1// -*- C++ -*-
2//===----------------------------------------------------------------------===//
3//
4// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
5// See https://llvm.org/LICENSE.txt for license information.
6// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
7//
8//===----------------------------------------------------------------------===//
9
10#ifndef _LIBCPP___MEMORY_ARRAY_COOKIE_H
11#define _LIBCPP___MEMORY_ARRAY_COOKIE_H
12
13#include <__config>
14#include <__configuration/abi.h>
15#include <__cstddef/size_t.h>
16#include <__memory/addressof.h>
17#include <__type_traits/integral_constant.h>
18#include <__type_traits/is_trivially_destructible.h>
19#include <__type_traits/negation.h>
20
21#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
22# pragma GCC system_header
23#endif
24
25_LIBCPP_BEGIN_NAMESPACE_STD
26
27// Trait representing whether a type requires an array cookie at the start of its allocation when
28// allocated as `new T[n]` and deallocated as `delete[] array`.
29//
30// Under the Itanium C++ ABI [1] and the ARM ABI which derives from it, we know that an array cookie is available
31// unless `T` is trivially destructible and the call to `operator delete[]` is not a sized operator delete. Under
32// other ABIs, we assume there are no array cookies.
33//
34// [1]: https://itanium-cxx-abi.github.io/cxx-abi/abi.html#array-cookies
35#if defined(_LIBCPP_ABI_ITANIUM) || defined(_LIBCPP_ABI_ITANIUM_WITH_ARM_DIFFERENCES)
36// TODO: Use a builtin instead
37// TODO: We should factor in the choice of the usual deallocation function in this determination:
38// a cookie may be available in more cases but we ignore those for now.
39template <class _Tp>
40struct __has_array_cookie : _Not<is_trivially_destructible<_Tp> > {};
41#else
42template <class _Tp>
43struct __has_array_cookie : false_type {};
44#endif
45
46struct __itanium_array_cookie {
47 size_t __element_count;
48};
49
50template <class _Tp>
51struct [[__gnu__::__aligned__(_LIBCPP_ALIGNOF(_Tp))]] __arm_array_cookie {
52 size_t __element_size;
53 size_t __element_count;
54};
55
56// Return the element count in the array cookie located before the given pointer.
57//
58// In the Itanium ABI [1]
59// ----------------------
60// The element count is stored immediately before the first element of the array. If the preferred alignment
61// of array elements (which is different from the ABI alignment) is more than that of size_t, additional
62// padding bytes exist before the array cookie. Assuming array elements of size and alignment 16 bytes, that
63// gives us the following layout:
64//
65// |ooooooooxxxxxxxxaaaaaaaaaaaaaaaabbbbbbbbbbbbbbbbccccccccccccccccdddddddddddddddd|
66// ^^^^^^^^ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
67// | ^^^^^^^^ |
68// | | array elements
69// padding |
70// element count
71//
72//
73// In the Itanium ABI with ARM differences [2]
74// -------------------------------------------
75// The array cookie is stored at the very start of the allocation and it has the following form:
76//
77// struct array_cookie {
78// std::size_t element_size; // element_size != 0
79// std::size_t element_count;
80// };
81//
82// Assuming elements of size and alignment 32 bytes, this gives us the following layout:
83//
84// |xxxxxxxxXXXXXXXXooooooooooooooooaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaabbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb|
85// ^^^^^^^^ ^^^^^^^^^^^^^^^^
86// | ^^^^^^^^ | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
87// element size | padding |
88// element count array elements
89//
90// We must be careful to take into account the alignment of the array cookie, which may result in padding
91// bytes between the element count and the first element of the array. Note that for ARM, the compiler
92// aligns the array cookie using the ABI alignment, not the preferred alignment of array elements.
93//
94// [1]: https://itanium-cxx-abi.github.io/cxx-abi/abi.html#array-cookies
95// [2]: https://developer.apple.com/documentation/xcode/writing-arm64-code-for-apple-platforms#Handle-C++-differences
96template <class _Tp>
97// Avoid failures when -fsanitize-address-poison-custom-array-cookie is enabled
98_LIBCPP_HIDE_FROM_ABI _LIBCPP_NO_SANITIZE("address") size_t __get_array_cookie([[__maybe_unused__]] _Tp const* __ptr) {
99 static_assert(
100 __has_array_cookie<_Tp>::value, "Trying to access the array cookie of a type that is not guaranteed to have one");
101
102#if defined(_LIBCPP_ABI_ITANIUM)
103 using _ArrayCookie = __itanium_array_cookie;
104#elif defined(_LIBCPP_ABI_ITANIUM_WITH_ARM_DIFFERENCES)
105 using _ArrayCookie = __arm_array_cookie<_Tp>;
106#else
107 static_assert(false, "The array cookie layout is unknown on this ABI");
108 struct _ArrayCookie { // dummy definition required to make the function parse
109 size_t element_count;
110 };
111#endif
112
113 char const* __array_cookie_start = reinterpret_cast<char const*>(__ptr) - sizeof(_ArrayCookie);
114 _ArrayCookie __cookie;
115 // This is necessary to avoid violating strict aliasing. It's valid because _ArrayCookie is an
116 // implicit lifetime type.
117 __builtin_memcpy(std::addressof(x&: __cookie), __array_cookie_start, sizeof(_ArrayCookie));
118 return __cookie.__element_count;
119}
120
121_LIBCPP_END_NAMESPACE_STD
122
123#endif // _LIBCPP___MEMORY_ARRAY_COOKIE_H
124