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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-exception7//8//===----------------------------------------------------------------------===//9 10#ifndef _LIBCPP___MEMORY_ARRAY_COOKIE_H11#define _LIBCPP___MEMORY_ARRAY_COOKIE_H12 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_header23#endif24 25_LIBCPP_BEGIN_NAMESPACE_STD26 27// Trait representing whether a type requires an array cookie at the start of its allocation when28// 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 available31// unless `T` is trivially destructible and the call to `operator delete[]` is not a sized operator delete. Under32// other ABIs, we assume there are no array cookies.33//34// [1]: https://itanium-cxx-abi.github.io/cxx-abi/abi.html#array-cookies35#if defined(_LIBCPP_ABI_ITANIUM) || defined(_LIBCPP_ABI_ITANIUM_WITH_ARM_DIFFERENCES)36// TODO: Use a builtin instead37// 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#else42template <class _Tp>43struct __has_array_cookie : false_type {};44#endif45 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 alignment61// of array elements (which is different from the ABI alignment) is more than that of size_t, additional62// padding bytes exist before the array cookie. Assuming array elements of size and alignment 16 bytes, that63// gives us the following layout:64//65// |ooooooooxxxxxxxxaaaaaaaaaaaaaaaabbbbbbbbbbbbbbbbccccccccccccccccdddddddddddddddd|66// ^^^^^^^^ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^67// | ^^^^^^^^ |68// | | array elements69// padding |70// element count71//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 != 079// 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 elements89//90// We must be careful to take into account the alignment of the array cookie, which may result in padding91// bytes between the element count and the first element of the array. Note that for ARM, the compiler92// 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-cookies95// [2]: https://developer.apple.com/documentation/xcode/writing-arm64-code-for-apple-platforms#Handle-C++-differences96template <class _Tp>97// Avoid failures when -fsanitize-address-poison-custom-array-cookie is enabled98_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#else107 static_assert(false, "The array cookie layout is unknown on this ABI");108 struct _ArrayCookie { // dummy definition required to make the function parse109 size_t element_count;110 };111#endif112 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 an116 // implicit lifetime type.117 __builtin_memcpy(std::addressof(__cookie), __array_cookie_start, sizeof(_ArrayCookie));118 return __cookie.__element_count;119}120 121_LIBCPP_END_NAMESPACE_STD122 123#endif // _LIBCPP___MEMORY_ARRAY_COOKIE_H124