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BranchProbabilityInfo.h
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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"
27 #include "llvm/Support/Casting.h"
28 #include <algorithm>
29 #include <cassert>
30 #include <cstdint>
31 #include <utility>
32 
33 namespace llvm {
34 
35 class Function;
36 class LoopInfo;
37 class raw_ostream;
38 class TargetLibraryInfo;
39 class 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.
54 public:
55  BranchProbabilityInfo() = default;
56 
58  const TargetLibraryInfo *TLI = nullptr) {
59  calculate(F, LI, TLI);
60  }
61 
63  : Probs(std::move(Arg.Probs)), LastF(Arg.LastF),
64  PostDominatedByUnreachable(std::move(Arg.PostDominatedByUnreachable)),
65  PostDominatedByColdCall(std::move(Arg.PostDominatedByColdCall)) {}
66 
69 
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.
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.
95  const BasicBlock *Dst) const;
96 
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.
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 
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.
143  using SccHeaderMaps = std::vector<SccHeaderMap>;
144  struct SccInfo {
147  };
148 
149 private:
150  // We need to store CallbackVH's in order to correctly handle basic block
151  // removal.
152  class BasicBlockCallbackVH final : public CallbackVH {
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 
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 
181 
182  /// \brief Track the last function we run over for printing.
183  const Function *LastF;
184 
185  /// \brief Track the set of blocks directly succeeded by a returning block.
186  SmallPtrSet<const BasicBlock *, 16> PostDominatedByUnreachable;
187 
188  /// \brief Track the set of blocks that always lead to a cold call.
189  SmallPtrSet<const BasicBlock *, 16> PostDominatedByColdCall;
190 
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,
198  SccInfo &SccI);
199  bool calcZeroHeuristics(const BasicBlock *BB, const TargetLibraryInfo *TLI);
200  bool calcFloatingPointHeuristics(const BasicBlock *BB);
201  bool calcInvokeHeuristics(const BasicBlock *BB);
202 };
203 
204 /// \brief Analysis pass which computes \c BranchProbabilityInfo.
206  : public AnalysisInfoMixin<BranchProbabilityAnalysis> {
208 
209  static AnalysisKey Key;
210 
211 public:
212  /// \brief Provide the result type for this analysis pass.
214 
215  /// \brief Run the analysis pass over a function and produce BPI.
217 };
218 
219 /// \brief Printer pass for the \c BranchProbabilityAnalysis results.
221  : public PassInfoMixin<BranchProbabilityPrinterPass> {
222  raw_ostream &OS;
223 
224 public:
225  explicit BranchProbabilityPrinterPass(raw_ostream &OS) : OS(OS) {}
226 
228 };
229 
230 /// \brief Legacy analysis pass which computes \c BranchProbabilityInfo.
233 
234 public:
235  static char ID;
236 
240  }
241 
242  BranchProbabilityInfo &getBPI() { return BPI; }
243  const BranchProbabilityInfo &getBPI() const { return BPI; }
244 
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;
249 };
250 
251 } // end namespace llvm
252 
253 #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
Compute iterated dominance frontiers using a linear time algorithm.
Definition: AllocatorList.h:24
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:63
void calculate(const Function &F, const LoopInfo &LI, const TargetLibraryInfo *TLI=nullptr)
Implements a dense probed hash-table based set.
Definition: DenseSet.h:221
F(f)
Definition: BitVector.h:920
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:59
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:285
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:418
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
amdgpu Simplify well known AMD library false Value Value * Arg
Analysis providing branch probability information.
assert(ImpDefSCC.getReg()==AMDGPU::SCC &&ImpDefSCC.isDef())
LLVM Value Representation.
Definition: Value.h:73
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:44
Value handle with callbacks on RAUW and destruction.
Definition: ValueHandle.h:389
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)