1 //===- llvm/ADT/PointerEmbeddedInt.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_POINTEREMBEDDEDINT_H
11 #define LLVM_ADT_POINTEREMBEDDEDINT_H
12 
13 #include "llvm/ADT/DenseMapInfo.h"
14 #include "llvm/Support/MathExtras.h"
15 #include "llvm/Support/PointerLikeTypeTraits.h"
16 #include <cassert>
17 #include <climits>
18 #include <cstdint>
19 #include <type_traits>
20 
21 namespace llvm {
22 
23 /// Utility to embed an integer into a pointer-like type. This is specifically
24 /// intended to allow embedding integers where fewer bits are required than
25 /// exist in a pointer, and the integer can participate in abstractions along
26 /// side other pointer-like types. For example it can be placed into a \c
27 /// PointerSumType or \c PointerUnion.
28 ///
29 /// Note that much like pointers, an integer value of zero has special utility
30 /// due to boolean conversions. For example, a non-null value can be tested for
31 /// in the above abstractions without testing the particular active member.
32 /// Also, the default constructed value zero initializes the integer.
33 template <typename IntT, int Bits = sizeof(IntT) * CHAR_BIT>
34 class PointerEmbeddedInt {
35   uintptr_t Value = 0;
36 
37   // Note: This '<' is correct; using '<=' would result in some shifts
38   // overflowing their storage types.
39   static_assert(Bits < sizeof(uintptr_t) * CHAR_BIT,
40                 "Cannot embed more bits than we have in a pointer!");
41 
42   enum : uintptr_t {
43     // We shift as many zeros into the value as we can while preserving the
44     // number of bits desired for the integer.
45     Shift = sizeof(uintptr_t) * CHAR_BIT - Bits,
46 
47     // We also want to be able to mask out the preserved bits for asserts.
48     Mask = static_cast<uintptr_t>(-1) << Bits
49   };
50 
51   struct RawValueTag {
52     explicit RawValueTag() = default;
53   };
54 
55   friend struct PointerLikeTypeTraits<PointerEmbeddedInt>;
56 
57   explicit PointerEmbeddedInt(uintptr_t Value, RawValueTag) : Value(Value) {}
58 
59 public:
60   PointerEmbeddedInt() = default;
61 
62   PointerEmbeddedInt(IntT I) { *this = I; }
63 
64   PointerEmbeddedInt &operator=(IntT I) {
65     assert((std::is_signed<IntT>::value ? isInt<Bits>(I) : isUInt<Bits>(I)) &&
66            "Integer has bits outside those preserved!");
67     Value = static_cast<uintptr_t>(I) << Shift;
68     return *this;
69   }
70 
71   // Note that this implicit conversion additionally allows all of the basic
72   // comparison operators to work transparently, etc.
73   operator IntT() const {
74     if (std::is_signed<IntT>::value)
75       return static_cast<IntT>(static_cast<intptr_t>(Value) >> Shift);
76     return static_cast<IntT>(Value >> Shift);
77   }
78 };
79 
80 // Provide pointer like traits to support use with pointer unions and sum
81 // types.
82 template <typename IntT, int Bits>
83 struct PointerLikeTypeTraits<PointerEmbeddedInt<IntT, Bits>> {
84   using T = PointerEmbeddedInt<IntT, Bits>;
85 
86   static inline void *getAsVoidPointer(const T &P) {
87     return reinterpret_cast<void *>(P.Value);
88   }
89 
90   static inline T getFromVoidPointer(void *P) {
91     return T(reinterpret_cast<uintptr_t>(P), typename T::RawValueTag());
92   }
93 
94   static inline T getFromVoidPointer(const void *P) {
95     return T(reinterpret_cast<uintptr_t>(P), typename T::RawValueTag());
96   }
97 
98   enum { NumLowBitsAvailable = T::Shift };
99 };
100 
101 // Teach DenseMap how to use PointerEmbeddedInt objects as keys if the Int type
102 // itself can be a key.
103 template <typename IntT, int Bits>
104 struct DenseMapInfo<PointerEmbeddedInt<IntT, Bits>> {
105   using T = PointerEmbeddedInt<IntT, Bits>;
106   using IntInfo = DenseMapInfo<IntT>;
107 
108   static inline T getEmptyKey() { return IntInfo::getEmptyKey(); }
109   static inline T getTombstoneKey() { return IntInfo::getTombstoneKey(); }
110 
111   static unsigned getHashValue(const T &Arg) {
112     return IntInfo::getHashValue(Arg);
113   }
114 
115   static bool isEqual(const T &LHS, const T &RHS) { return LHS == RHS; }
116 };
117 
118 } // end namespace llvm
119 
120 #endif // LLVM_ADT_POINTEREMBEDDEDINT_H
121