1 //===- llvm/Analysis/DemandedBits.h - Determine demanded bits ---*- 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 implements a demanded bits analysis. A demanded bit is one that 10 // contributes to a result; bits that are not demanded can be either zero or 11 // one without affecting control or data flow. For example in this sequence: 12 // 13 // %1 = add i32 %x, %y 14 // %2 = trunc i32 %1 to i16 15 // 16 // Only the lowest 16 bits of %1 are demanded; the rest are removed by the 17 // trunc. 18 // 19 //===----------------------------------------------------------------------===// 20 21 #ifndef LLVM_ANALYSIS_DEMANDED_BITS_H 22 #define LLVM_ANALYSIS_DEMANDED_BITS_H 23 24 #include "llvm/ADT/APInt.h" 25 #include "llvm/ADT/DenseMap.h" 26 #include "llvm/ADT/Optional.h" 27 #include "llvm/ADT/SmallPtrSet.h" 28 #include "llvm/IR/PassManager.h" 29 #include "llvm/Pass.h" 30 31 namespace llvm { 32 33 class AssumptionCache; 34 class DominatorTree; 35 class Function; 36 class Instruction; 37 struct KnownBits; 38 class raw_ostream; 39 40 class DemandedBits { 41 public: DemandedBits(Function & F,AssumptionCache & AC,DominatorTree & DT)42 DemandedBits(Function &F, AssumptionCache &AC, DominatorTree &DT) : 43 F(F), AC(AC), DT(DT) {} 44 45 /// Return the bits demanded from instruction I. 46 /// 47 /// For vector instructions individual vector elements are not distinguished: 48 /// A bit is demanded if it is demanded for any of the vector elements. The 49 /// size of the return value corresponds to the type size in bits of the 50 /// scalar type. 51 /// 52 /// Instructions that do not have integer or vector of integer type are 53 /// accepted, but will always produce a mask with all bits set. 54 APInt getDemandedBits(Instruction *I); 55 56 /// Return true if, during analysis, I could not be reached. 57 bool isInstructionDead(Instruction *I); 58 59 /// Return whether this use is dead by means of not having any demanded bits. 60 bool isUseDead(Use *U); 61 62 void print(raw_ostream &OS); 63 64 private: 65 void performAnalysis(); 66 void determineLiveOperandBits(const Instruction *UserI, 67 const Value *Val, unsigned OperandNo, 68 const APInt &AOut, APInt &AB, 69 KnownBits &Known, KnownBits &Known2, bool &KnownBitsComputed); 70 71 Function &F; 72 AssumptionCache &AC; 73 DominatorTree &DT; 74 75 bool Analyzed = false; 76 77 // The set of visited instructions (non-integer-typed only). 78 SmallPtrSet<Instruction*, 32> Visited; 79 DenseMap<Instruction *, APInt> AliveBits; 80 // Uses with no demanded bits. If the user also has no demanded bits, the use 81 // might not be stored explicitly in this map, to save memory during analysis. 82 SmallPtrSet<Use *, 16> DeadUses; 83 }; 84 85 class DemandedBitsWrapperPass : public FunctionPass { 86 private: 87 mutable Optional<DemandedBits> DB; 88 89 public: 90 static char ID; // Pass identification, replacement for typeid 91 92 DemandedBitsWrapperPass(); 93 94 bool runOnFunction(Function &F) override; 95 void getAnalysisUsage(AnalysisUsage &AU) const override; 96 97 /// Clean up memory in between runs 98 void releaseMemory() override; 99 getDemandedBits()100 DemandedBits &getDemandedBits() { return *DB; } 101 102 void print(raw_ostream &OS, const Module *M) const override; 103 }; 104 105 /// An analysis that produces \c DemandedBits for a function. 106 class DemandedBitsAnalysis : public AnalysisInfoMixin<DemandedBitsAnalysis> { 107 friend AnalysisInfoMixin<DemandedBitsAnalysis>; 108 109 static AnalysisKey Key; 110 111 public: 112 /// Provide the result type for this analysis pass. 113 using Result = DemandedBits; 114 115 /// Run the analysis pass over a function and produce demanded bits 116 /// information. 117 DemandedBits run(Function &F, FunctionAnalysisManager &AM); 118 }; 119 120 /// Printer pass for DemandedBits 121 class DemandedBitsPrinterPass : public PassInfoMixin<DemandedBitsPrinterPass> { 122 raw_ostream &OS; 123 124 public: DemandedBitsPrinterPass(raw_ostream & OS)125 explicit DemandedBitsPrinterPass(raw_ostream &OS) : OS(OS) {} 126 127 PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM); 128 }; 129 130 /// Create a demanded bits analysis pass. 131 FunctionPass *createDemandedBitsWrapperPass(); 132 133 } // end namespace llvm 134 135 #endif // LLVM_ANALYSIS_DEMANDED_BITS_H 136