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 "llvm/Pass.h"
29#include <optional>
30
31namespace llvm {
32
33class AssumptionCache;
34class DominatorTree;
35class Function;
36class Instruction;
37struct KnownBits;
38class raw_ostream;
39
40class DemandedBits {
41public:
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
81private:
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
102class DemandedBitsWrapperPass : public FunctionPass {
103private:
104  mutable std::optional<DemandedBits> DB;
105
106public:
107  static char ID; // Pass identification, replacement for typeid
108
109  DemandedBitsWrapperPass();
110
111  bool runOnFunction(Function &F) override;
112  void getAnalysisUsage(AnalysisUsage &AU) const override;
113
114  /// Clean up memory in between runs
115  void releaseMemory() override;
116
117  DemandedBits &getDemandedBits() { return *DB; }
118
119  void print(raw_ostream &OS, const Module *M) const override;
120};
121
122/// An analysis that produces \c DemandedBits for a function.
123class DemandedBitsAnalysis : public AnalysisInfoMixin<DemandedBitsAnalysis> {
124  friend AnalysisInfoMixin<DemandedBitsAnalysis>;
125
126  static AnalysisKey Key;
127
128public:
129  /// Provide the result type for this analysis pass.
130  using Result = DemandedBits;
131
132  /// Run the analysis pass over a function and produce demanded bits
133  /// information.
134  DemandedBits run(Function &F, FunctionAnalysisManager &AM);
135};
136
137/// Printer pass for DemandedBits
138class DemandedBitsPrinterPass : public PassInfoMixin<DemandedBitsPrinterPass> {
139  raw_ostream &OS;
140
141public:
142  explicit DemandedBitsPrinterPass(raw_ostream &OS) : OS(OS) {}
143
144  PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM);
145};
146
147/// Create a demanded bits analysis pass.
148FunctionPass *createDemandedBitsWrapperPass();
149
150} // end namespace llvm
151
152#endif // LLVM_ANALYSIS_DEMANDEDBITS_H
153