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//===- DeadArgumentElimination.h - Eliminate Dead Args ----------*- C++ -*-===//

//

// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.

// See https://llvm.org/LICENSE.txt for license information.

// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception

//

//===----------------------------------------------------------------------===//

//

// This pass deletes dead arguments from internal functions.  Dead argument

// elimination removes arguments which are directly dead, as well as arguments

// only passed into function calls as dead arguments of other functions.  This

// pass also deletes dead return values in a similar way.

//

// This pass is often useful as a cleanup pass to run after aggressive

// interprocedural passes, which add possibly-dead arguments or return values.

//

//===----------------------------------------------------------------------===//

#ifndef LLVM_TRANSFORMS_IPO_DEADARGUMENTELIMINATION_H

#define LLVM_TRANSFORMS_IPO_DEADARGUMENTELIMINATION_H

#include "llvm/ADT/SmallVector.h"

#include "llvm/ADT/Twine.h"

#include "llvm/IR/Function.h"

#include "llvm/IR/PassManager.h"

#include <map>

#include <set>

#include <string>

#include <tuple>

namespace llvm {

class Module;

class Use;

class Value;

/// Eliminate dead arguments (and return values) from functions.

class DeadArgumentEliminationPass

    : public PassInfoMixin<DeadArgumentEliminationPass> {

public:

  /// Struct that represents (part of) either a return value or a function

  /// argument.  Used so that arguments and return values can be used

  /// interchangeably.

  struct RetOrArg {

    const Function *F;

    unsigned Idx;

    bool IsArg;

    RetOrArg(const Function *F, unsigned Idx, bool IsArg)

        : F(F), Idx(Idx), IsArg(IsArg) {}

    /// Make RetOrArg comparable, so we can put it into a map.

    bool operator<(const RetOrArg &O) const {

      return std::tie(F, Idx, IsArg) < std::tie(O.F, O.Idx, O.IsArg);

    }

    /// Make RetOrArg comparable, so we can easily iterate the multimap.

    bool operator==(const RetOrArg &O) const {

      return F == O.F && Idx == O.Idx && IsArg == O.IsArg;

    }

    std::string getDescription() const {

      return (Twine(IsArg ? "Argument #" : "Return value #") + Twine(Idx) +

              " of function " + F->getName())

          .str();

    }

  };

  /// During our initial pass over the program, we determine that things are

  /// either alive or maybe alive. We don't mark anything explicitly dead (even

  /// if we know they are), since anything not alive with no registered uses

  /// (in Uses) will never be marked alive and will thus become dead in the end.

  enum Liveness { Live, MaybeLive };

  DeadArgumentEliminationPass(bool ShouldHackArguments = false)

      : ShouldHackArguments(ShouldHackArguments) {}

  PreservedAnalyses run(Module &M, ModuleAnalysisManager &);

  /// Convenience wrapper

  RetOrArg createRet(const Function *F, unsigned Idx) {

    return RetOrArg(F, Idx, false);

  }

  /// Convenience wrapper

  RetOrArg createArg(const Function *F, unsigned Idx) {

    return RetOrArg(F, Idx, true);

  }

  using UseMap = std::multimap<RetOrArg, RetOrArg>;

  /// This maps a return value or argument to any MaybeLive return values or

  /// arguments it uses. This allows the MaybeLive values to be marked live

  /// when any of its users is marked live.

  /// For example (indices are left out for clarity):

  ///  - Uses[ret F] = ret G

  ///    This means that F calls G, and F returns the value returned by G.

  ///  - Uses[arg F] = ret G

  ///    This means that some function calls G and passes its result as an

  ///    argument to F.

  ///  - Uses[ret F] = arg F

  ///    This means that F returns one of its own arguments.

  ///  - Uses[arg F] = arg G

  ///    This means that G calls F and passes one of its own (G's) arguments

  ///    directly to F.

  UseMap Uses;

  using LiveSet = std::set<RetOrArg>;

  using LiveFuncSet = std::set<const Function *>;

  /// This set contains all values that have been determined to be live.

  LiveSet LiveValues;

  /// This set contains all values that are cannot be changed in any way.

  LiveFuncSet LiveFunctions;

  using UseVector = SmallVector<RetOrArg, 5>;

  /// This allows this pass to do double-duty as the dead arg hacking pass

  /// (used only by bugpoint).

  bool ShouldHackArguments = false;

private:

  Liveness markIfNotLive(RetOrArg Use, UseVector &MaybeLiveUses);

  Liveness surveyUse(const Use *U, UseVector &MaybeLiveUses,

                     unsigned RetValNum = -1U);

  Liveness surveyUses(const Value *V, UseVector &MaybeLiveUses);

  void surveyFunction(const Function &F);

  bool isLive(const RetOrArg &RA);

  void markValue(const RetOrArg &RA, Liveness L,

                 const UseVector &MaybeLiveUses);

  void markLive(const RetOrArg &RA);

  void markLive(const Function &F);

  void propagateLiveness(const RetOrArg &RA);

  bool removeDeadStuffFromFunction(Function *F);

  bool deleteDeadVarargs(Function &F);

  bool removeDeadArgumentsFromCallers(Function &F);

};

} // end namespace llvm

#endif // LLVM_TRANSFORMS_IPO_DEADARGUMENTELIMINATION_H