PointerUnion.h

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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 WPIUTIL_WPI_POINTERUNION_H
#define WPIUTIL_WPI_POINTERUNION_H
 
#include "wpi/DenseMapInfo.h"
#include "wpi/PointerIntPair.h"
#include "wpi/PointerLikeTypeTraits.h"
#include <algorithm>
#include <cassert>
#include <cstddef>
#include <cstdint>
#include <type_traits>
 
namespace wpi {
 
namespace detail {
template <typename T, typename... Us> struct TypesAreDistinct;
template <typename T, typename... Us>
struct TypesAreDistinct
    : std::integral_constant<bool, !std::disjunction_v<std::is_same<T, Us>...> &&
                                       TypesAreDistinct<Us...>::value> {};
template <typename T> struct TypesAreDistinct<T> : std::true_type {};
} // namespace detail
 
/// Determine if all types in Ts are distinct.
///
/// Useful to statically assert when Ts is intended to describe a non-multi set
/// of types.
///
/// Expensive (currently quadratic in sizeof(Ts...)), and so should only be
/// asserted once per instantiation of a type which requires it.
template <typename... Ts> struct TypesAreDistinct;
template <> struct TypesAreDistinct<> : std::true_type {};
template <typename... Ts>
struct TypesAreDistinct
    : std::integral_constant<bool, detail::TypesAreDistinct<Ts...>::value> {};
 
/// Find the first index where a type appears in a list of types.
///
/// FirstIndexOfType<T, Us...>::value is the first index of T in Us.
///
/// Typically only meaningful when it is otherwise statically known that the
/// type pack has no duplicate types. This should be guaranteed explicitly with
/// static_assert(TypesAreDistinct<Us...>::value).
///
/// It is a compile-time error to instantiate when T is not present in Us, i.e.
/// if is_one_of<T, Us...>::value is false.
template <typename T, typename... Us> struct FirstIndexOfType;
template <typename T, typename U, typename... Us>
struct FirstIndexOfType<T, U, Us...>
    : std::integral_constant<size_t, 1 + FirstIndexOfType<T, Us...>::value> {};
template <typename T, typename... Us>
struct FirstIndexOfType<T, T, Us...> : std::integral_constant<size_t, 0> {};
 
/// Find the type at a given index in a list of types.
///
/// TypeAtIndex<I, Ts...> is the type at index I in Ts.
template <size_t I, typename... Ts>
using TypeAtIndex = std::tuple_element_t<I, std::tuple<Ts...>>;
 
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);
    };
  };
}
 
/// 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;
 
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(); }
 
  /// Test if the Union currently holds the type matching T.
  template <typename T> bool is() const {
    return this->Val.getInt() == FirstIndexOfType<T, PTs...>::value;
  }
 
  /// Returns the value of the specified pointer type.
  ///
  /// If the specified pointer type is incorrect, assert.
  template <typename T> T get() const {
    assert(is<T>() && "Invalid accessor called");
    return PointerLikeTypeTraits<T>::getFromVoidPointer(this->Val.getPointer());
  }
 
  /// Returns the current pointer if it is of the specified pointer type,
  /// otherwise returns null.
  template <typename T> T dyn_cast() const {
    if (is<T>())
      return get<T>();
    return T();
  }
 
  /// 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();
}
 
// 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 wpi
 
#endif // WPIUTIL_WPI_POINTERUNION_H