include/llvm/ADT/PointerUnion.h

   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   /// Implement assigment in terms of construction.
  97   template <typename Derived, typename T> struct AssignableFrom {
  98     Derived &operator=(T t) {
  99       return static_cast<Derived &>(*this) = Derived(t);
 100     }
 101   };
 102
 103   template <typename Derived, typename ValTy, int I, typename ...Types>
 104   class PointerUnionMembers;
 105
 106   template <typename Derived, typename ValTy, int I>
 107   class PointerUnionMembers<Derived, ValTy, I> {
 108   protected:
 109     ValTy Val;
 110     PointerUnionMembers() = default;
 111     PointerUnionMembers(ValTy Val) : Val(Val) {}
 112
 113     friend struct PointerLikeTypeTraits<Derived>;
 114   };
 115
 116   template <typename Derived, typename ValTy, int I, typename Type,
 117             typename ...Types>
 118   class PointerUnionMembers<Derived, ValTy, I, Type, Types...>
 119       : public PointerUnionMembers<Derived, ValTy, I + 1, Types...> {
 120     using Base = PointerUnionMembers<Derived, ValTy, I + 1, Types...>;
 121   public:
 122     using Base::Base;
 123     PointerUnionMembers() = default;
 124     PointerUnionMembers(Type V)
 125         : Base(ValTy(const_cast<void *>(
 126                          PointerLikeTypeTraits<Type>::getAsVoidPointer(V)),
 127                      I)) {}
 128
 129     using Base::operator=;
 130     Derived &operator=(Type V) {
 131       this->Val = ValTy(
 132           const_cast<void *>(PointerLikeTypeTraits<Type>::getAsVoidPointer(V)),
 133           I);
 134       return static_cast<Derived &>(*this);
 135     };
 136   };
 137 }
 138
 139 /// A discriminated union of two or more pointer types, with the discriminator
 140 /// in the low bit of the pointer.
 141 ///
 142 /// This implementation is extremely efficient in space due to leveraging the
 143 /// low bits of the pointer, while exposing a natural and type-safe API.
 144 ///
 145 /// Common use patterns would be something like this:
 146 ///    PointerUnion<int*, float*> P;
 147 ///    P = (int*)0;
 148 ///    printf("%d %d", P.is<int*>(), P.is<float*>());  // prints "1 0"
 149 ///    X = P.get<int*>();     // ok.
 150 ///    Y = P.get<float*>();   // runtime assertion failure.
 151 ///    Z = P.get<double*>();  // compile time failure.
 152 ///    P = (float*)0;
 153 ///    Y = P.get<float*>();   // ok.
 154 ///    X = P.get<int*>();     // runtime assertion failure.
 155 template <typename... PTs>
 156 class PointerUnion
 157     : public pointer_union_detail::PointerUnionMembers<
 158           PointerUnion<PTs...>,
 159           PointerIntPair<
 160               void *, pointer_union_detail::bitsRequired(sizeof...(PTs)), int,
 161               pointer_union_detail::PointerUnionUIntTraits<PTs...>>,
 162           0, PTs...> {
 163   // The first type is special because we want to directly cast a pointer to a
 164   // default-initialized union to a pointer to the first type. But we don't
 165   // want PointerUnion to be a 'template <typename First, typename ...Rest>'
 166   // because it's much more convenient to have a name for the whole pack. So
 167   // split off the first type here.
 168   using First = typename pointer_union_detail::GetFirstType<PTs...>::type;
 169   using Base = typename PointerUnion::PointerUnionMembers;
 170
 171 public:
 172   PointerUnion() = default;
 173
 174   PointerUnion(std::nullptr_t) : PointerUnion() {}
 175   using Base::Base;
 176
 177   /// Test if the pointer held in the union is null, regardless of
 178   /// which type it is.
 179   bool isNull() const { return !this->Val.getPointer(); }
 180
 181   explicit operator bool() const { return !isNull(); }
 182
 183   /// Test if the Union currently holds the type matching T.
 184   template <typename T> int is() const {
 185     constexpr int Index = pointer_union_detail::TypeIndex<T, PTs...>::Index;
 186     static_assert(Index < sizeof...(PTs),
 187                   "PointerUnion::is<T> given type not in the union");
 188     return this->Val.getInt() == Index;
 189   }
 190
 191   /// Returns the value of the specified pointer type.
 192   ///
 193   /// If the specified pointer type is incorrect, assert.
 194   template <typename T> T get() const {
 195     assert(is<T>() && "Invalid accessor called");
 196     return PointerLikeTypeTraits<T>::getFromVoidPointer(this->Val.getPointer());
 197   }
 198
 199   /// Returns the current pointer if it is of the specified pointer type,
 200   /// otherwises returns null.
 201   template <typename T> T dyn_cast() const {
 202     if (is<T>())
 203       return get<T>();
 204     return T();
 205   }
 206
 207   /// If the union is set to the first pointer type get an address pointing to
 208   /// it.
 209   First const *getAddrOfPtr1() const {
 210     return const_cast<PointerUnion *>(this)->getAddrOfPtr1();
 211   }
 212
 213   /// If the union is set to the first pointer type get an address pointing to
 214   /// it.
 215   First *getAddrOfPtr1() {
 216     assert(is<First>() && "Val is not the first pointer");
 217     assert(
 218         PointerLikeTypeTraits<First>::getAsVoidPointer(get<First>()) ==
 219             this->Val.getPointer() &&
 220         "Can't get the address because PointerLikeTypeTraits changes the ptr");
 221     return const_cast<First *>(
 222         reinterpret_cast<const First *>(this->Val.getAddrOfPointer()));
 223   }
 224
 225   /// Assignment from nullptr which just clears the union.
 226   const PointerUnion &operator=(std::nullptr_t) {
 227     this->Val.initWithPointer(nullptr);
 228     return *this;
 229   }
 230
 231   /// Assignment from elements of the union.
 232   using Base::operator=;
 233
 234   void *getOpaqueValue() const { return this->Val.getOpaqueValue(); }
 235   static inline PointerUnion getFromOpaqueValue(void *VP) {
 236     PointerUnion V;
 237     V.Val = decltype(V.Val)::getFromOpaqueValue(VP);
 238     return V;
 239   }
 240 };
 241
 242 template <typename ...PTs>
 243 bool operator==(PointerUnion<PTs...> lhs, PointerUnion<PTs...> rhs) {
 244   return lhs.getOpaqueValue() == rhs.getOpaqueValue();
 245 }
 246
 247 template <typename ...PTs>
 248 bool operator!=(PointerUnion<PTs...> lhs, PointerUnion<PTs...> rhs) {
 249   return lhs.getOpaqueValue() != rhs.getOpaqueValue();
 250 }
 251
 252 template <typename ...PTs>
 253 bool operator<(PointerUnion<PTs...> lhs, PointerUnion<PTs...> rhs) {
 254   return lhs.getOpaqueValue() < rhs.getOpaqueValue();
 255 }
 256
 257 // Teach SmallPtrSet that PointerUnion is "basically a pointer", that has
 258 // # low bits available = min(PT1bits,PT2bits)-1.
 259 template <typename ...PTs>
 260 struct PointerLikeTypeTraits<PointerUnion<PTs...>> {
 261   static inline void *getAsVoidPointer(const PointerUnion<PTs...> &P) {
 262     return P.getOpaqueValue();
 263   }
 264
 265   static inline PointerUnion<PTs...> getFromVoidPointer(void *P) {
 266     return PointerUnion<PTs...>::getFromOpaqueValue(P);
 267   }
 268
 269   // The number of bits available are the min of the pointer types minus the
 270   // bits needed for the discriminator.
 271   static constexpr int NumLowBitsAvailable = PointerLikeTypeTraits<decltype(
 272       PointerUnion<PTs...>::Val)>::NumLowBitsAvailable;
 273 };
 274
 275 /// A pointer union of three pointer types. See documentation for PointerUnion
 276 /// for usage.
 277 template <typename PT1, typename PT2, typename PT3>
 278 using PointerUnion3 = PointerUnion<PT1, PT2, PT3>;
 279
 280 /// A pointer union of four pointer types. See documentation for PointerUnion
 281 /// for usage.
 282 template <typename PT1, typename PT2, typename PT3, typename PT4>
 283 using PointerUnion4 = PointerUnion<PT1, PT2, PT3, PT4>;
 284
 285 // Teach DenseMap how to use PointerUnions as keys.
 286 template <typename ...PTs> struct DenseMapInfo<PointerUnion<PTs...>> {
 287   using Union = PointerUnion<PTs...>;
 288   using FirstInfo =
 289       DenseMapInfo<typename pointer_union_detail::GetFirstType<PTs...>::type>;
 290
 291   static inline Union getEmptyKey() { return Union(FirstInfo::getEmptyKey()); }
 292
 293   static inline Union getTombstoneKey() {
 294     return Union(FirstInfo::getTombstoneKey());
 295   }
 296
 297   static unsigned getHashValue(const Union &UnionVal) {
 298     intptr_t key = (intptr_t)UnionVal.getOpaqueValue();
 299     return DenseMapInfo<intptr_t>::getHashValue(key);
 300   }
 301
 302   static bool isEqual(const Union &LHS, const Union &RHS) {
 303     return LHS == RHS;
 304   }
 305 };
 306
 307 } // end namespace llvm
 308
 309 #endif // LLVM_ADT_POINTERUNION_H