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 
32 namespace llvm {
33 
34 class Function;
35 class LoopInfo;
36 class raw_ostream;
37 class PostDominatorTree;
38 class TargetLibraryInfo;
39 class 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.
53 class BranchProbabilityInfo {
54 public:
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 
149 private:
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.
207 class BranchProbabilityAnalysis
208     : public AnalysisInfoMixin<BranchProbabilityAnalysis> {
209   friend AnalysisInfoMixin<BranchProbabilityAnalysis>;
210 
211   static AnalysisKey Key;
212 
213 public:
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.
222 class BranchProbabilityPrinterPass
223     : public PassInfoMixin<BranchProbabilityPrinterPass> {
224   raw_ostream &OS;
225 
226 public:
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.
233 class BranchProbabilityInfoWrapperPass : public FunctionPass {
234   BranchProbabilityInfo BPI;
235 
236 public:
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