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_DEMANDEDBITS_H 22 #define LLVM_ANALYSIS_DEMANDEDBITS_H 23 24 #include "llvm/ADT/APInt.h" 25 #include "llvm/ADT/DenseMap.h" 26 #include "llvm/ADT/SmallPtrSet.h" 27 #include "llvm/IR/PassManager.h" 28 29 namespace llvm { 30 31 class AssumptionCache; 32 class DominatorTree; 33 class Function; 34 class Instruction; 35 struct KnownBits; 36 class raw_ostream; 37 class Use; 38 class Value; 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 the bits demanded from use U. 57 APInt getDemandedBits(Use *U); 58 59 /// Return true if, during analysis, I could not be reached. 60 bool isInstructionDead(Instruction *I); 61 62 /// Return whether this use is dead by means of not having any demanded bits. 63 bool isUseDead(Use *U); 64 65 void print(raw_ostream &OS); 66 67 /// Compute alive bits of one addition operand from alive output and known 68 /// operand bits 69 static APInt determineLiveOperandBitsAdd(unsigned OperandNo, 70 const APInt &AOut, 71 const KnownBits &LHS, 72 const KnownBits &RHS); 73 74 /// Compute alive bits of one subtraction operand from alive output and known 75 /// operand bits 76 static APInt determineLiveOperandBitsSub(unsigned OperandNo, 77 const APInt &AOut, 78 const KnownBits &LHS, 79 const KnownBits &RHS); 80 81 private: 82 void performAnalysis(); 83 void determineLiveOperandBits(const Instruction *UserI, 84 const Value *Val, unsigned OperandNo, 85 const APInt &AOut, APInt &AB, 86 KnownBits &Known, KnownBits &Known2, bool &KnownBitsComputed); 87 88 Function &F; 89 AssumptionCache &AC; 90 DominatorTree &DT; 91 92 bool Analyzed = false; 93 94 // The set of visited instructions (non-integer-typed only). 95 SmallPtrSet<Instruction*, 32> Visited; 96 DenseMap<Instruction *, APInt> AliveBits; 97 // Uses with no demanded bits. If the user also has no demanded bits, the use 98 // might not be stored explicitly in this map, to save memory during analysis. 99 SmallPtrSet<Use *, 16> DeadUses; 100 }; 101 102 /// An analysis that produces \c DemandedBits for a function. 103 class DemandedBitsAnalysis : public AnalysisInfoMixin<DemandedBitsAnalysis> { 104 friend AnalysisInfoMixin<DemandedBitsAnalysis>; 105 106 static AnalysisKey Key; 107 108 public: 109 /// Provide the result type for this analysis pass. 110 using Result = DemandedBits; 111 112 /// Run the analysis pass over a function and produce demanded bits 113 /// information. 114 DemandedBits run(Function &F, FunctionAnalysisManager &AM); 115 }; 116 117 /// Printer pass for DemandedBits 118 class DemandedBitsPrinterPass : public PassInfoMixin<DemandedBitsPrinterPass> { 119 raw_ostream &OS; 120 121 public: DemandedBitsPrinterPass(raw_ostream & OS)122 explicit DemandedBitsPrinterPass(raw_ostream &OS) : OS(OS) {} 123 124 PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM); 125 isRequired()126 static bool isRequired() { return true; } 127 }; 128 129 } // end namespace llvm 130 131 #endif // LLVM_ANALYSIS_DEMANDEDBITS_H 132