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      1 //===- LazyBranchProbabilityInfo.h - Lazy Branch Probability ----*- 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 is an alternative analysis pass to BranchProbabilityInfoWrapperPass.
     11 // The difference is that with this pass the branch probabilities are not
     12 // computed when the analysis pass is executed but rather when the BPI results
     13 // is explicitly requested by the analysis client.
     14 //
     15 //===----------------------------------------------------------------------===//
     16 
     17 #ifndef LLVM_ANALYSIS_LAZYBRANCHPROBABILITYINFO_H
     18 #define LLVM_ANALYSIS_LAZYBRANCHPROBABILITYINFO_H
     19 
     20 #include "llvm/Analysis/BranchProbabilityInfo.h"
     21 #include "llvm/Pass.h"
     22 
     23 namespace llvm {
     24 class AnalysisUsage;
     25 class Function;
     26 class LoopInfo;
     27 class TargetLibraryInfo;
     28 
     29 /// \brief This is an alternative analysis pass to
     30 /// BranchProbabilityInfoWrapperPass.  The difference is that with this pass the
     31 /// branch probabilities are not computed when the analysis pass is executed but
     32 /// rather when the BPI results is explicitly requested by the analysis client.
     33 ///
     34 /// There are some additional requirements for any client pass that wants to use
     35 /// the analysis:
     36 ///
     37 /// 1. The pass needs to initialize dependent passes with:
     38 ///
     39 ///   INITIALIZE_PASS_DEPENDENCY(LazyBPIPass)
     40 ///
     41 /// 2. Similarly, getAnalysisUsage should call:
     42 ///
     43 ///   LazyBranchProbabilityInfoPass::getLazyBPIAnalysisUsage(AU)
     44 ///
     45 /// 3. The computed BPI should be requested with
     46 ///    getAnalysis<LazyBranchProbabilityInfoPass>().getBPI() before LoopInfo
     47 ///    could be invalidated for example by changing the CFG.
     48 ///
     49 /// Note that it is expected that we wouldn't need this functionality for the
     50 /// new PM since with the new PM, analyses are executed on demand.
     51 class LazyBranchProbabilityInfoPass : public FunctionPass {
     52 
     53   /// Wraps a BPI to allow lazy computation of the branch probabilities.
     54   ///
     55   /// A pass that only conditionally uses BPI can uncondtionally require the
     56   /// analysis without paying for the overhead if BPI doesn't end up being used.
     57   class LazyBranchProbabilityInfo {
     58   public:
     59     LazyBranchProbabilityInfo(const Function *F, const LoopInfo *LI,
     60                               const TargetLibraryInfo *TLI)
     61         : Calculated(false), F(F), LI(LI), TLI(TLI) {}
     62 
     63     /// Retrieve the BPI with the branch probabilities computed.
     64     BranchProbabilityInfo &getCalculated() {
     65       if (!Calculated) {
     66         assert(F && LI && "call setAnalysis");
     67         BPI.calculate(*F, *LI, TLI);
     68         Calculated = true;
     69       }
     70       return BPI;
     71     }
     72 
     73     const BranchProbabilityInfo &getCalculated() const {
     74       return const_cast<LazyBranchProbabilityInfo *>(this)->getCalculated();
     75     }
     76 
     77   private:
     78     BranchProbabilityInfo BPI;
     79     bool Calculated;
     80     const Function *F;
     81     const LoopInfo *LI;
     82     const TargetLibraryInfo *TLI;
     83   };
     84 
     85   std::unique_ptr<LazyBranchProbabilityInfo> LBPI;
     86 
     87 public:
     88   static char ID;
     89 
     90   LazyBranchProbabilityInfoPass();
     91 
     92   /// \brief Compute and return the branch probabilities.
     93   BranchProbabilityInfo &getBPI() { return LBPI->getCalculated(); }
     94 
     95   /// \brief Compute and return the branch probabilities.
     96   const BranchProbabilityInfo &getBPI() const { return LBPI->getCalculated(); }
     97 
     98   void getAnalysisUsage(AnalysisUsage &AU) const override;
     99 
    100   /// Helper for client passes to set up the analysis usage on behalf of this
    101   /// pass.
    102   static void getLazyBPIAnalysisUsage(AnalysisUsage &AU);
    103 
    104   bool runOnFunction(Function &F) override;
    105   void releaseMemory() override;
    106   void print(raw_ostream &OS, const Module *M) const override;
    107 };
    108 
    109 /// \brief Helper for client passes to initialize dependent passes for LBPI.
    110 void initializeLazyBPIPassPass(PassRegistry &Registry);
    111 
    112 /// \brief Simple trait class that provides a mapping between BPI passes and the
    113 /// corresponding BPInfo.
    114 template <typename PassT> struct BPIPassTrait {
    115   static PassT &getBPI(PassT *P) { return *P; }
    116 };
    117 
    118 template <> struct BPIPassTrait<LazyBranchProbabilityInfoPass> {
    119   static BranchProbabilityInfo &getBPI(LazyBranchProbabilityInfoPass *P) {
    120     return P->getBPI();
    121   }
    122 };
    123 }
    124 #endif
    125