1//===- llvm/Support/PointerLikeTypeTraits.h - Pointer Traits ----*- 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 PointerLikeTypeTraits class. This allows data
10// structures to reason about pointers and other things that are pointer sized.
11//
12//===----------------------------------------------------------------------===//
13
14#ifndef LLVM_SUPPORT_POINTERLIKETYPETRAITS_H
15#define LLVM_SUPPORT_POINTERLIKETYPETRAITS_H
16
17#include "llvm/Support/DataTypes.h"
18#include <cassert>
19#include <type_traits>
20
21namespace llvm {
22
23/// A traits type that is used to handle pointer types and things that are just
24/// wrappers for pointers as a uniform entity.
25template <typename T> struct PointerLikeTypeTraits;
26
27namespace detail {
28/// A tiny meta function to compute the log2 of a compile time constant.
29template <size_t N>
30struct ConstantLog2
31 : std::integral_constant<size_t, ConstantLog2<N / 2>::value + 1> {};
32template <> struct ConstantLog2<1> : std::integral_constant<size_t, 0> {};
33
34// Provide a trait to check if T is pointer-like.
35template <typename T, typename U = void> struct HasPointerLikeTypeTraits {
36 static const bool value = false;
37};
38
39// sizeof(T) is valid only for a complete T.
40template <typename T>
41struct 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 static constexpr int NumLowBitsAvailable =
61 detail::ConstantLog2<alignof(T)>::value;
62};
63
64template <> struct PointerLikeTypeTraits<void *> {
65 static inline void *getAsVoidPointer(void *P) { return P; }
66 static inline void *getFromVoidPointer(void *P) { return P; }
67
68 /// Note, we assume here that void* is related to raw malloc'ed memory and
69 /// that malloc returns objects at least 4-byte aligned. However, this may be
70 /// wrong, or pointers may be from something other than malloc. In this case,
71 /// you should specify a real typed pointer or avoid this template.
72 ///
73 /// All clients should use assertions to do a run-time check to ensure that
74 /// this is actually true.
75 static constexpr int NumLowBitsAvailable = 2;
76};
77
78// Provide PointerLikeTypeTraits for const things.
79template <typename T> struct PointerLikeTypeTraits<const T> {
80 typedef PointerLikeTypeTraits<T> NonConst;
81
82 static inline const void *getAsVoidPointer(const T P) {
83 return NonConst::getAsVoidPointer(P);
84 }
85 static inline const T getFromVoidPointer(const void *P) {
86 return NonConst::getFromVoidPointer(const_cast<void *>(P));
87 }
88 static constexpr int NumLowBitsAvailable = NonConst::NumLowBitsAvailable;
89};
90
91// Provide PointerLikeTypeTraits for const pointers.
92template <typename T> struct PointerLikeTypeTraits<const T *> {
93 typedef PointerLikeTypeTraits<T *> NonConst;
94
95 static inline const void *getAsVoidPointer(const T *P) {
96 return NonConst::getAsVoidPointer(const_cast<T *>(P));
97 }
98 static inline const T *getFromVoidPointer(const void *P) {
99 return NonConst::getFromVoidPointer(const_cast<void *>(P));
100 }
101 static constexpr int NumLowBitsAvailable = NonConst::NumLowBitsAvailable;
102};
103
104// Provide PointerLikeTypeTraits for uintptr_t.
105template <> struct PointerLikeTypeTraits<uintptr_t> {
106 static inline void *getAsVoidPointer(uintptr_t P) {
107 return reinterpret_cast<void *>(P);
108 }
109 static inline uintptr_t getFromVoidPointer(void *P) {
110 return reinterpret_cast<uintptr_t>(P);
111 }
112 // No bits are available!
113 static constexpr int NumLowBitsAvailable = 0;
114};
115
116/// Provide suitable custom traits struct for function pointers.
117///
118/// Function pointers can't be directly given these traits as functions can't
119/// have their alignment computed with `alignof` and we need different casting.
120///
121/// To rely on higher alignment for a specialized use, you can provide a
122/// customized form of this template explicitly with higher alignment, and
123/// potentially use alignment attributes on functions to satisfy that.
124template <int Alignment, typename FunctionPointerT>
125struct FunctionPointerLikeTypeTraits {
126 static constexpr int NumLowBitsAvailable =
127 detail::ConstantLog2<Alignment>::value;
128 static inline void *getAsVoidPointer(FunctionPointerT P) {
129 assert((reinterpret_cast<uintptr_t>(P) &
130 ~((uintptr_t)-1 << NumLowBitsAvailable)) == 0 &&
131 "Alignment not satisfied for an actual function pointer!");
132 return reinterpret_cast<void *>(P);
133 }
134 static inline FunctionPointerT getFromVoidPointer(void *P) {
135 return reinterpret_cast<FunctionPointerT>(P);
136 }
137};
138
139/// Provide a default specialization for function pointers that assumes 4-byte
140/// alignment.
141///
142/// We assume here that functions used with this are always at least 4-byte
143/// aligned. This means that, for example, thumb functions won't work or systems
144/// with weird unaligned function pointers won't work. But all practical systems
145/// we support satisfy this requirement.
146template <typename ReturnT, typename... ParamTs>
147struct PointerLikeTypeTraits<ReturnT (*)(ParamTs...)>
148 : FunctionPointerLikeTypeTraits<4, ReturnT (*)(ParamTs...)> {};
149
150} // end namespace llvm
151
152#endif
153