PointerLikeTypeTraits.h revision 341825 
1//===- llvm/Support/PointerLikeTypeTraits.h - Pointer Traits ----*- 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// This file defines the PointerLikeTypeTraits class.  This allows data
11// structures to reason about pointers and other things that are pointer sized.
12//
13//===----------------------------------------------------------------------===//
14
15#ifndef LLVM_SUPPORT_POINTERLIKETYPETRAITS_H
16#define LLVM_SUPPORT_POINTERLIKETYPETRAITS_H
17
18#include "llvm/Support/DataTypes.h"
19#include <assert.h>
20#include <type_traits>
21
22namespace llvm {
23
24/// A traits type that is used to handle pointer types and things that are just
25/// wrappers for pointers as a uniform entity.
26template <typename T> struct PointerLikeTypeTraits;
27
28namespace detail {
29/// A tiny meta function to compute the log2 of a compile time constant.
30template <size_t N>
31struct ConstantLog2
32    : std::integral_constant<size_t, ConstantLog2<N / 2>::value + 1> {};
33template <> struct ConstantLog2<1> : std::integral_constant<size_t, 0> {};
34
35// Provide a trait to check if T is pointer-like.
36template <typename T, typename U = void> struct HasPointerLikeTypeTraits {
37  static const bool value = false;
38};
39
40// sizeof(T) is valid only for a complete T.
41template <typename T> struct HasPointerLikeTypeTraits<
42  T, decltype((sizeof(PointerLikeTypeTraits<T>) + sizeof(T)), void())> {
43  static const bool value = true;
44};
45
46template <typename T> struct IsPointerLike {
47  static const bool value = HasPointerLikeTypeTraits<T>::value;
48};
49
50template <typename T> struct IsPointerLike<T *> {
51  static const bool value = true;
52};
53} // namespace detail
54
55// Provide PointerLikeTypeTraits for non-cvr pointers.
56template <typename T> struct PointerLikeTypeTraits<T *> {
57  static inline void *getAsVoidPointer(T *P) { return P; }
58  static inline T *getFromVoidPointer(void *P) { return static_cast<T *>(P); }
59
60  enum { NumLowBitsAvailable = detail::ConstantLog2<alignof(T)>::value };
61};
62
63template <> struct PointerLikeTypeTraits<void *> {
64  static inline void *getAsVoidPointer(void *P) { return P; }
65  static inline void *getFromVoidPointer(void *P) { return P; }
66
67  /// Note, we assume here that void* is related to raw malloc'ed memory and
68  /// that malloc returns objects at least 4-byte aligned. However, this may be
69  /// wrong, or pointers may be from something other than malloc. In this case,
70  /// you should specify a real typed pointer or avoid this template.
71  ///
72  /// All clients should use assertions to do a run-time check to ensure that
73  /// this is actually true.
74  enum { NumLowBitsAvailable = 2 };
75};
76
77// Provide PointerLikeTypeTraits for const things.
78template <typename T> struct PointerLikeTypeTraits<const T> {
79  typedef PointerLikeTypeTraits<T> NonConst;
80
81  static inline const void *getAsVoidPointer(const T P) {
82    return NonConst::getAsVoidPointer(P);
83  }
84  static inline const T getFromVoidPointer(const void *P) {
85    return NonConst::getFromVoidPointer(const_cast<void *>(P));
86  }
87  enum { NumLowBitsAvailable = NonConst::NumLowBitsAvailable };
88};
89
90// Provide PointerLikeTypeTraits for const pointers.
91template <typename T> struct PointerLikeTypeTraits<const T *> {
92  typedef PointerLikeTypeTraits<T *> NonConst;
93
94  static inline const void *getAsVoidPointer(const T *P) {
95    return NonConst::getAsVoidPointer(const_cast<T *>(P));
96  }
97  static inline const T *getFromVoidPointer(const void *P) {
98    return NonConst::getFromVoidPointer(const_cast<void *>(P));
99  }
100  enum { NumLowBitsAvailable = NonConst::NumLowBitsAvailable };
101};
102
103// Provide PointerLikeTypeTraits for uintptr_t.
104template <> struct PointerLikeTypeTraits<uintptr_t> {
105  static inline void *getAsVoidPointer(uintptr_t P) {
106    return reinterpret_cast<void *>(P);
107  }
108  static inline uintptr_t getFromVoidPointer(void *P) {
109    return reinterpret_cast<uintptr_t>(P);
110  }
111  // No bits are available!
112  enum { NumLowBitsAvailable = 0 };
113};
114
115/// Provide suitable custom traits struct for function pointers.
116///
117/// Function pointers can't be directly given these traits as functions can't
118/// have their alignment computed with `alignof` and we need different casting.
119///
120/// To rely on higher alignment for a specialized use, you can provide a
121/// customized form of this template explicitly with higher alignment, and
122/// potentially use alignment attributes on functions to satisfy that.
123template <int Alignment, typename FunctionPointerT>
124struct FunctionPointerLikeTypeTraits {
125  enum { NumLowBitsAvailable = detail::ConstantLog2<Alignment>::value };
126  static inline void *getAsVoidPointer(FunctionPointerT P) {
127    assert((reinterpret_cast<uintptr_t>(P) &
128            ~((uintptr_t)-1 << NumLowBitsAvailable)) == 0 &&
129           "Alignment not satisfied for an actual function pointer!");
130    return reinterpret_cast<void *>(P);
131  }
132  static inline FunctionPointerT getFromVoidPointer(void *P) {
133    return reinterpret_cast<FunctionPointerT>(P);
134  }
135};
136
137/// Provide a default specialization for function pointers that assumes 4-byte
138/// alignment.
139///
140/// We assume here that functions used with this are always at least 4-byte
141/// aligned. This means that, for example, thumb functions won't work or systems
142/// with weird unaligned function pointers won't work. But all practical systems
143/// we support satisfy this requirement.
144template <typename ReturnT, typename... ParamTs>
145struct PointerLikeTypeTraits<ReturnT (*)(ParamTs...)>
146    : FunctionPointerLikeTypeTraits<4, ReturnT (*)(ParamTs...)> {};
147
148} // end namespace llvm
149
150#endif
151