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      1 //===-- llvm/Analysis/DemandedBits.h - Determine demanded bits --*- 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 pass implements a demanded bits analysis. A demanded bit is one that
     11 // contributes to a result; bits that are not demanded can be either zero or
     12 // one without affecting control or data flow. For example in this sequence:
     13 //
     14 //   %1 = add i32 %x, %y
     15 //   %2 = trunc i32 %1 to i16
     16 //
     17 // Only the lowest 16 bits of %1 are demanded; the rest are removed by the
     18 // trunc.
     19 //
     20 //===----------------------------------------------------------------------===//
     21 
     22 #ifndef LLVM_ANALYSIS_DEMANDED_BITS_H
     23 #define LLVM_ANALYSIS_DEMANDED_BITS_H
     24 
     25 #include "llvm/Pass.h"
     26 #include "llvm/ADT/APInt.h"
     27 #include "llvm/ADT/DenseMap.h"
     28 #include "llvm/ADT/SmallPtrSet.h"
     29 #include "llvm/IR/PassManager.h"
     30 
     31 namespace llvm {
     32 
     33 class FunctionPass;
     34 class Function;
     35 class Instruction;
     36 class DominatorTree;
     37 class AssumptionCache;
     38 
     39 class DemandedBits {
     40 public:
     41   DemandedBits(Function &F, AssumptionCache &AC, DominatorTree &DT) :
     42     F(F), AC(AC), DT(DT), Analyzed(false) {}
     43 
     44   /// Return the bits demanded from instruction I.
     45   APInt getDemandedBits(Instruction *I);
     46 
     47   /// Return true if, during analysis, I could not be reached.
     48   bool isInstructionDead(Instruction *I);
     49 
     50   void print(raw_ostream &OS);
     51 
     52 private:
     53   Function &F;
     54   AssumptionCache ∾
     55   DominatorTree &DT;
     56 
     57   void performAnalysis();
     58   void determineLiveOperandBits(const Instruction *UserI,
     59     const Instruction *I, unsigned OperandNo,
     60     const APInt &AOut, APInt &AB,
     61     APInt &KnownZero, APInt &KnownOne,
     62     APInt &KnownZero2, APInt &KnownOne2);
     63 
     64   bool Analyzed;
     65 
     66   // The set of visited instructions (non-integer-typed only).
     67   SmallPtrSet<Instruction*, 32> Visited;
     68   DenseMap<Instruction *, APInt> AliveBits;
     69 };
     70 
     71 class DemandedBitsWrapperPass : public FunctionPass {
     72 private:
     73   mutable Optional<DemandedBits> DB;
     74 public:
     75   static char ID; // Pass identification, replacement for typeid
     76   DemandedBitsWrapperPass();
     77 
     78   bool runOnFunction(Function &F) override;
     79   void getAnalysisUsage(AnalysisUsage &AU) const override;
     80 
     81   /// Clean up memory in between runs
     82   void releaseMemory() override;
     83 
     84   DemandedBits &getDemandedBits() { return *DB; }
     85 
     86   void print(raw_ostream &OS, const Module *M) const override;
     87 };
     88 
     89 /// An analysis that produces \c DemandedBits for a function.
     90 class DemandedBitsAnalysis : public AnalysisInfoMixin<DemandedBitsAnalysis> {
     91   friend AnalysisInfoMixin<DemandedBitsAnalysis>;
     92   static char PassID;
     93 
     94 public:
     95   /// \brief Provide the result typedef for this analysis pass.
     96   typedef DemandedBits Result;
     97 
     98   /// \brief Run the analysis pass over a function and produce demanded bits
     99   /// information.
    100   DemandedBits run(Function &F, AnalysisManager<Function> &AM);
    101 };
    102 
    103 /// \brief Printer pass for DemandedBits
    104 class DemandedBitsPrinterPass : public PassInfoMixin<DemandedBitsPrinterPass> {
    105   raw_ostream &OS;
    106 
    107 public:
    108   explicit DemandedBitsPrinterPass(raw_ostream &OS) : OS(OS) {}
    109   PreservedAnalyses run(Function &F, AnalysisManager<Function> &AM);
    110 };
    111 
    112 /// Create a demanded bits analysis pass.
    113 FunctionPass *createDemandedBitsWrapperPass();
    114 
    115 } // End llvm namespace
    116 
    117 #endif
    118