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1 //===- llvm/ADT/PointerSumType.h --------------------------------*- 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 #ifndef LLVM_ADT_POINTERSUMTYPE_H
11 #define LLVM_ADT_POINTERSUMTYPE_H
12 
13 #include "llvm/ADT/DenseMapInfo.h"
14 #include "llvm/Support/Compiler.h"
15 #include "llvm/Support/PointerLikeTypeTraits.h"
16 
17 namespace llvm {
18 
19 /// A compile time pair of an integer tag and the pointer-like type which it
20 /// indexes within a sum type. Also allows the user to specify a particular
21 /// traits class for pointer types with custom behavior such as over-aligned
22 /// allocation.
23 template <uintptr_t N, typename PointerArgT,
24           typename TraitsArgT = PointerLikeTypeTraits<PointerArgT>>
25 struct PointerSumTypeMember {
26   enum { Tag = N };
27   typedef PointerArgT PointerT;
28   typedef TraitsArgT TraitsT;
29 };
30 
31 namespace detail {
32 
33 template <typename TagT, typename... MemberTs>
34 struct PointerSumTypeHelper;
35 
36 }
37 
38 /// A sum type over pointer-like types.
39 ///
40 /// This is a normal tagged union across pointer-like types that uses the low
41 /// bits of the pointers to store the tag.
42 ///
43 /// Each member of the sum type is specified by passing a \c
44 /// PointerSumTypeMember specialization in the variadic member argument list.
45 /// This allows the user to control the particular tag value associated with
46 /// a particular type, use the same type for multiple different tags, and
47 /// customize the pointer-like traits used for a particular member. Note that
48 /// these *must* be specializations of \c PointerSumTypeMember, no other type
49 /// will suffice, even if it provides a compatible interface.
50 ///
51 /// This type implements all of the comparison operators and even hash table
52 /// support by comparing the underlying storage of the pointer values. It
53 /// doesn't support delegating to particular members for comparisons.
54 ///
55 /// It also default constructs to a zero tag with a null pointer, whatever that
56 /// would be. This means that the zero value for the tag type is significant
57 /// and may be desireable to set to a state that is particularly desirable to
58 /// default construct.
59 ///
60 /// There is no support for constructing or accessing with a dynamic tag as
61 /// that would fundamentally violate the type safety provided by the sum type.
62 template <typename TagT, typename... MemberTs> class PointerSumType {
63   uintptr_t Value;
64 
65   typedef detail::PointerSumTypeHelper<TagT, MemberTs...> HelperT;
66 
67 public:
PointerSumType()68   PointerSumType() : Value(0) {}
69 
70   /// A typed constructor for a specific tagged member of the sum type.
71   template <TagT N>
72   static PointerSumType
create(typename HelperT::template Lookup<N>::PointerT Pointer)73   create(typename HelperT::template Lookup<N>::PointerT Pointer) {
74     PointerSumType Result;
75     void *V = HelperT::template Lookup<N>::TraitsT::getAsVoidPointer(Pointer);
76     assert((reinterpret_cast<uintptr_t>(V) & HelperT::TagMask) == 0 &&
77            "Pointer is insufficiently aligned to store the discriminant!");
78     Result.Value = reinterpret_cast<uintptr_t>(V) | N;
79     return Result;
80   }
81 
getTag()82   TagT getTag() const { return static_cast<TagT>(Value & HelperT::TagMask); }
83 
is()84   template <TagT N> bool is() const { return N == getTag(); }
85 
get()86   template <TagT N> typename HelperT::template Lookup<N>::PointerT get() const {
87     void *P = is<N>() ? getImpl() : nullptr;
88     return HelperT::template Lookup<N>::TraitsT::getFromVoidPointer(P);
89   }
90 
91   template <TagT N>
cast()92   typename HelperT::template Lookup<N>::PointerT cast() const {
93     assert(is<N>() && "This instance has a different active member.");
94     return HelperT::template Lookup<N>::TraitsT::getFromVoidPointer(getImpl());
95   }
96 
97   operator bool() const { return Value & HelperT::PointerMask; }
98   bool operator==(const PointerSumType &R) const { return Value == R.Value; }
99   bool operator!=(const PointerSumType &R) const { return Value != R.Value; }
100   bool operator<(const PointerSumType &R) const { return Value < R.Value; }
101   bool operator>(const PointerSumType &R) const { return Value > R.Value; }
102   bool operator<=(const PointerSumType &R) const { return Value <= R.Value; }
103   bool operator>=(const PointerSumType &R) const { return Value >= R.Value; }
104 
getOpaqueValue()105   uintptr_t getOpaqueValue() const { return Value; }
106 
107 protected:
getImpl()108   void *getImpl() const {
109     return reinterpret_cast<void *>(Value & HelperT::PointerMask);
110   }
111 };
112 
113 namespace detail {
114 
115 /// A helper template for implementing \c PointerSumType. It provides fast
116 /// compile-time lookup of the member from a particular tag value, along with
117 /// useful constants and compile time checking infrastructure..
118 template <typename TagT, typename... MemberTs>
119 struct PointerSumTypeHelper : MemberTs... {
120   // First we use a trick to allow quickly looking up information about
121   // a particular member of the sum type. This works because we arranged to
122   // have this type derive from all of the member type templates. We can select
123   // the matching member for a tag using type deduction during overload
124   // resolution.
125   template <TagT N, typename PointerT, typename TraitsT>
126   static PointerSumTypeMember<N, PointerT, TraitsT>
127   LookupOverload(PointerSumTypeMember<N, PointerT, TraitsT> *);
128   template <TagT N> static void LookupOverload(...);
129   template <TagT N> struct Lookup {
130     // Compute a particular member type by resolving the lookup helper ovorload.
131     typedef decltype(LookupOverload<N>(
132         static_cast<PointerSumTypeHelper *>(nullptr))) MemberT;
133 
134     /// The Nth member's pointer type.
135     typedef typename MemberT::PointerT PointerT;
136 
137     /// The Nth member's traits type.
138     typedef typename MemberT::TraitsT TraitsT;
139   };
140 
141   // Next we need to compute the number of bits available for the discriminant
142   // by taking the min of the bits available for each member. Much of this
143   // would be amazingly easier with good constexpr support.
144   template <uintptr_t V, uintptr_t... Vs>
145   struct Min : std::integral_constant<
146                    uintptr_t, (V < Min<Vs...>::value ? V : Min<Vs...>::value)> {
147   };
148   template <uintptr_t V>
149   struct Min<V> : std::integral_constant<uintptr_t, V> {};
150   enum { NumTagBits = Min<MemberTs::TraitsT::NumLowBitsAvailable...>::value };
151 
152   // Also compute the smallest discriminant and various masks for convenience.
153   enum : uint64_t {
154     MinTag = Min<MemberTs::Tag...>::value,
155     PointerMask = static_cast<uint64_t>(-1) << NumTagBits,
156     TagMask = ~PointerMask
157   };
158 
159   // Finally we need a recursive template to do static checks of each
160   // member.
161   template <typename MemberT, typename... InnerMemberTs>
162   struct Checker : Checker<InnerMemberTs...> {
163     static_assert(MemberT::Tag < (1 << NumTagBits),
164                   "This discriminant value requires too many bits!");
165   };
166   template <typename MemberT> struct Checker<MemberT> : std::true_type {
167     static_assert(MemberT::Tag < (1 << NumTagBits),
168                   "This discriminant value requires too many bits!");
169   };
170   static_assert(Checker<MemberTs...>::value,
171                 "Each member must pass the checker.");
172 };
173 
174 }
175 
176 // Teach DenseMap how to use PointerSumTypes as keys.
177 template <typename TagT, typename... MemberTs>
178 struct DenseMapInfo<PointerSumType<TagT, MemberTs...>> {
179   typedef PointerSumType<TagT, MemberTs...> SumType;
180 
181   typedef detail::PointerSumTypeHelper<TagT, MemberTs...> HelperT;
182   enum { SomeTag = HelperT::MinTag };
183   typedef typename HelperT::template Lookup<HelperT::MinTag>::PointerT
184       SomePointerT;
185   typedef DenseMapInfo<SomePointerT> SomePointerInfo;
186 
187   static inline SumType getEmptyKey() {
188     return SumType::create<SomeTag>(SomePointerInfo::getEmptyKey());
189   }
190   static inline SumType getTombstoneKey() {
191     return SumType::create<SomeTag>(
192         SomePointerInfo::getTombstoneKey());
193   }
194   static unsigned getHashValue(const SumType &Arg) {
195     uintptr_t OpaqueValue = Arg.getOpaqueValue();
196     return DenseMapInfo<uintptr_t>::getHashValue(OpaqueValue);
197   }
198   static bool isEqual(const SumType &LHS, const SumType &RHS) {
199     return LHS == RHS;
200   }
201 };
202 
203 }
204 
205 #endif
206