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      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 {
     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 
    110 public:
    111   DeadArgumentEliminationPass(bool ShouldHackArguments_ = false)
    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