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 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.
52 class BranchProbabilityInfo {
53 public:
54   BranchProbabilityInfo() = default;
55 
56   BranchProbabilityInfo(const Function &F, const LoopInfo &LI,
57                         const TargetLibraryInfo *TLI = nullptr) {
58     calculate(F, LI, TLI);
59   }
60 
61   BranchProbabilityInfo(BranchProbabilityInfo &&Arg)
62       : Probs(std::move(Arg.Probs)), LastF(Arg.LastF),
63         PostDominatedByUnreachable(std::move(Arg.PostDominatedByUnreachable)),
64         PostDominatedByColdCall(std::move(Arg.PostDominatedByColdCall)) {}
65 
66   BranchProbabilityInfo(const BranchProbabilityInfo &) = delete;
67   BranchProbabilityInfo &operator=(const BranchProbabilityInfo &) = delete;
68 
69   BranchProbabilityInfo &operator=(BranchProbabilityInfo &&RHS) {
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.
87   BranchProbability getEdgeProbability(const BasicBlock *Src,
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.
93   BranchProbability getEdgeProbability(const BasicBlock *Src,
94                                        const BasicBlock *Dst) const;
95 
96   BranchProbability getEdgeProbability(const BasicBlock *Src,
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.
116   raw_ostream &printEdgeProbability(raw_ostream &OS, const BasicBlock *Src,
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 
128   static BranchProbability getBranchProbStackProtector(bool IsLikely) {
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.
140   using SccMap = DenseMap<const BasicBlock *, int>;
141   using SccHeaderMap = DenseMap<const BasicBlock *, bool>;
142   using SccHeaderMaps = std::vector<SccHeaderMap>;
143   struct SccInfo {
144     SccMap SccNums;
145     SccHeaderMaps SccHeaders;
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 {
152     BranchProbabilityInfo *BPI;
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 
165   DenseSet<BasicBlockCallbackVH, DenseMapInfo<Value*>> Handles;
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 
179   DenseMap<Edge, BranchProbability> Probs;
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.
204 class BranchProbabilityAnalysis
205     : public AnalysisInfoMixin<BranchProbabilityAnalysis> {
206   friend AnalysisInfoMixin<BranchProbabilityAnalysis>;
207 
208   static AnalysisKey Key;
209 
210 public:
211   /// Provide the result type for this analysis pass.
212   using Result = BranchProbabilityInfo;
213 
214   /// Run the analysis pass over a function and produce BPI.
215   BranchProbabilityInfo run(Function &F, FunctionAnalysisManager &AM);
216 };
217 
218 /// Printer pass for the \c BranchProbabilityAnalysis results.
219 class BranchProbabilityPrinterPass
220     : public PassInfoMixin<BranchProbabilityPrinterPass> {
221   raw_ostream &OS;
222 
223 public:
224   explicit BranchProbabilityPrinterPass(raw_ostream &OS) : OS(OS) {}
225 
226   PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM);
227 };
228 
229 /// Legacy analysis pass which computes \c BranchProbabilityInfo.
230 class BranchProbabilityInfoWrapperPass : public FunctionPass {
231   BranchProbabilityInfo BPI;
232 
233 public:
234   static char ID;
235 
236   BranchProbabilityInfoWrapperPass() : FunctionPass(ID) {
237     initializeBranchProbabilityInfoWrapperPassPass(
238         *PassRegistry::getPassRegistry());
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
253