1 //===- llvm/ADT/PointerUnion.h - Discriminated Union of 2 Ptrs --*- C++ -*-===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 // This file defines the PointerUnion class, which is a discriminated union of
10 // pointer types.
11 //
12 //===----------------------------------------------------------------------===//
13 
14 #ifndef LLVM_ADT_POINTERUNION_H
15 #define LLVM_ADT_POINTERUNION_H
16 
17 #include "llvm/ADT/DenseMapInfo.h"
18 #include "llvm/ADT/PointerIntPair.h"
19 #include "llvm/Support/PointerLikeTypeTraits.h"
20 #include <cassert>
21 #include <cstddef>
22 #include <cstdint>
23 
24 namespace llvm {
25 
26 template <typename T> struct PointerUnionTypeSelectorReturn {
27   using Return = T;
28 };
29 
30 /// Get a type based on whether two types are the same or not.
31 ///
32 /// For:
33 ///
34 /// \code
35 ///   using Ret = typename PointerUnionTypeSelector<T1, T2, EQ, NE>::Return;
36 /// \endcode
37 ///
38 /// Ret will be EQ type if T1 is same as T2 or NE type otherwise.
39 template <typename T1, typename T2, typename RET_EQ, typename RET_NE>
40 struct PointerUnionTypeSelector {
41   using Return = typename PointerUnionTypeSelectorReturn<RET_NE>::Return;
42 };
43 
44 template <typename T, typename RET_EQ, typename RET_NE>
45 struct PointerUnionTypeSelector<T, T, RET_EQ, RET_NE> {
46   using Return = typename PointerUnionTypeSelectorReturn<RET_EQ>::Return;
47 };
48 
49 template <typename T1, typename T2, typename RET_EQ, typename RET_NE>
50 struct PointerUnionTypeSelectorReturn<
51     PointerUnionTypeSelector<T1, T2, RET_EQ, RET_NE>> {
52   using Return =
53       typename PointerUnionTypeSelector<T1, T2, RET_EQ, RET_NE>::Return;
54 };
55 
56 namespace pointer_union_detail {
57   /// Determine the number of bits required to store integers with values < n.
58   /// This is ceil(log2(n)).
59   constexpr int bitsRequired(unsigned n) {
60     return n > 1 ? 1 + bitsRequired((n + 1) / 2) : 0;
61   }
62 
63   template <typename... Ts> constexpr int lowBitsAvailable() {
64     return std::min<int>({PointerLikeTypeTraits<Ts>::NumLowBitsAvailable...});
65   }
66 
67   /// Find the index of a type in a list of types. TypeIndex<T, Us...>::Index
68   /// is the index of T in Us, or sizeof...(Us) if T does not appear in the
69   /// list.
70   template <typename T, typename ...Us> struct TypeIndex;
71   template <typename T, typename ...Us> struct TypeIndex<T, T, Us...> {
72     static constexpr int Index = 0;
73   };
74   template <typename T, typename U, typename... Us>
75   struct TypeIndex<T, U, Us...> {
76     static constexpr int Index = 1 + TypeIndex<T, Us...>::Index;
77   };
78   template <typename T> struct TypeIndex<T> {
79     static constexpr int Index = 0;
80   };
81 
82   /// Find the first type in a list of types.
83   template <typename T, typename...> struct GetFirstType {
84     using type = T;
85   };
86 
87   /// Provide PointerLikeTypeTraits for void* that is used by PointerUnion
88   /// for the template arguments.
89   template <typename ...PTs> class PointerUnionUIntTraits {
90   public:
91     static inline void *getAsVoidPointer(void *P) { return P; }
92     static inline void *getFromVoidPointer(void *P) { return P; }
93     static constexpr int NumLowBitsAvailable = lowBitsAvailable<PTs...>();
94   };
95 
96   template <typename Derived, typename ValTy, int I, typename ...Types>
97   class PointerUnionMembers;
98 
99   template <typename Derived, typename ValTy, int I>
100   class PointerUnionMembers<Derived, ValTy, I> {
101   protected:
102     ValTy Val;
103     PointerUnionMembers() = default;
104     PointerUnionMembers(ValTy Val) : Val(Val) {}
105 
106     friend struct PointerLikeTypeTraits<Derived>;
107   };
108 
109   template <typename Derived, typename ValTy, int I, typename Type,
110             typename ...Types>
111   class PointerUnionMembers<Derived, ValTy, I, Type, Types...>
112       : public PointerUnionMembers<Derived, ValTy, I + 1, Types...> {
113     using Base = PointerUnionMembers<Derived, ValTy, I + 1, Types...>;
114   public:
115     using Base::Base;
116     PointerUnionMembers() = default;
117     PointerUnionMembers(Type V)
118         : Base(ValTy(const_cast<void *>(
119                          PointerLikeTypeTraits<Type>::getAsVoidPointer(V)),
120                      I)) {}
121 
122     using Base::operator=;
123     Derived &operator=(Type V) {
124       this->Val = ValTy(
125           const_cast<void *>(PointerLikeTypeTraits<Type>::getAsVoidPointer(V)),
126           I);
127       return static_cast<Derived &>(*this);
128     };
129   };
130 }
131 
132 /// A discriminated union of two or more pointer types, with the discriminator
133 /// in the low bit of the pointer.
134 ///
135 /// This implementation is extremely efficient in space due to leveraging the
136 /// low bits of the pointer, while exposing a natural and type-safe API.
137 ///
138 /// Common use patterns would be something like this:
139 ///    PointerUnion<int*, float*> P;
140 ///    P = (int*)0;
141 ///    printf("%d %d", P.is<int*>(), P.is<float*>());  // prints "1 0"
142 ///    X = P.get<int*>();     // ok.
143 ///    Y = P.get<float*>();   // runtime assertion failure.
144 ///    Z = P.get<double*>();  // compile time failure.
145 ///    P = (float*)0;
146 ///    Y = P.get<float*>();   // ok.
147 ///    X = P.get<int*>();     // runtime assertion failure.
148 template <typename... PTs>
149 class PointerUnion
150     : public pointer_union_detail::PointerUnionMembers<
151           PointerUnion<PTs...>,
152           PointerIntPair<
153               void *, pointer_union_detail::bitsRequired(sizeof...(PTs)), int,
154               pointer_union_detail::PointerUnionUIntTraits<PTs...>>,
155           0, PTs...> {
156   // The first type is special because we want to directly cast a pointer to a
157   // default-initialized union to a pointer to the first type. But we don't
158   // want PointerUnion to be a 'template <typename First, typename ...Rest>'
159   // because it's much more convenient to have a name for the whole pack. So
160   // split off the first type here.
161   using First = typename pointer_union_detail::GetFirstType<PTs...>::type;
162   using Base = typename PointerUnion::PointerUnionMembers;
163 
164 public:
165   PointerUnion() = default;
166 
167   PointerUnion(std::nullptr_t) : PointerUnion() {}
168   using Base::Base;
169 
170   /// Test if the pointer held in the union is null, regardless of
171   /// which type it is.
172   bool isNull() const { return !this->Val.getPointer(); }
173 
174   explicit operator bool() const { return !isNull(); }
175 
176   /// Test if the Union currently holds the type matching T.
177   template <typename T> bool is() const {
178     constexpr int Index = pointer_union_detail::TypeIndex<T, PTs...>::Index;
179     static_assert(Index < sizeof...(PTs),
180                   "PointerUnion::is<T> given type not in the union");
181     return this->Val.getInt() == Index;
182   }
183 
184   /// Returns the value of the specified pointer type.
185   ///
186   /// If the specified pointer type is incorrect, assert.
187   template <typename T> T get() const {
188     assert(is<T>() && "Invalid accessor called");
189     return PointerLikeTypeTraits<T>::getFromVoidPointer(this->Val.getPointer());
190   }
191 
192   /// Returns the current pointer if it is of the specified pointer type,
193   /// otherwise returns null.
194   template <typename T> T dyn_cast() const {
195     if (is<T>())
196       return get<T>();
197     return T();
198   }
199 
200   /// If the union is set to the first pointer type get an address pointing to
201   /// it.
202   First const *getAddrOfPtr1() const {
203     return const_cast<PointerUnion *>(this)->getAddrOfPtr1();
204   }
205 
206   /// If the union is set to the first pointer type get an address pointing to
207   /// it.
208   First *getAddrOfPtr1() {
209     assert(is<First>() && "Val is not the first pointer");
210     assert(
211         PointerLikeTypeTraits<First>::getAsVoidPointer(get<First>()) ==
212             this->Val.getPointer() &&
213         "Can't get the address because PointerLikeTypeTraits changes the ptr");
214     return const_cast<First *>(
215         reinterpret_cast<const First *>(this->Val.getAddrOfPointer()));
216   }
217 
218   /// Assignment from nullptr which just clears the union.
219   const PointerUnion &operator=(std::nullptr_t) {
220     this->Val.initWithPointer(nullptr);
221     return *this;
222   }
223 
224   /// Assignment from elements of the union.
225   using Base::operator=;
226 
227   void *getOpaqueValue() const { return this->Val.getOpaqueValue(); }
228   static inline PointerUnion getFromOpaqueValue(void *VP) {
229     PointerUnion V;
230     V.Val = decltype(V.Val)::getFromOpaqueValue(VP);
231     return V;
232   }
233 };
234 
235 template <typename ...PTs>
236 bool operator==(PointerUnion<PTs...> lhs, PointerUnion<PTs...> rhs) {
237   return lhs.getOpaqueValue() == rhs.getOpaqueValue();
238 }
239 
240 template <typename ...PTs>
241 bool operator!=(PointerUnion<PTs...> lhs, PointerUnion<PTs...> rhs) {
242   return lhs.getOpaqueValue() != rhs.getOpaqueValue();
243 }
244 
245 template <typename ...PTs>
246 bool operator<(PointerUnion<PTs...> lhs, PointerUnion<PTs...> rhs) {
247   return lhs.getOpaqueValue() < rhs.getOpaqueValue();
248 }
249 
250 // Teach SmallPtrSet that PointerUnion is "basically a pointer", that has
251 // # low bits available = min(PT1bits,PT2bits)-1.
252 template <typename ...PTs>
253 struct PointerLikeTypeTraits<PointerUnion<PTs...>> {
254   static inline void *getAsVoidPointer(const PointerUnion<PTs...> &P) {
255     return P.getOpaqueValue();
256   }
257 
258   static inline PointerUnion<PTs...> getFromVoidPointer(void *P) {
259     return PointerUnion<PTs...>::getFromOpaqueValue(P);
260   }
261 
262   // The number of bits available are the min of the pointer types minus the
263   // bits needed for the discriminator.
264   static constexpr int NumLowBitsAvailable = PointerLikeTypeTraits<decltype(
265       PointerUnion<PTs...>::Val)>::NumLowBitsAvailable;
266 };
267 
268 // Teach DenseMap how to use PointerUnions as keys.
269 template <typename ...PTs> struct DenseMapInfo<PointerUnion<PTs...>> {
270   using Union = PointerUnion<PTs...>;
271   using FirstInfo =
272       DenseMapInfo<typename pointer_union_detail::GetFirstType<PTs...>::type>;
273 
274   static inline Union getEmptyKey() { return Union(FirstInfo::getEmptyKey()); }
275 
276   static inline Union getTombstoneKey() {
277     return Union(FirstInfo::getTombstoneKey());
278   }
279 
280   static unsigned getHashValue(const Union &UnionVal) {
281     intptr_t key = (intptr_t)UnionVal.getOpaqueValue();
282     return DenseMapInfo<intptr_t>::getHashValue(key);
283   }
284 
285   static bool isEqual(const Union &LHS, const Union &RHS) {
286     return LHS == RHS;
287   }
288 };
289 
290 } // end namespace llvm
291 
292 #endif // LLVM_ADT_POINTERUNION_H
293