1 //===- DeadArgumentElimination.h - Eliminate Dead Args ----------*- 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 deletes dead arguments from internal functions.  Dead argument
10 // elimination removes arguments which are directly dead, as well as arguments
11 // only passed into function calls as dead arguments of other functions.  This
12 // pass also deletes dead return values in a similar way.
13 //
14 // This pass is often useful as a cleanup pass to run after aggressive
15 // interprocedural passes, which add possibly-dead arguments or return values.
16 //
17 //===----------------------------------------------------------------------===//
18 
19 #ifndef LLVM_TRANSFORMS_IPO_DEADARGUMENTELIMINATION_H
20 #define LLVM_TRANSFORMS_IPO_DEADARGUMENTELIMINATION_H
21 
22 #include "llvm/ADT/SmallVector.h"
23 #include "llvm/ADT/Twine.h"
24 #include "llvm/IR/Function.h"
25 #include "llvm/IR/PassManager.h"
26 #include <map>
27 #include <set>
28 #include <string>
29 #include <tuple>
30 
31 namespace llvm {
32 
33 class Module;
34 class Use;
35 class Value;
36 
37 /// Eliminate dead arguments (and return values) from functions.
38 class DeadArgumentEliminationPass
39     : public PassInfoMixin<DeadArgumentEliminationPass> {
40 public:
41   /// Struct that represents (part of) either a return value or a function
42   /// argument.  Used so that arguments and return values can be used
43   /// interchangeably.
44   struct RetOrArg {
45     const Function *F;
46     unsigned Idx;
47     bool IsArg;
48 
49     RetOrArg(const Function *F, unsigned Idx, bool IsArg)
50         : F(F), Idx(Idx), IsArg(IsArg) {}
51 
52     /// Make RetOrArg comparable, so we can put it into a map.
53     bool operator<(const RetOrArg &O) const {
54       return std::tie(F, Idx, IsArg) < std::tie(O.F, O.Idx, O.IsArg);
55     }
56 
57     /// Make RetOrArg comparable, so we can easily iterate the multimap.
58     bool operator==(const RetOrArg &O) const {
59       return F == O.F && Idx == O.Idx && IsArg == O.IsArg;
60     }
61 
62     std::string getDescription() const {
63       return (Twine(IsArg ? "Argument #" : "Return value #") + Twine(Idx) +
64               " of function " + F->getName())
65           .str();
66     }
67   };
68 
69   /// Liveness enum - During our initial pass over the program, we determine
70   /// that things are either alive or maybe alive. We don't mark anything
71   /// explicitly dead (even if we know they are), since anything not alive
72   /// with no registered uses (in Uses) will never be marked alive and will
73   /// thus become dead in the end.
74   enum Liveness { Live, MaybeLive };
75 
76   DeadArgumentEliminationPass(bool ShouldHackArguments_ = false)
77       : ShouldHackArguments(ShouldHackArguments_) {}
78 
79   PreservedAnalyses run(Module &M, ModuleAnalysisManager &);
80 
81   /// Convenience wrapper
82   RetOrArg CreateRet(const Function *F, unsigned Idx) {
83     return RetOrArg(F, Idx, false);
84   }
85 
86   /// Convenience wrapper
87   RetOrArg CreateArg(const Function *F, unsigned Idx) {
88     return RetOrArg(F, Idx, true);
89   }
90 
91   using UseMap = std::multimap<RetOrArg, RetOrArg>;
92 
93   /// This maps a return value or argument to any MaybeLive return values or
94   /// arguments it uses. This allows the MaybeLive values to be marked live
95   /// when any of its users is marked live.
96   /// For example (indices are left out for clarity):
97   ///  - Uses[ret F] = ret G
98   ///    This means that F calls G, and F returns the value returned by G.
99   ///  - Uses[arg F] = ret G
100   ///    This means that some function calls G and passes its result as an
101   ///    argument to F.
102   ///  - Uses[ret F] = arg F
103   ///    This means that F returns one of its own arguments.
104   ///  - Uses[arg F] = arg G
105   ///    This means that G calls F and passes one of its own (G's) arguments
106   ///    directly to F.
107   UseMap Uses;
108 
109   using LiveSet = std::set<RetOrArg>;
110   using LiveFuncSet = std::set<const Function *>;
111 
112   /// This set contains all values that have been determined to be live.
113   LiveSet LiveValues;
114 
115   /// This set contains all values that are cannot be changed in any way.
116   LiveFuncSet LiveFunctions;
117 
118   using UseVector = SmallVector<RetOrArg, 5>;
119 
120   /// This allows this pass to do double-duty as the dead arg hacking pass
121   /// (used only by bugpoint).
122   bool ShouldHackArguments = false;
123 
124 private:
125   Liveness MarkIfNotLive(RetOrArg Use, UseVector &MaybeLiveUses);
126   Liveness SurveyUse(const Use *U, UseVector &MaybeLiveUses,
127                      unsigned RetValNum = -1U);
128   Liveness SurveyUses(const Value *V, UseVector &MaybeLiveUses);
129 
130   void SurveyFunction(const Function &F);
131   void MarkValue(const RetOrArg &RA, Liveness L,
132                  const UseVector &MaybeLiveUses);
133   void MarkLive(const RetOrArg &RA);
134   void MarkLive(const Function &F);
135   void PropagateLiveness(const RetOrArg &RA);
136   bool RemoveDeadStuffFromFunction(Function *F);
137   bool DeleteDeadVarargs(Function &Fn);
138   bool RemoveDeadArgumentsFromCallers(Function &Fn);
139 };
140 
141 } // end namespace llvm
142 
143 #endif // LLVM_TRANSFORMS_IPO_DEADARGUMENTELIMINATION_H
144