[libc++] Simplify the implementation of aligned_storage #162459
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@llvm/pr-subscribers-libcxx Author: Nikolas Klauser (philnik777) ChangesFull diff: https://github.com/llvm/llvm-project/pull/162459.diff 2 Files Affected:
diff --git a/libcxx/include/__config b/libcxx/include/__config
index b4c081dcdff1b..a77d40636838a 100644
--- a/libcxx/include/__config
+++ b/libcxx/include/__config
@@ -339,7 +339,7 @@ _LIBCPP_HARDENING_MODE_DEBUG
# ifndef _LIBCPP_CXX03_LANG
-# define _LIBCPP_ALIGNOF(_Tp) alignof(_Tp)
+# define _LIBCPP_ALIGNOF(...) alignof(__VA_ARGS__)
# define _ALIGNAS_TYPE(x) alignas(x)
# define _ALIGNAS(x) alignas(x)
# define _NOEXCEPT noexcept
@@ -348,7 +348,7 @@ _LIBCPP_HARDENING_MODE_DEBUG
# else
-# define _LIBCPP_ALIGNOF(_Tp) _Alignof(_Tp)
+# define _LIBCPP_ALIGNOF(...) _Alignof(__VA_ARGS__)
# define _ALIGNAS_TYPE(x) __attribute__((__aligned__(_LIBCPP_ALIGNOF(x))))
# define _ALIGNAS(x) __attribute__((__aligned__(x)))
# define nullptr __nullptr
diff --git a/libcxx/include/__type_traits/aligned_storage.h b/libcxx/include/__type_traits/aligned_storage.h
index 5c2208ae0c70a..33c0368d0c3c8 100644
--- a/libcxx/include/__type_traits/aligned_storage.h
+++ b/libcxx/include/__type_traits/aligned_storage.h
@@ -11,8 +11,6 @@
#include <__config>
#include <__cstddef/size_t.h>
-#include <__type_traits/integral_constant.h>
-#include <__type_traits/type_list.h>
#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
# pragma GCC system_header
@@ -21,10 +19,10 @@
_LIBCPP_BEGIN_NAMESPACE_STD
template <class _Tp>
-struct __align_type {
- static const size_t value = _LIBCPP_PREFERRED_ALIGNOF(_Tp);
- typedef _Tp type;
-};
+struct _ALIGNAS(_LIBCPP_PREFERRED_ALIGNOF(_Tp)) _AlignedAsT {};
+
+template <class... _Args>
+struct __max_align_impl : _AlignedAsT<_Args>... {};
struct __struct_double {
long double __lx;
@@ -33,41 +31,16 @@ struct __struct_double4 {
double __lx[4];
};
-using __all_types _LIBCPP_NODEBUG =
- __type_list<__align_type<unsigned char>,
- __align_type<unsigned short>,
- __align_type<unsigned int>,
- __align_type<unsigned long>,
- __align_type<unsigned long long>,
- __align_type<double>,
- __align_type<long double>,
- __align_type<__struct_double>,
- __align_type<__struct_double4>,
- __align_type<int*> >;
-
-template <class _TL, size_t _Len>
-struct __find_max_align;
-
-template <class _Head, size_t _Len>
-struct __find_max_align<__type_list<_Head>, _Len> : public integral_constant<size_t, _Head::value> {};
-
-template <size_t _Len, size_t _A1, size_t _A2>
-struct __select_align {
-private:
- static const size_t __min = _A2 < _A1 ? _A2 : _A1;
- static const size_t __max = _A1 < _A2 ? _A2 : _A1;
-
-public:
- static const size_t value = _Len < __max ? __min : __max;
-};
+inline const size_t __aligned_storage_max_align =
+ _LIBCPP_ALIGNOF(__max_align_impl<unsigned long long, double, long double, __struct_double, __struct_double4, int*>);
-template <class _Head, class... _Tail, size_t _Len>
-struct __find_max_align<__type_list<_Head, _Tail...>, _Len>
- : public integral_constant<
- size_t,
- __select_align<_Len, _Head::value, __find_max_align<__type_list<_Tail...>, _Len>::value>::value> {};
+template <size_t _Len>
+inline const size_t __aligned_storage_alignment =
+ _Len > __aligned_storage_max_align
+ ? __aligned_storage_max_align
+ : size_t(1) << ((sizeof(size_t) * __CHAR_BIT__) - __builtin_clzg(_Len) - 1);
-template <size_t _Len, size_t _Align = __find_max_align<__all_types, _Len>::value>
+template <size_t _Len, size_t _Align = __aligned_storage_alignment<_Len> >
struct _LIBCPP_DEPRECATED_IN_CXX23 _LIBCPP_NO_SPECIALIZATIONS aligned_storage {
union _ALIGNAS(_Align) type {
unsigned char __data[(_Len + _Align - 1) / _Align * _Align];
@@ -77,7 +50,7 @@ struct _LIBCPP_DEPRECATED_IN_CXX23 _LIBCPP_NO_SPECIALIZATIONS aligned_storage {
#if _LIBCPP_STD_VER >= 14
_LIBCPP_SUPPRESS_DEPRECATED_PUSH
-template <size_t _Len, size_t _Align = __find_max_align<__all_types, _Len>::value>
+template <size_t _Len, size_t _Align = __aligned_storage_alignment<_Len> >
using aligned_storage_t _LIBCPP_DEPRECATED_IN_CXX23 = typename aligned_storage<_Len, _Align>::type;
_LIBCPP_SUPPRESS_DEPRECATED_POP
|
ldionne
reviewed
Nov 12, 2025
| inline const size_t __aligned_storage_alignment = | ||
| _Len > __aligned_storage_max_align | ||
| ? __aligned_storage_max_align | ||
| : size_t(1) << ((sizeof(size_t) * __CHAR_BIT__) - __builtin_clzg(_Len) - 1); |
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IIUC, this computes the largest power of two that is less than or equal to the given _Len. So for example, for sizeof(T) == 3, we get an alignment of 2. For sizeof(T) == 4, we get 4.
I don't understand how that's correct. I would have thought that sizeof(T) implies that the alignment of T >= 4, doesn't it?
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I think you convinced me that this was the right thing since the size of a struct is always a multiple of its alignment.
ldionne
approved these changes
Nov 12, 2025
| inline const size_t __aligned_storage_alignment = | ||
| _Len > __aligned_storage_max_align | ||
| ? __aligned_storage_max_align | ||
| : size_t(1) << ((sizeof(size_t) * __CHAR_BIT__) - __builtin_clzg(_Len) - 1); |
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I think you convinced me that this was the right thing since the size of a struct is always a multiple of its alignment.
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Nov 13, 2025
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