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 
30 namespace llvm {
31 
32 /// Eliminate dead arguments (and return values) from functions.
33 class DeadArgumentEliminationPass
34     : public PassInfoMixin<DeadArgumentEliminationPass> {
35 public:
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 {
RetOrArgRetOrArg40     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 
getDescriptionRetOrArg56     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
CreateRet(const Function * F,unsigned Idx)71   RetOrArg CreateRet(const Function *F, unsigned Idx) {
72     return RetOrArg(F, Idx, false);
73   }
74   /// Convenience wrapper
CreateArg(const Function * F,unsigned Idx)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 
110 public:
111   DeadArgumentEliminationPass(bool ShouldHackArguments_ = false)
ShouldHackArguments(ShouldHackArguments_)112       : ShouldHackArguments(ShouldHackArguments_) {}
113   PreservedAnalyses run(Module &M, ModuleAnalysisManager &);
114 
115 private:
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