include/llvm/Analysis/BranchProbabilityInfo.h

   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