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//===- llvm/ADT/PointerUnion.h - Discriminated Union of 2 Ptrs --*- 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

//

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

///

/// \file

/// This file defines the PointerUnion class, which is a discriminated union of

/// pointer types.

///

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

#ifndef LLVM_ADT_POINTERUNION_H

#define LLVM_ADT_POINTERUNION_H

#include "llvm/ADT/DenseMapInfo.h"

#include "llvm/ADT/PointerIntPair.h"

#include "llvm/ADT/STLExtras.h"

#include "llvm/Support/Casting.h"

#include "llvm/Support/PointerLikeTypeTraits.h"

#include <algorithm>

#include <cassert>

#include <cstddef>

#include <cstdint>

namespace llvm {

namespace pointer_union_detail {

  /// Determine the number of bits required to store integers with values < n.

  /// This is ceil(log2(n)).

  constexpr int bitsRequired(unsigned n) {

    return n > 1 ? 1 + bitsRequired((n + 1) / 2) : 0;

  }

  template <typename... Ts> constexpr int lowBitsAvailable() {

    return std::min<int>({PointerLikeTypeTraits<Ts>::NumLowBitsAvailable...});

  }

  /// Find the first type in a list of types.

  template <typename T, typename...> struct GetFirstType {

    using type = T;

  };

  /// Provide PointerLikeTypeTraits for void* that is used by PointerUnion

  /// for the template arguments.

  template <typename ...PTs> class PointerUnionUIntTraits {

  public:

    static inline void *getAsVoidPointer(void *P) { return P; }

    static inline void *getFromVoidPointer(void *P) { return P; }

    static constexpr int NumLowBitsAvailable = lowBitsAvailable<PTs...>();

  };

  template <typename Derived, typename ValTy, int I, typename ...Types>

  class PointerUnionMembers;

  template <typename Derived, typename ValTy, int I>

  class PointerUnionMembers<Derived, ValTy, I> {

  protected:

    ValTy Val;

    PointerUnionMembers() = default;

    PointerUnionMembers(ValTy Val) : Val(Val) {}

    friend struct PointerLikeTypeTraits<Derived>;

  };

  template <typename Derived, typename ValTy, int I, typename Type,

            typename ...Types>

  class PointerUnionMembers<Derived, ValTy, I, Type, Types...>

      : public PointerUnionMembers<Derived, ValTy, I + 1, Types...> {

    using Base = PointerUnionMembers<Derived, ValTy, I + 1, Types...>;

  public:

    using Base::Base;

    PointerUnionMembers() = default;

    PointerUnionMembers(Type V)

        : Base(ValTy(const_cast<void *>(

                         PointerLikeTypeTraits<Type>::getAsVoidPointer(V)),

                     I)) {}

    using Base::operator=;

    Derived &operator=(Type V) {

      this->Val = ValTy(

          const_cast<void *>(PointerLikeTypeTraits<Type>::getAsVoidPointer(V)),

          I);

      return static_cast<Derived &>(*this);

    };

  };

}

// This is a forward declaration of CastInfoPointerUnionImpl

// Refer to its definition below for further details

template <typename... PTs> struct CastInfoPointerUnionImpl;

/// A discriminated union of two or more pointer types, with the discriminator

/// in the low bit of the pointer.

///

/// This implementation is extremely efficient in space due to leveraging the

/// low bits of the pointer, while exposing a natural and type-safe API.

///

/// Common use patterns would be something like this:

///    PointerUnion<int*, float*> P;

///    P = (int*)0;

///    printf("%d %d", P.is<int*>(), P.is<float*>());  // prints "1 0"

///    X = P.get<int*>();     // ok.

///    Y = P.get<float*>();   // runtime assertion failure.

///    Z = P.get<double*>();  // compile time failure.

///    P = (float*)0;

///    Y = P.get<float*>();   // ok.

///    X = P.get<int*>();     // runtime assertion failure.

///    PointerUnion<int*, int*> Q; // compile time failure.

template <typename... PTs>

class PointerUnion

    : public pointer_union_detail::PointerUnionMembers<

          PointerUnion<PTs...>,

          PointerIntPair<

              void *, pointer_union_detail::bitsRequired(sizeof...(PTs)), int,

              pointer_union_detail::PointerUnionUIntTraits<PTs...>>,

          0, PTs...> {

  static_assert(TypesAreDistinct<PTs...>::value,

                "PointerUnion alternative types cannot be repeated");

  // The first type is special because we want to directly cast a pointer to a

  // default-initialized union to a pointer to the first type. But we don't

  // want PointerUnion to be a 'template <typename First, typename ...Rest>'

  // because it's much more convenient to have a name for the whole pack. So

  // split off the first type here.

  using First = TypeAtIndex<0, PTs...>;

  using Base = typename PointerUnion::PointerUnionMembers;

  /// This is needed to give the CastInfo implementation below access

  /// to protected members.

  /// Refer to its definition for further details.

  friend struct CastInfoPointerUnionImpl<PTs...>;

public:

  PointerUnion() = default;

  PointerUnion(std::nullptr_t) : PointerUnion() {}

  using Base::Base;

  /// Test if the pointer held in the union is null, regardless of

  /// which type it is.

  bool isNull() const { return !this->Val.getPointer(); }

  explicit operator bool() const { return !isNull(); }

  // FIXME: Replace the uses of is(), get() and dyn_cast() with

  //        isa<T>, cast<T> and the llvm::dyn_cast<T>

  /// Test if the Union currently holds the type matching T.

  template <typename T> inline bool is() const { return isa<T>(*this); }

  /// Returns the value of the specified pointer type.

  ///

  /// If the specified pointer type is incorrect, assert.

  template <typename T> inline T get() const {

    assert(isa<T>(*this) && "Invalid accessor called");

    return cast<T>(*this);

  }

  /// Returns the current pointer if it is of the specified pointer type,

  /// otherwise returns null.

  template <typename T> inline T dyn_cast() const {

    return llvm::dyn_cast_if_present<T>(*this);

  }

  /// If the union is set to the first pointer type get an address pointing to

  /// it.

  First const *getAddrOfPtr1() const {

    return const_cast<PointerUnion *>(this)->getAddrOfPtr1();

  }

  /// If the union is set to the first pointer type get an address pointing to

  /// it.

  First *getAddrOfPtr1() {

    assert(is<First>() && "Val is not the first pointer");

    assert(

        PointerLikeTypeTraits<First>::getAsVoidPointer(get<First>()) ==

            this->Val.getPointer() &&

        "Can't get the address because PointerLikeTypeTraits changes the ptr");

    return const_cast<First *>(

        reinterpret_cast<const First *>(this->Val.getAddrOfPointer()));

  }

  /// Assignment from nullptr which just clears the union.

  const PointerUnion &operator=(std::nullptr_t) {

    this->Val.initWithPointer(nullptr);

    return *this;

  }

  /// Assignment from elements of the union.

  using Base::operator=;

  void *getOpaqueValue() const { return this->Val.getOpaqueValue(); }

  static inline PointerUnion getFromOpaqueValue(void *VP) {

    PointerUnion V;

    V.Val = decltype(V.Val)::getFromOpaqueValue(VP);

    return V;

  }

};

template <typename ...PTs>

bool operator==(PointerUnion<PTs...> lhs, PointerUnion<PTs...> rhs) {

  return lhs.getOpaqueValue() == rhs.getOpaqueValue();

}

template <typename ...PTs>

bool operator!=(PointerUnion<PTs...> lhs, PointerUnion<PTs...> rhs) {

  return lhs.getOpaqueValue() != rhs.getOpaqueValue();

}

template <typename ...PTs>

bool operator<(PointerUnion<PTs...> lhs, PointerUnion<PTs...> rhs) {

  return lhs.getOpaqueValue() < rhs.getOpaqueValue();

}

/// We can't (at least, at this moment with C++14) declare CastInfo

/// as a friend of PointerUnion like this:

/// ```

///   template<typename To>

///   friend struct CastInfo<To, PointerUnion<PTs...>>;

/// ```

/// The compiler complains 'Partial specialization cannot be declared as a

/// friend'.

/// So we define this struct to be a bridge between CastInfo and

/// PointerUnion.

template <typename... PTs> struct CastInfoPointerUnionImpl {

  using From = PointerUnion<PTs...>;

  template <typename To> static inline bool isPossible(From &F) {

    return F.Val.getInt() == FirstIndexOfType<To, PTs...>::value;

  }

  template <typename To> static To doCast(From &F) {

    assert(isPossible<To>(F) && "cast to an incompatible type !");

    return PointerLikeTypeTraits<To>::getFromVoidPointer(F.Val.getPointer());

  }

};

// Specialization of CastInfo for PointerUnion

template <typename To, typename... PTs>

struct CastInfo<To, PointerUnion<PTs...>>

    : public DefaultDoCastIfPossible<To, PointerUnion<PTs...>,

                                     CastInfo<To, PointerUnion<PTs...>>> {

  using From = PointerUnion<PTs...>;

  using Impl = CastInfoPointerUnionImpl<PTs...>;

  static inline bool isPossible(From &f) {

    return Impl::template isPossible<To>(f);

  }

  static To doCast(From &f) { return Impl::template doCast<To>(f); }

  static inline To castFailed() { return To(); }

};

template <typename To, typename... PTs>

struct CastInfo<To, const PointerUnion<PTs...>>

    : public ConstStrippingForwardingCast<To, const PointerUnion<PTs...>,

                                          CastInfo<To, PointerUnion<PTs...>>> {

};

// Teach SmallPtrSet that PointerUnion is "basically a pointer", that has

// # low bits available = min(PT1bits,PT2bits)-1.

template <typename ...PTs>

struct PointerLikeTypeTraits<PointerUnion<PTs...>> {

  static inline void *getAsVoidPointer(const PointerUnion<PTs...> &P) {

    return P.getOpaqueValue();

  }

  static inline PointerUnion<PTs...> getFromVoidPointer(void *P) {

    return PointerUnion<PTs...>::getFromOpaqueValue(P);

  }

  // The number of bits available are the min of the pointer types minus the

  // bits needed for the discriminator.

  static constexpr int NumLowBitsAvailable = PointerLikeTypeTraits<decltype(

      PointerUnion<PTs...>::Val)>::NumLowBitsAvailable;

};

// Teach DenseMap how to use PointerUnions as keys.

template <typename ...PTs> struct DenseMapInfo<PointerUnion<PTs...>> {

  using Union = PointerUnion<PTs...>;

  using FirstInfo =

      DenseMapInfo<typename pointer_union_detail::GetFirstType<PTs...>::type>;

  static inline Union getEmptyKey() { return Union(FirstInfo::getEmptyKey()); }

  static inline Union getTombstoneKey() {

    return Union(FirstInfo::getTombstoneKey());

  }

  static unsigned getHashValue(const Union &UnionVal) {

    intptr_t key = (intptr_t)UnionVal.getOpaqueValue();

    return DenseMapInfo<intptr_t>::getHashValue(key);

  }

  static bool isEqual(const Union &LHS, const Union &RHS) {

    return LHS == RHS;

  }

};

} // end namespace llvm

#endif // LLVM_ADT_POINTERUNION_H