DeadArgumentElimination.h revision de2d8694e25a814696358e95141f4b1aa4d8847e 
1//===- DeadArgumentElimination.h - Eliminate Dead Args ----------*- C++ -*-===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
10// This pass deletes dead arguments from internal functions.  Dead argument
11// elimination removes arguments which are directly dead, as well as arguments
12// only passed into function calls as dead arguments of other functions.  This
13// pass also deletes dead return values in a similar way.
14//
15// This pass is often useful as a cleanup pass to run after aggressive
16// interprocedural passes, which add possibly-dead arguments or return values.
17//
18//===----------------------------------------------------------------------===//
19
20#ifndef LLVM_TRANSFORMS_IPO_DEADARGUMENTELIMINATION_H
21#define LLVM_TRANSFORMS_IPO_DEADARGUMENTELIMINATION_H
22
23#include "llvm/IR/Module.h"
24#include "llvm/IR/PassManager.h"
25
26#include <map>
27#include <set>
28#include <string>
29
30namespace llvm {
31
32/// Eliminate dead arguments (and return values) from functions.
33class DeadArgumentEliminationPass
34    : public PassInfoMixin<DeadArgumentEliminationPass> {
35public:
36  /// Struct that represents (part of) either a return value or a function
37  /// argument.  Used so that arguments and return values can be used
38  /// interchangeably.
39  struct RetOrArg {
40    RetOrArg(const Function *F, unsigned Idx, bool IsArg)
41        : F(F), Idx(Idx), IsArg(IsArg) {}
42    const Function *F;
43    unsigned Idx;
44    bool IsArg;
45
46    /// Make RetOrArg comparable, so we can put it into a map.
47    bool operator<(const RetOrArg &O) const {
48      return std::tie(F, Idx, IsArg) < std::tie(O.F, O.Idx, O.IsArg);
49    }
50
51    /// Make RetOrArg comparable, so we can easily iterate the multimap.
52    bool operator==(const RetOrArg &O) const {
53      return F == O.F && Idx == O.Idx && IsArg == O.IsArg;
54    }
55
56    std::string getDescription() const {
57      return (Twine(IsArg ? "Argument #" : "Return value #") + Twine(Idx) +
58              " of function " + F->getName())
59          .str();
60    }
61  };
62
63  /// Liveness enum - During our initial pass over the program, we determine
64  /// that things are either alive or maybe alive. We don't mark anything
65  /// explicitly dead (even if we know they are), since anything not alive
66  /// with no registered uses (in Uses) will never be marked alive and will
67  /// thus become dead in the end.
68  enum Liveness { Live, MaybeLive };
69
70  /// Convenience wrapper
71  RetOrArg CreateRet(const Function *F, unsigned Idx) {
72    return RetOrArg(F, Idx, false);
73  }
74  /// Convenience wrapper
75  RetOrArg CreateArg(const Function *F, unsigned Idx) {
76    return RetOrArg(F, Idx, true);
77  }
78
79  typedef std::multimap<RetOrArg, RetOrArg> UseMap;
80  /// This maps a return value or argument to any MaybeLive return values or
81  /// arguments it uses. This allows the MaybeLive values to be marked live
82  /// when any of its users is marked live.
83  /// For example (indices are left out for clarity):
84  ///  - Uses[ret F] = ret G
85  ///    This means that F calls G, and F returns the value returned by G.
86  ///  - Uses[arg F] = ret G
87  ///    This means that some function calls G and passes its result as an
88  ///    argument to F.
89  ///  - Uses[ret F] = arg F
90  ///    This means that F returns one of its own arguments.
91  ///  - Uses[arg F] = arg G
92  ///    This means that G calls F and passes one of its own (G's) arguments
93  ///    directly to F.
94  UseMap Uses;
95
96  typedef std::set<RetOrArg> LiveSet;
97  typedef std::set<const Function *> LiveFuncSet;
98
99  /// This set contains all values that have been determined to be live.
100  LiveSet LiveValues;
101  /// This set contains all values that are cannot be changed in any way.
102  LiveFuncSet LiveFunctions;
103
104  typedef SmallVector<RetOrArg, 5> UseVector;
105
106  /// This allows this pass to do double-duty as the dead arg hacking pass
107  /// (used only by bugpoint).
108  bool ShouldHackArguments = false;
109
110public:
111  DeadArgumentEliminationPass(bool ShouldHackArguments_ = false)
112      : ShouldHackArguments(ShouldHackArguments_) {}
113  PreservedAnalyses run(Module &M, ModuleAnalysisManager &);
114
115private:
116  Liveness MarkIfNotLive(RetOrArg Use, UseVector &MaybeLiveUses);
117  Liveness SurveyUse(const Use *U, UseVector &MaybeLiveUses,
118                     unsigned RetValNum = -1U);
119  Liveness SurveyUses(const Value *V, UseVector &MaybeLiveUses);
120
121  void SurveyFunction(const Function &F);
122  void MarkValue(const RetOrArg &RA, Liveness L,
123                 const UseVector &MaybeLiveUses);
124  void MarkLive(const RetOrArg &RA);
125  void MarkLive(const Function &F);
126  void PropagateLiveness(const RetOrArg &RA);
127  bool RemoveDeadStuffFromFunction(Function *F);
128  bool DeleteDeadVarargs(Function &Fn);
129  bool RemoveDeadArgumentsFromCallers(Function &Fn);
130};
131}
132
133#endif // LLVM_TRANSFORMS_IPO_DEADARGUMENTELIMINATION_H
134