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 /// \file
10 /// This file defines the PointerUnion class, which is a discriminated union of
11 /// pointer types.
12 ///
13 //===----------------------------------------------------------------------===//
14 
15 #ifndef LLVM_ADT_POINTERUNION_H
16 #define LLVM_ADT_POINTERUNION_H
17 
18 #include "llvm/ADT/DenseMapInfo.h"
19 #include "llvm/ADT/PointerIntPair.h"
20 #include "llvm/ADT/STLExtras.h"
21 #include "llvm/Support/Casting.h"
22 #include "llvm/Support/PointerLikeTypeTraits.h"
23 #include <algorithm>
24 #include <cassert>
25 #include <cstddef>
26 #include <cstdint>
27 
28 namespace llvm {
29 
30 namespace pointer_union_detail {
31   /// Determine the number of bits required to store integers with values < n.
32   /// This is ceil(log2(n)).
33   constexpr int bitsRequired(unsigned n) {
34     return n > 1 ? 1 + bitsRequired((n + 1) / 2) : 0;
35   }
36 
37   template <typename... Ts> constexpr int lowBitsAvailable() {
38     return std::min<int>({PointerLikeTypeTraits<Ts>::NumLowBitsAvailable...});
39   }
40 
41   /// Find the first type in a list of types.
42   template <typename T, typename...> struct GetFirstType {
43     using type = T;
44   };
45 
46   /// Provide PointerLikeTypeTraits for void* that is used by PointerUnion
47   /// for the template arguments.
48   template <typename ...PTs> class PointerUnionUIntTraits {
49   public:
50     static inline void *getAsVoidPointer(void *P) { return P; }
51     static inline void *getFromVoidPointer(void *P) { return P; }
52     static constexpr int NumLowBitsAvailable = lowBitsAvailable<PTs...>();
53   };
54 
55   template <typename Derived, typename ValTy, int I, typename ...Types>
56   class PointerUnionMembers;
57 
58   template <typename Derived, typename ValTy, int I>
59   class PointerUnionMembers<Derived, ValTy, I> {
60   protected:
61     ValTy Val;
62     PointerUnionMembers() = default;
63     PointerUnionMembers(ValTy Val) : Val(Val) {}
64 
65     friend struct PointerLikeTypeTraits<Derived>;
66   };
67 
68   template <typename Derived, typename ValTy, int I, typename Type,
69             typename ...Types>
70   class PointerUnionMembers<Derived, ValTy, I, Type, Types...>
71       : public PointerUnionMembers<Derived, ValTy, I + 1, Types...> {
72     using Base = PointerUnionMembers<Derived, ValTy, I + 1, Types...>;
73   public:
74     using Base::Base;
75     PointerUnionMembers() = default;
76     PointerUnionMembers(Type V)
77         : Base(ValTy(const_cast<void *>(
78                          PointerLikeTypeTraits<Type>::getAsVoidPointer(V)),
79                      I)) {}
80 
81     using Base::operator=;
82     Derived &operator=(Type V) {
83       this->Val = ValTy(
84           const_cast<void *>(PointerLikeTypeTraits<Type>::getAsVoidPointer(V)),
85           I);
86       return static_cast<Derived &>(*this);
87     };
88   };
89 }
90 
91 // This is a forward declaration of CastInfoPointerUnionImpl
92 // Refer to its definition below for further details
93 template <typename... PTs> struct CastInfoPointerUnionImpl;
94 /// A discriminated union of two or more pointer types, with the discriminator
95 /// in the low bit of the pointer.
96 ///
97 /// This implementation is extremely efficient in space due to leveraging the
98 /// low bits of the pointer, while exposing a natural and type-safe API.
99 ///
100 /// Common use patterns would be something like this:
101 ///    PointerUnion<int*, float*> P;
102 ///    P = (int*)0;
103 ///    printf("%d %d", P.is<int*>(), P.is<float*>());  // prints "1 0"
104 ///    X = P.get<int*>();     // ok.
105 ///    Y = P.get<float*>();   // runtime assertion failure.
106 ///    Z = P.get<double*>();  // compile time failure.
107 ///    P = (float*)0;
108 ///    Y = P.get<float*>();   // ok.
109 ///    X = P.get<int*>();     // runtime assertion failure.
110 ///    PointerUnion<int*, int*> Q; // compile time failure.
111 template <typename... PTs>
112 class PointerUnion
113     : public pointer_union_detail::PointerUnionMembers<
114           PointerUnion<PTs...>,
115           PointerIntPair<
116               void *, pointer_union_detail::bitsRequired(sizeof...(PTs)), int,
117               pointer_union_detail::PointerUnionUIntTraits<PTs...>>,
118           0, PTs...> {
119   static_assert(TypesAreDistinct<PTs...>::value,
120                 "PointerUnion alternative types cannot be repeated");
121   // The first type is special because we want to directly cast a pointer to a
122   // default-initialized union to a pointer to the first type. But we don't
123   // want PointerUnion to be a 'template <typename First, typename ...Rest>'
124   // because it's much more convenient to have a name for the whole pack. So
125   // split off the first type here.
126   using First = TypeAtIndex<0, PTs...>;
127   using Base = typename PointerUnion::PointerUnionMembers;
128 
129   /// This is needed to give the CastInfo implementation below access
130   /// to protected members.
131   /// Refer to its definition for further details.
132   friend struct CastInfoPointerUnionImpl<PTs...>;
133 
134 public:
135   PointerUnion() = default;
136 
137   PointerUnion(std::nullptr_t) : PointerUnion() {}
138   using Base::Base;
139 
140   /// Test if the pointer held in the union is null, regardless of
141   /// which type it is.
142   bool isNull() const { return !this->Val.getPointer(); }
143 
144   explicit operator bool() const { return !isNull(); }
145 
146   // FIXME: Replace the uses of is(), get() and dyn_cast() with
147   //        isa<T>, cast<T> and the llvm::dyn_cast<T>
148 
149   /// Test if the Union currently holds the type matching T.
150   template <typename T> inline bool is() const { return isa<T>(*this); }
151 
152   /// Returns the value of the specified pointer type.
153   ///
154   /// If the specified pointer type is incorrect, assert.
155   template <typename T> inline T get() const {
156     assert(isa<T>(*this) && "Invalid accessor called");
157     return cast<T>(*this);
158   }
159 
160   /// Returns the current pointer if it is of the specified pointer type,
161   /// otherwise returns null.
162   template <typename T> inline T dyn_cast() const {
163     return llvm::dyn_cast<T>(*this);
164   }
165 
166   /// If the union is set to the first pointer type get an address pointing to
167   /// it.
168   First const *getAddrOfPtr1() const {
169     return const_cast<PointerUnion *>(this)->getAddrOfPtr1();
170   }
171 
172   /// If the union is set to the first pointer type get an address pointing to
173   /// it.
174   First *getAddrOfPtr1() {
175     assert(is<First>() && "Val is not the first pointer");
176     assert(
177         PointerLikeTypeTraits<First>::getAsVoidPointer(get<First>()) ==
178             this->Val.getPointer() &&
179         "Can't get the address because PointerLikeTypeTraits changes the ptr");
180     return const_cast<First *>(
181         reinterpret_cast<const First *>(this->Val.getAddrOfPointer()));
182   }
183 
184   /// Assignment from nullptr which just clears the union.
185   const PointerUnion &operator=(std::nullptr_t) {
186     this->Val.initWithPointer(nullptr);
187     return *this;
188   }
189 
190   /// Assignment from elements of the union.
191   using Base::operator=;
192 
193   void *getOpaqueValue() const { return this->Val.getOpaqueValue(); }
194   static inline PointerUnion getFromOpaqueValue(void *VP) {
195     PointerUnion V;
196     V.Val = decltype(V.Val)::getFromOpaqueValue(VP);
197     return V;
198   }
199 };
200 
201 template <typename ...PTs>
202 bool operator==(PointerUnion<PTs...> lhs, PointerUnion<PTs...> rhs) {
203   return lhs.getOpaqueValue() == rhs.getOpaqueValue();
204 }
205 
206 template <typename ...PTs>
207 bool operator!=(PointerUnion<PTs...> lhs, PointerUnion<PTs...> rhs) {
208   return lhs.getOpaqueValue() != rhs.getOpaqueValue();
209 }
210 
211 template <typename ...PTs>
212 bool operator<(PointerUnion<PTs...> lhs, PointerUnion<PTs...> rhs) {
213   return lhs.getOpaqueValue() < rhs.getOpaqueValue();
214 }
215 
216 /// We can't (at least, at this moment with C++14) declare CastInfo
217 /// as a friend of PointerUnion like this:
218 /// ```
219 ///   template<typename To>
220 ///   friend struct CastInfo<To, PointerUnion<PTs...>>;
221 /// ```
222 /// The compiler complains 'Partial specialization cannot be declared as a
223 /// friend'.
224 /// So we define this struct to be a bridge between CastInfo and
225 /// PointerUnion.
226 template <typename... PTs> struct CastInfoPointerUnionImpl {
227   using From = PointerUnion<PTs...>;
228 
229   template <typename To> static inline bool isPossible(From &F) {
230     return F.Val.getInt() == FirstIndexOfType<To, PTs...>::value;
231   }
232 
233   template <typename To> static To doCast(From &F) {
234     assert(isPossible<To>(F) && "cast to an incompatible type !");
235     return PointerLikeTypeTraits<To>::getFromVoidPointer(F.Val.getPointer());
236   }
237 };
238 
239 // Specialization of CastInfo for PointerUnion
240 template <typename To, typename... PTs>
241 struct CastInfo<To, PointerUnion<PTs...>>
242     : public DefaultDoCastIfPossible<To, PointerUnion<PTs...>,
243                                      CastInfo<To, PointerUnion<PTs...>>> {
244   using From = PointerUnion<PTs...>;
245   using Impl = CastInfoPointerUnionImpl<PTs...>;
246 
247   static inline bool isPossible(From &f) {
248     return Impl::template isPossible<To>(f);
249   }
250 
251   static To doCast(From &f) { return Impl::template doCast<To>(f); }
252 
253   static inline To castFailed() { return To(); }
254 };
255 
256 template <typename To, typename... PTs>
257 struct CastInfo<To, const PointerUnion<PTs...>>
258     : public ConstStrippingForwardingCast<To, const PointerUnion<PTs...>,
259                                           CastInfo<To, PointerUnion<PTs...>>> {
260 };
261 
262 // Teach SmallPtrSet that PointerUnion is "basically a pointer", that has
263 // # low bits available = min(PT1bits,PT2bits)-1.
264 template <typename ...PTs>
265 struct PointerLikeTypeTraits<PointerUnion<PTs...>> {
266   static inline void *getAsVoidPointer(const PointerUnion<PTs...> &P) {
267     return P.getOpaqueValue();
268   }
269 
270   static inline PointerUnion<PTs...> getFromVoidPointer(void *P) {
271     return PointerUnion<PTs...>::getFromOpaqueValue(P);
272   }
273 
274   // The number of bits available are the min of the pointer types minus the
275   // bits needed for the discriminator.
276   static constexpr int NumLowBitsAvailable = PointerLikeTypeTraits<decltype(
277       PointerUnion<PTs...>::Val)>::NumLowBitsAvailable;
278 };
279 
280 // Teach DenseMap how to use PointerUnions as keys.
281 template <typename ...PTs> struct DenseMapInfo<PointerUnion<PTs...>> {
282   using Union = PointerUnion<PTs...>;
283   using FirstInfo =
284       DenseMapInfo<typename pointer_union_detail::GetFirstType<PTs...>::type>;
285 
286   static inline Union getEmptyKey() { return Union(FirstInfo::getEmptyKey()); }
287 
288   static inline Union getTombstoneKey() {
289     return Union(FirstInfo::getTombstoneKey());
290   }
291 
292   static unsigned getHashValue(const Union &UnionVal) {
293     intptr_t key = (intptr_t)UnionVal.getOpaqueValue();
294     return DenseMapInfo<intptr_t>::getHashValue(key);
295   }
296 
297   static bool isEqual(const Union &LHS, const Union &RHS) {
298     return LHS == RHS;
299   }
300 };
301 
302 } // end namespace llvm
303 
304 #endif // LLVM_ADT_POINTERUNION_H
305