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 #include <optional> 29 30 namespace llvm { 31 32 class AssumptionCache; 33 class DominatorTree; 34 class Function; 35 class Instruction; 36 struct KnownBits; 37 class raw_ostream; 38 39 class DemandedBits { 40 public: 41 DemandedBits(Function &F, AssumptionCache &AC, DominatorTree &DT) : 42 F(F), AC(AC), DT(DT) {} 43 44 /// Return the bits demanded from instruction I. 45 /// 46 /// For vector instructions individual vector elements are not distinguished: 47 /// A bit is demanded if it is demanded for any of the vector elements. The 48 /// size of the return value corresponds to the type size in bits of the 49 /// scalar type. 50 /// 51 /// Instructions that do not have integer or vector of integer type are 52 /// accepted, but will always produce a mask with all bits set. 53 APInt getDemandedBits(Instruction *I); 54 55 /// Return the bits demanded from use U. 56 APInt getDemandedBits(Use *U); 57 58 /// Return true if, during analysis, I could not be reached. 59 bool isInstructionDead(Instruction *I); 60 61 /// Return whether this use is dead by means of not having any demanded bits. 62 bool isUseDead(Use *U); 63 64 void print(raw_ostream &OS); 65 66 /// Compute alive bits of one addition operand from alive output and known 67 /// operand bits 68 static APInt determineLiveOperandBitsAdd(unsigned OperandNo, 69 const APInt &AOut, 70 const KnownBits &LHS, 71 const KnownBits &RHS); 72 73 /// Compute alive bits of one subtraction operand from alive output and known 74 /// operand bits 75 static APInt determineLiveOperandBitsSub(unsigned OperandNo, 76 const APInt &AOut, 77 const KnownBits &LHS, 78 const KnownBits &RHS); 79 80 private: 81 void performAnalysis(); 82 void determineLiveOperandBits(const Instruction *UserI, 83 const Value *Val, unsigned OperandNo, 84 const APInt &AOut, APInt &AB, 85 KnownBits &Known, KnownBits &Known2, bool &KnownBitsComputed); 86 87 Function &F; 88 AssumptionCache &AC; 89 DominatorTree &DT; 90 91 bool Analyzed = false; 92 93 // The set of visited instructions (non-integer-typed only). 94 SmallPtrSet<Instruction*, 32> Visited; 95 DenseMap<Instruction *, APInt> AliveBits; 96 // Uses with no demanded bits. If the user also has no demanded bits, the use 97 // might not be stored explicitly in this map, to save memory during analysis. 98 SmallPtrSet<Use *, 16> DeadUses; 99 }; 100 101 /// An analysis that produces \c DemandedBits for a function. 102 class DemandedBitsAnalysis : public AnalysisInfoMixin<DemandedBitsAnalysis> { 103 friend AnalysisInfoMixin<DemandedBitsAnalysis>; 104 105 static AnalysisKey Key; 106 107 public: 108 /// Provide the result type for this analysis pass. 109 using Result = DemandedBits; 110 111 /// Run the analysis pass over a function and produce demanded bits 112 /// information. 113 DemandedBits run(Function &F, FunctionAnalysisManager &AM); 114 }; 115 116 /// Printer pass for DemandedBits 117 class DemandedBitsPrinterPass : public PassInfoMixin<DemandedBitsPrinterPass> { 118 raw_ostream &OS; 119 120 public: 121 explicit DemandedBitsPrinterPass(raw_ostream &OS) : OS(OS) {} 122 123 PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM); 124 }; 125 126 } // end namespace llvm 127 128 #endif // LLVM_ANALYSIS_DEMANDEDBITS_H 129