58  calculate(F, LI, TLI);
59  }
60 
62  : Probs(std::move(Arg.Probs)), LastF(Arg.LastF),
63  PostDominatedByUnreachable(std::move(Arg.PostDominatedByUnreachable)),
64  PostDominatedByColdCall(std::move(Arg.PostDominatedByColdCall)) {}
65 
68 
70  releaseMemory();
71  Probs = std::move(RHS.Probs);
72  PostDominatedByColdCall = std::move(RHS.PostDominatedByColdCall);
73  PostDominatedByUnreachable = std::move(RHS.PostDominatedByUnreachable);
74  return *this;
75  }
76 
77  void releaseMemory();
78 
79  void print(raw_ostream &OS) const;
80 
81  /// Get an edge's probability, relative to other out-edges of the Src.
82  ///
83  /// This routine provides access to the fractional probability between zero
84  /// (0%) and one (100%) of this edge executing, relative to other edges
85  /// leaving the 'Src' block. The returned probability is never zero, and can
86  /// only be one if the source block has only one successor.
88  unsigned IndexInSuccessors) const;
89 
90  /// Get the probability of going from Src to Dst.
91  ///
92  /// It returns the sum of all probabilities for edges from Src to Dst.
94  const BasicBlock *Dst) const;
95 
97  succ_const_iterator Dst) const;
98 
99  /// Test if an edge is hot relative to other out-edges of the Src.
100  ///
101  /// Check whether this edge out of the source block is 'hot'. We define hot
102  /// as having a relative probability >= 80%.
103  bool isEdgeHot(const BasicBlock *Src, const BasicBlock *Dst) const;
104 
105  /// Retrieve the hot successor of a block if one exists.
106  ///
107  /// Given a basic block, look through its successors and if one exists for
108  /// which \see isEdgeHot would return true, return that successor block.
109  const BasicBlock *getHotSucc(const BasicBlock *BB) const;
110 
111  /// Print an edge's probability.
112  ///
113  /// Retrieves an edge's probability similarly to \see getEdgeProbability, but
114  /// then prints that probability to the provided stream. That stream is then
115  /// returned.
117  const BasicBlock *Dst) const;
118 
119  /// Set the raw edge probability for the given edge.
120  ///
121  /// This allows a pass to explicitly set the edge probability for an edge. It
122  /// can be used when updating the CFG to update and preserve the branch
123  /// probability information. Read the implementation of how these edge
124  /// probabilities are calculated carefully before using!
125  void setEdgeProbability(const BasicBlock *Src, unsigned IndexInSuccessors,
126  BranchProbability Prob);
127 
129  static const BranchProbability LikelyProb((1u << 20) - 1, 1u << 20);
130  return IsLikely ? LikelyProb : LikelyProb.getCompl();
131  }
132 
133  void calculate(const Function &F, const LoopInfo &LI,
134  const TargetLibraryInfo *TLI = nullptr);
135 
136  /// Forget analysis results for the given basic block.
137  void eraseBlock(const BasicBlock *BB);
138 
139  // Use to track SCCs for handling irreducible loops.
142  using SccHeaderMaps = std::vector<SccHeaderMap>;
143  struct SccInfo {
146  };
147 
148 private:
149  // We need to store CallbackVH's in order to correctly handle basic block
150  // removal.
151  class BasicBlockCallbackVH final : public CallbackVH {
153 
154  void deleted() override {
155  assert(BPI != nullptr);
156  BPI->eraseBlock(cast<BasicBlock>(getValPtr()));
157  BPI->Handles.erase(*this);
158  }
159 
160  public:
161  BasicBlockCallbackVH(const Value *V, BranchProbabilityInfo *BPI = nullptr)
162  : CallbackVH(const_cast<Value *>(V)), BPI(BPI) {}
163  };
164 
166 
167  // Since we allow duplicate edges from one basic block to another, we use
168  // a pair (PredBlock and an index in the successors) to specify an edge.
169  using Edge = std::pair<const BasicBlock *, unsigned>;
170 
171  // Default weight value. Used when we don't have information about the edge.
172  // TODO: DEFAULT_WEIGHT makes sense during static predication, when none of
173  // the successors have a weight yet. But it doesn't make sense when providing
174  // weight to an edge that may have siblings with non-zero weights. This can
175  // be handled various ways, but it's probably fine for an edge with unknown
176  // weight to just "inherit" the non-zero weight of an adjacent successor.
177  static const uint32_t DEFAULT_WEIGHT = 16;
178 
180 
181  /// Track the last function we run over for printing.
182  const Function *LastF;
183 
184  /// Track the set of blocks directly succeeded by a returning block.
185  SmallPtrSet<const BasicBlock *, 16> PostDominatedByUnreachable;
186 
187  /// Track the set of blocks that always lead to a cold call.
188  SmallPtrSet<const BasicBlock *, 16> PostDominatedByColdCall;
189 
190  void updatePostDominatedByUnreachable(const BasicBlock *BB);
191  void updatePostDominatedByColdCall(const BasicBlock *BB);
192  bool calcUnreachableHeuristics(const BasicBlock *BB);
193  bool calcMetadataWeights(const BasicBlock *BB);
194  bool calcColdCallHeuristics(const BasicBlock *BB);
195  bool calcPointerHeuristics(const BasicBlock *BB);
196  bool calcLoopBranchHeuristics(const BasicBlock *BB, const LoopInfo &LI,
197  SccInfo &SccI);
198  bool calcZeroHeuristics(const BasicBlock *BB, const TargetLibraryInfo *TLI);
199  bool calcFloatingPointHeuristics(const BasicBlock *BB);
200  bool calcInvokeHeuristics(const BasicBlock *BB);
201 };
202 
203 /// Analysis pass which computes \c BranchProbabilityInfo.
205  : public AnalysisInfoMixin<BranchProbabilityAnalysis> {
207 
208  static AnalysisKey Key;
209 
210 public:
211  /// Provide the result type for this analysis pass.
213 
214  /// Run the analysis pass over a function and produce BPI.
216 };
217 
218 /// Printer pass for the \c BranchProbabilityAnalysis results.
220  : public PassInfoMixin<BranchProbabilityPrinterPass> {
221  raw_ostream &OS;
222 
223 public:
224  explicit BranchProbabilityPrinterPass(raw_ostream &OS) : OS(OS) {}
225 
227 };
228 
229 /// Legacy analysis pass which computes \c BranchProbabilityInfo.
232 
233 public:
234  static char ID;
235 
239  }
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
BranchProbabilityInfo & operator=(BranchProbabilityInfo &&RHS)
static PassRegistry * getPassRegistry()
getPassRegistry - Access the global registry object, which is automatically initialized at applicatio...
BranchProbability getCompl() const
This class represents lattice values for constants.
Definition: AllocatorList.h:23
BranchProbabilityInfo(const Function &F, const LoopInfo &LI, const TargetLibraryInfo *TLI=nullptr)
A Module instance is used to store all the information related to an LLVM module. ...
Definition: Module.h:65
void calculate(const Function &F, const LoopInfo &LI, const TargetLibraryInfo *TLI=nullptr)
Implements a dense probed hash-table based set.
Definition: DenseSet.h:249
F(f)
Definition: BitVector.h:937
const BasicBlock * getHotSucc(const BasicBlock *BB) const
Retrieve the hot successor of a block if one exists.
std::vector< SccHeaderMap > SccHeaderMaps
Analysis pass which computes BranchProbabilityInfo.
Key
PAL metadata keys.
BranchProbabilityInfo(BranchProbabilityInfo &&Arg)
const BranchProbabilityInfo & getBPI() const
A CRTP mix-in to automatically provide informational APIs needed for passes.
Definition: PassManager.h:365
Legacy analysis pass which computes BranchProbabilityInfo.
static bool runOnFunction(Function &F, bool PostInlining)
Printer pass for the BranchProbabilityAnalysis results.
void initializeBranchProbabilityInfoWrapperPassPass(PassRegistry &)
A set of analyses that are preserved following a run of a transformation pass.
Definition: PassManager.h:153
LLVM Basic Block Representation.
Definition: BasicBlock.h:57
A CRTP mix-in that provides informational APIs needed for analysis passes.
Definition: PassManager.h:382
void eraseBlock(const BasicBlock *BB)
Forget analysis results for the given basic block.
Represent the analysis usage information of a pass.
FunctionPass class - This class is used to implement most global optimizations.
Definition: Pass.h:284
amdgpu Simplify well known AMD library false FunctionCallee Value * Arg
BranchProbability getEdgeProbability(const BasicBlock *Src, unsigned IndexInSuccessors) const
Get an edge&#39;s probability, relative to other out-edges of the Src.
SmallPtrSet - This class implements a set which is optimized for holding SmallSize or less elements...
Definition: SmallPtrSet.h:417
Provides information about what library functions are available for the current target.
void setEdgeProbability(const BasicBlock *Src, unsigned IndexInSuccessors, BranchProbability Prob)
Set the raw edge probability for the given edge.
void print(raw_ostream &OS) const
This file provides various utilities for inspecting and working with the control flow graph in LLVM I...
Analysis providing branch probability information.
assert(ImpDefSCC.getReg()==AMDGPU::SCC &&ImpDefSCC.isDef())
LLVM Value Representation.
Definition: Value.h:72
bool isEdgeHot(const BasicBlock *Src, const BasicBlock *Dst) const
Test if an edge is hot relative to other out-edges of the Src.
raw_ostream & printEdgeProbability(raw_ostream &OS, const BasicBlock *Src, const BasicBlock *Dst) const
Print an edge&#39;s probability.
BranchProbabilityInfo & operator=(const BranchProbabilityInfo &)=delete
This class implements an extremely fast bulk output stream that can only output to a stream...
Definition: raw_ostream.h:45
Value handle with callbacks on RAUW and destruction.
Definition: ValueHandle.h:379
A container for analyses that lazily runs them and caches their results.
This header defines various interfaces for pass management in LLVM.
A special type used by analysis passes to provide an address that identifies that particular analysis...
Definition: PassManager.h:70
static BranchProbability getBranchProbStackProtector(bool IsLikely)