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+//===- llvm/ADT/PointerSumType.h --------------------------------*- 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_ADT_POINTERSUMTYPE_H
+#define LLVM_ADT_POINTERSUMTYPE_H
+
+#include "llvm/ADT/bit.h"
+#include "llvm/ADT/DenseMapInfo.h"
+#include "llvm/Support/PointerLikeTypeTraits.h"
+#include <cassert>
+#include <cstdint>
+#include <type_traits>
+
+namespace llvm {
+
+/// A compile time pair of an integer tag and the pointer-like type which it
+/// indexes within a sum type. Also allows the user to specify a particular
+/// traits class for pointer types with custom behavior such as over-aligned
+/// allocation.
+template <uintptr_t N, typename PointerArgT,
+          typename TraitsArgT = PointerLikeTypeTraits<PointerArgT>>
+struct PointerSumTypeMember {
+  enum { Tag = N };
+  using PointerT = PointerArgT;
+  using TraitsT = TraitsArgT;
+};
+
+namespace detail {
+
+template <typename TagT, typename... MemberTs> struct PointerSumTypeHelper;
+
+} // end namespace detail
+
+/// A sum type over pointer-like types.
+///
+/// This is a normal tagged union across pointer-like types that uses the low
+/// bits of the pointers to store the tag.
+///
+/// Each member of the sum type is specified by passing a \c
+/// PointerSumTypeMember specialization in the variadic member argument list.
+/// This allows the user to control the particular tag value associated with
+/// a particular type, use the same type for multiple different tags, and
+/// customize the pointer-like traits used for a particular member. Note that
+/// these *must* be specializations of \c PointerSumTypeMember, no other type
+/// will suffice, even if it provides a compatible interface.
+///
+/// This type implements all of the comparison operators and even hash table
+/// support by comparing the underlying storage of the pointer values. It
+/// doesn't support delegating to particular members for comparisons.
+///
+/// It also default constructs to a zero tag with a null pointer, whatever that
+/// would be. This means that the zero value for the tag type is significant
+/// and may be desirable to set to a state that is particularly desirable to
+/// default construct.
+///
+/// Having a supported zero-valued tag also enables getting the address of a
+/// pointer stored with that tag provided it is stored in its natural bit
+/// representation. This works because in the case of a zero-valued tag, the
+/// pointer's value is directly stored into this object and we can expose the
+/// address of that internal storage. This is especially useful when building an
+/// `ArrayRef` of a single pointer stored in a sum type.
+///
+/// There is no support for constructing or accessing with a dynamic tag as
+/// that would fundamentally violate the type safety provided by the sum type.
+template <typename TagT, typename... MemberTs> class PointerSumType {
+  using HelperT = detail::PointerSumTypeHelper<TagT, MemberTs...>;
+
+  // We keep both the raw value and the min tag value's pointer in a union. When
+  // the minimum tag value is zero, this allows code below to cleanly expose the
+  // address of the zero-tag pointer instead of just the zero-tag pointer
+  // itself. This is especially useful when building `ArrayRef`s out of a single
+  // pointer. However, we have to carefully access the union due to the active
+  // member potentially changing. When we *store* a new value, we directly
+  // access the union to allow us to store using the obvious types. However,
+  // when we *read* a value, we copy the underlying storage out to avoid relying
+  // on one member or the other being active.
+  union StorageT {
+    // Ensure we get a null default constructed value. We don't use a member
+    // initializer because some compilers seem to not implement those correctly
+    // for a union.
+    StorageT() : Value(0) {}
+
+    uintptr_t Value;
+
+    typename HelperT::template Lookup<HelperT::MinTag>::PointerT MinTagPointer;
+  };
+
+  StorageT Storage;
+
+public:
+  constexpr PointerSumType() = default;
+
+  /// A typed setter to a given tagged member of the sum type.
+  template <TagT N>
+  void set(typename HelperT::template Lookup<N>::PointerT Pointer) {
+    void *V = HelperT::template Lookup<N>::TraitsT::getAsVoidPointer(Pointer);
+    assert((reinterpret_cast<uintptr_t>(V) & HelperT::TagMask) == 0 &&
+           "Pointer is insufficiently aligned to store the discriminant!");
+    Storage.Value = reinterpret_cast<uintptr_t>(V) | N;
+  }
+
+  /// A typed constructor for a specific tagged member of the sum type.
+  template <TagT N>
+  static PointerSumType
+  create(typename HelperT::template Lookup<N>::PointerT Pointer) {
+    PointerSumType Result;
+    Result.set<N>(Pointer);
+    return Result;
+  }
+
+  /// Clear the value to null with the min tag type.
+  void clear() { set<HelperT::MinTag>(nullptr); }
+
+  TagT getTag() const {
+    return static_cast<TagT>(getOpaqueValue() & HelperT::TagMask);
+  }
+
+  template <TagT N> bool is() const { return N == getTag(); }
+
+  template <TagT N> typename HelperT::template Lookup<N>::PointerT get() const {
+    void *P = is<N>() ? getVoidPtr() : nullptr;
+    return HelperT::template Lookup<N>::TraitsT::getFromVoidPointer(P);
+  }
+
+  template <TagT N>
+  typename HelperT::template Lookup<N>::PointerT cast() const {
+    assert(is<N>() && "This instance has a different active member.");
+    return HelperT::template Lookup<N>::TraitsT::getFromVoidPointer(
+        getVoidPtr());
+  }
+
+  /// If the tag is zero and the pointer's value isn't changed when being
+  /// stored, get the address of the stored value type-punned to the zero-tag's
+  /// pointer type.
+  typename HelperT::template Lookup<HelperT::MinTag>::PointerT const *
+  getAddrOfZeroTagPointer() const {
+    return const_cast<PointerSumType *>(this)->getAddrOfZeroTagPointer();
+  }
+
+  /// If the tag is zero and the pointer's value isn't changed when being
+  /// stored, get the address of the stored value type-punned to the zero-tag's
+  /// pointer type.
+  typename HelperT::template Lookup<HelperT::MinTag>::PointerT *
+  getAddrOfZeroTagPointer() {
+    static_assert(HelperT::MinTag == 0, "Non-zero minimum tag value!");
+    assert(is<HelperT::MinTag>() && "The active tag is not zero!");
+    // Store the initial value of the pointer when read out of our storage.
+    auto InitialPtr = get<HelperT::MinTag>();
+    // Now update the active member of the union to be the actual pointer-typed
+    // member so that accessing it indirectly through the returned address is
+    // valid.
+    Storage.MinTagPointer = InitialPtr;
+    // Finally, validate that this was a no-op as expected by reading it back
+    // out using the same underlying-storage read as above.
+    assert(InitialPtr == get<HelperT::MinTag>() &&
+           "Switching to typed storage changed the pointer returned!");
+    // Now we can correctly return an address to typed storage.
+    return &Storage.MinTagPointer;
+  }
+
+  explicit operator bool() const {
+    return getOpaqueValue() & HelperT::PointerMask;
+  }
+  bool operator==(const PointerSumType &R) const {
+    return getOpaqueValue() == R.getOpaqueValue();
+  }
+  bool operator!=(const PointerSumType &R) const {
+    return getOpaqueValue() != R.getOpaqueValue();
+  }
+  bool operator<(const PointerSumType &R) const {
+    return getOpaqueValue() < R.getOpaqueValue();
+  }
+  bool operator>(const PointerSumType &R) const {
+    return getOpaqueValue() > R.getOpaqueValue();
+  }
+  bool operator<=(const PointerSumType &R) const {
+    return getOpaqueValue() <= R.getOpaqueValue();
+  }
+  bool operator>=(const PointerSumType &R) const {
+    return getOpaqueValue() >= R.getOpaqueValue();
+  }
+
+  uintptr_t getOpaqueValue() const {
+    // Read the underlying storage of the union, regardless of the active
+    // member.
+    return bit_cast<uintptr_t>(Storage);
+  }
+
+protected:
+  void *getVoidPtr() const {
+    return reinterpret_cast<void *>(getOpaqueValue() & HelperT::PointerMask);
+  }
+};
+
+namespace detail {
+
+/// A helper template for implementing \c PointerSumType. It provides fast
+/// compile-time lookup of the member from a particular tag value, along with
+/// useful constants and compile time checking infrastructure..
+template <typename TagT, typename... MemberTs>
+struct PointerSumTypeHelper : MemberTs... {
+  // First we use a trick to allow quickly looking up information about
+  // a particular member of the sum type. This works because we arranged to
+  // have this type derive from all of the member type templates. We can select
+  // the matching member for a tag using type deduction during overload
+  // resolution.
+  template <TagT N, typename PointerT, typename TraitsT>
+  static PointerSumTypeMember<N, PointerT, TraitsT>
+  LookupOverload(PointerSumTypeMember<N, PointerT, TraitsT> *);
+  template <TagT N> static void LookupOverload(...);
+  template <TagT N> struct Lookup {
+    // Compute a particular member type by resolving the lookup helper ovorload.
+    using MemberT = decltype(
+        LookupOverload<N>(static_cast<PointerSumTypeHelper *>(nullptr)));
+
+    /// The Nth member's pointer type.
+    using PointerT = typename MemberT::PointerT;
+
+    /// The Nth member's traits type.
+    using TraitsT = typename MemberT::TraitsT;
+  };
+
+  // Next we need to compute the number of bits available for the discriminant
+  // by taking the min of the bits available for each member. Much of this
+  // would be amazingly easier with good constexpr support.
+  template <uintptr_t V, uintptr_t... Vs>
+  struct Min : std::integral_constant<
+                   uintptr_t, (V < Min<Vs...>::value ? V : Min<Vs...>::value)> {
+  };
+  template <uintptr_t V>
+  struct Min<V> : std::integral_constant<uintptr_t, V> {};
+  enum { NumTagBits = Min<MemberTs::TraitsT::NumLowBitsAvailable...>::value };
+
+  // Also compute the smallest discriminant and various masks for convenience.
+  constexpr static TagT MinTag =
+      static_cast<TagT>(Min<MemberTs::Tag...>::value);
+  enum : uint64_t {
+    PointerMask = static_cast<uint64_t>(-1) << NumTagBits,
+    TagMask = ~PointerMask
+  };
+
+  // Finally we need a recursive template to do static checks of each
+  // member.
+  template <typename MemberT, typename... InnerMemberTs>
+  struct Checker : Checker<InnerMemberTs...> {
+    static_assert(MemberT::Tag < (1 << NumTagBits),
+                  "This discriminant value requires too many bits!");
+  };
+  template <typename MemberT> struct Checker<MemberT> : std::true_type {
+    static_assert(MemberT::Tag < (1 << NumTagBits),
+                  "This discriminant value requires too many bits!");
+  };
+  static_assert(Checker<MemberTs...>::value,
+                "Each member must pass the checker.");
+};
+
+} // end namespace detail
+
+// Teach DenseMap how to use PointerSumTypes as keys.
+template <typename TagT, typename... MemberTs>
+struct DenseMapInfo<PointerSumType<TagT, MemberTs...>> {
+  using SumType = PointerSumType<TagT, MemberTs...>;
+  using HelperT = detail::PointerSumTypeHelper<TagT, MemberTs...>;
+  enum { SomeTag = HelperT::MinTag };
+  using SomePointerT =
+      typename HelperT::template Lookup<HelperT::MinTag>::PointerT;
+  using SomePointerInfo = DenseMapInfo<SomePointerT>;
+
+  static inline SumType getEmptyKey() {
+    return SumType::create<SomeTag>(SomePointerInfo::getEmptyKey());
+  }
+
+  static inline SumType getTombstoneKey() {
+    return SumType::create<SomeTag>(SomePointerInfo::getTombstoneKey());
+  }
+
+  static unsigned getHashValue(const SumType &Arg) {
+    uintptr_t OpaqueValue = Arg.getOpaqueValue();
+    return DenseMapInfo<uintptr_t>::getHashValue(OpaqueValue);
+  }
+
+  static bool isEqual(const SumType &LHS, const SumType &RHS) {
+    return LHS == RHS;
+  }
+};
+
+} // end namespace llvm
+
+#endif // LLVM_ADT_POINTERSUMTYPE_H