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: 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 /// Compute alive bits of one addition operand from alive output and known 65 /// operand bits 66 static APInt determineLiveOperandBitsAdd(unsigned OperandNo, 67 const APInt &AOut, 68 const KnownBits &LHS, 69 const KnownBits &RHS); 70 71 /// Compute alive bits of one subtraction operand from alive output and known 72 /// operand bits 73 static APInt determineLiveOperandBitsSub(unsigned OperandNo, 74 const APInt &AOut, 75 const KnownBits &LHS, 76 const KnownBits &RHS); 77 78 private: 79 void performAnalysis(); 80 void determineLiveOperandBits(const Instruction *UserI, 81 const Value *Val, unsigned OperandNo, 82 const APInt &AOut, APInt &AB, 83 KnownBits &Known, KnownBits &Known2, bool &KnownBitsComputed); 84 85 Function &F; 86 AssumptionCache &AC; 87 DominatorTree &DT; 88 89 bool Analyzed = false; 90 91 // The set of visited instructions (non-integer-typed only). 92 SmallPtrSet<Instruction*, 32> Visited; 93 DenseMap<Instruction *, APInt> AliveBits; 94 // Uses with no demanded bits. If the user also has no demanded bits, the use 95 // might not be stored explicitly in this map, to save memory during analysis. 96 SmallPtrSet<Use *, 16> DeadUses; 97 }; 98 99 class DemandedBitsWrapperPass : public FunctionPass { 100 private: 101 mutable Optional<DemandedBits> DB; 102 103 public: 104 static char ID; // Pass identification, replacement for typeid 105 106 DemandedBitsWrapperPass(); 107 108 bool runOnFunction(Function &F) override; 109 void getAnalysisUsage(AnalysisUsage &AU) const override; 110 111 /// Clean up memory in between runs 112 void releaseMemory() override; 113 114 DemandedBits &getDemandedBits() { return *DB; } 115 116 void print(raw_ostream &OS, const Module *M) const override; 117 }; 118 119 /// An analysis that produces \c DemandedBits for a function. 120 class DemandedBitsAnalysis : public AnalysisInfoMixin<DemandedBitsAnalysis> { 121 friend AnalysisInfoMixin<DemandedBitsAnalysis>; 122 123 static AnalysisKey Key; 124 125 public: 126 /// Provide the result type for this analysis pass. 127 using Result = DemandedBits; 128 129 /// Run the analysis pass over a function and produce demanded bits 130 /// information. 131 DemandedBits run(Function &F, FunctionAnalysisManager &AM); 132 }; 133 134 /// Printer pass for DemandedBits 135 class DemandedBitsPrinterPass : public PassInfoMixin<DemandedBitsPrinterPass> { 136 raw_ostream &OS; 137 138 public: 139 explicit DemandedBitsPrinterPass(raw_ostream &OS) : OS(OS) {} 140 141 PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM); 142 }; 143 144 /// Create a demanded bits analysis pass. 145 FunctionPass *createDemandedBitsWrapperPass(); 146 147 } // end namespace llvm 148 149 #endif // LLVM_ANALYSIS_DEMANDED_BITS_H 150