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