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[libc++] Make std::pair trivially copyable if its members are (#89652)
This makes `std::pair` trivially copyable if its members are and we have a way to do so. We need either C++20 with requires clauses or support for `__attribute__((enable_if))`. Only Clang has support for this attribute, so it's effectively clang or C++20. Co-authored-by: Christopher Di Bella <[email protected]>
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‎libcxx/include/__configuration/abi.h

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@@ -98,6 +98,10 @@
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// and WCHAR_MAX. This ABI setting determines whether we should instead track whether the fill
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// value has been initialized using a separate boolean, which changes the ABI.
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# define _LIBCPP_ABI_IOS_ALLOW_ARBITRARY_FILL_VALUE
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// Make a std::pair of trivially copyable types trivially copyable.
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// While this technically doesn't change the layout of pair itself, other types may decide to programatically change
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// their representation based on whether something is trivially copyable.
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# define _LIBCPP_ABI_TRIVIALLY_COPYABLE_PAIR
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#elif _LIBCPP_ABI_VERSION == 1
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# if !(defined(_LIBCPP_OBJECT_FORMAT_COFF) || defined(_LIBCPP_OBJECT_FORMAT_XCOFF))
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// Enable compiling copies of now inline methods into the dylib to support

‎libcxx/include/__type_traits/datasizeof.h

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@@ -54,6 +54,7 @@ struct _FirstPaddingByte<_Tp, true> {
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// the use as an extension.
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_LIBCPP_DIAGNOSTIC_PUSH
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_LIBCPP_CLANG_DIAGNOSTIC_IGNORED("-Winvalid-offsetof")
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_LIBCPP_GCC_DIAGNOSTIC_IGNORED("-Winvalid-offsetof")
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template <class _Tp>
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inline const size_t __datasizeof_v = offsetof(_FirstPaddingByte<_Tp>, __first_padding_byte_);
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_LIBCPP_DIAGNOSTIC_POP

‎libcxx/include/__utility/pair.h

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@@ -32,6 +32,7 @@
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#include <__type_traits/is_implicitly_default_constructible.h>
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#include <__type_traits/is_nothrow_assignable.h>
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#include <__type_traits/is_nothrow_constructible.h>
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#include <__type_traits/is_reference.h>
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#include <__type_traits/is_same.h>
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#include <__type_traits/is_swappable.h>
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#include <__type_traits/is_trivially_relocatable.h>
@@ -80,6 +81,38 @@ struct _LIBCPP_TEMPLATE_VIS pair
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_LIBCPP_HIDE_FROM_ABI pair(pair const&) = default;
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_LIBCPP_HIDE_FROM_ABI pair(pair&&) = default;
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// When we are requested for pair to be trivially copyable by the ABI macro, we use defaulted members
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// if it is both legal to do it (i.e. no references) and we have a way to actually implement it, which requires
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// the __enable_if__ attribute before C++20.
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#ifdef _LIBCPP_ABI_TRIVIALLY_COPYABLE_PAIR
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// FIXME: This should really just be a static constexpr variable. It's in a struct to avoid gdb printing the value
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// when printing a pair
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struct __has_defaulted_members {
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static const bool value = !is_reference<first_type>::value && !is_reference<second_type>::value;
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};
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# if _LIBCPP_STD_VER >= 20
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_LIBCPP_HIDE_FROM_ABI constexpr pair& operator=(const pair&)
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requires __has_defaulted_members::value
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= default;
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_LIBCPP_HIDE_FROM_ABI constexpr pair& operator=(pair&&)
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requires __has_defaulted_members::value
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= default;
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# elif __has_attribute(__enable_if__)
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_LIBCPP_HIDE_FROM_ABI pair& operator=(const pair&)
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__attribute__((__enable_if__(__has_defaulted_members::value, ""))) = default;
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_LIBCPP_HIDE_FROM_ABI pair& operator=(pair&&)
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__attribute__((__enable_if__(__has_defaulted_members::value, ""))) = default;
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# else
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# error "_LIBCPP_ABI_TRIVIALLY_COPYABLE_PAIR isn't supported with this compiler"
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# endif
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#else
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struct __has_defaulted_members {
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static const bool value = false;
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};
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#endif // defined(_LIBCPP_ABI_TRIVIALLY_COPYABLE_PAIR) && __has_attribute(__enable_if__)
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#ifdef _LIBCPP_CXX03_LANG
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_LIBCPP_HIDE_FROM_ABI pair() : first(), second() {}
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@@ -225,7 +258,8 @@ struct _LIBCPP_TEMPLATE_VIS pair
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typename __make_tuple_indices<sizeof...(_Args2) >::type()) {}
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_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 pair&
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operator=(__conditional_t<is_copy_assignable<first_type>::value && is_copy_assignable<second_type>::value,
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operator=(__conditional_t<!__has_defaulted_members::value && is_copy_assignable<first_type>::value &&
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is_copy_assignable<second_type>::value,
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pair,
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__nat> const& __p) noexcept(is_nothrow_copy_assignable<first_type>::value &&
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is_nothrow_copy_assignable<second_type>::value) {
@@ -234,10 +268,12 @@ struct _LIBCPP_TEMPLATE_VIS pair
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return *this;
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}
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_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 pair& operator=(
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__conditional_t<is_move_assignable<first_type>::value && is_move_assignable<second_type>::value, pair, __nat>&&
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__p) noexcept(is_nothrow_move_assignable<first_type>::value &&
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is_nothrow_move_assignable<second_type>::value) {
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_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 pair&
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operator=(__conditional_t<!__has_defaulted_members::value && is_move_assignable<first_type>::value &&
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is_move_assignable<second_type>::value,
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pair,
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__nat>&& __p) noexcept(is_nothrow_move_assignable<first_type>::value &&
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is_nothrow_move_assignable<second_type>::value) {
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first = std::forward<first_type>(__p.first);
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second = std::forward<second_type>(__p.second);
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return *this;

‎libcxx/test/libcxx/utilities/utility/pairs/pairs.pair/abi.trivial_copy_move.pass.cpp

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@@ -162,8 +162,13 @@ void test_trivial()
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static_assert(!std::is_trivially_copy_constructible<P>::value, "");
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static_assert(!std::is_trivially_move_constructible<P>::value, "");
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#endif // TEST_STD_VER >= 11
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#ifndef _LIBCPP_ABI_TRIVIALLY_COPYABLE_PAIR
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static_assert(!std::is_trivially_copy_assignable<P>::value, "");
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static_assert(!std::is_trivially_move_assignable<P>::value, "");
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#else
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static_assert(std::is_trivially_copy_assignable<P>::value, "");
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static_assert(std::is_trivially_move_assignable<P>::value, "");
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#endif
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static_assert(std::is_trivially_destructible<P>::value, "");
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}
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}

‎libcxx/test/libcxx/utilities/utility/pairs/pairs.pair/abi.trivially_copyable.compile.pass.cpp

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@@ -47,22 +47,42 @@ static_assert(!std::is_trivially_copyable<std::pair<int&, int> >::value, "");
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static_assert(!std::is_trivially_copyable<std::pair<int, int&> >::value, "");
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static_assert(!std::is_trivially_copyable<std::pair<int&, int&> >::value, "");
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#ifdef _LIBCPP_ABI_TRIVIALLY_COPYABLE_PAIR
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static_assert(std::is_trivially_copyable<std::pair<int, int> >::value, "");
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static_assert(std::is_trivially_copyable<std::pair<int, char> >::value, "");
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static_assert(std::is_trivially_copyable<std::pair<char, int> >::value, "");
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static_assert(std::is_trivially_copyable<std::pair<std::pair<char, char>, int> >::value, "");
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static_assert(std::is_trivially_copyable<std::pair<trivially_copyable, int> >::value, "");
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#else
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static_assert(!std::is_trivially_copyable<std::pair<int, int> >::value, "");
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static_assert(!std::is_trivially_copyable<std::pair<int, char> >::value, "");
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static_assert(!std::is_trivially_copyable<std::pair<char, int> >::value, "");
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static_assert(!std::is_trivially_copyable<std::pair<std::pair<char, char>, int> >::value, "");
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static_assert(!std::is_trivially_copyable<std::pair<trivially_copyable, int> >::value, "");
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#endif // _LIBCPP_ABI_TRIVIALLY_COPYABLE_PAIR
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#if TEST_STD_VER == 03 // Known ABI difference
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static_assert(!std::is_trivially_copyable<std::pair<trivially_copyable_no_copy_assignment, int> >::value, "");
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static_assert(!std::is_trivially_copyable<std::pair<trivially_copyable_no_move_assignment, int> >::value, "");
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#else
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static_assert(std::is_trivially_copyable<std::pair<trivially_copyable_no_copy_assignment, int> >::value, "");
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static_assert(std::is_trivially_copyable<std::pair<trivially_copyable_no_move_assignment, int> >::value, "");
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#endif
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#ifdef _LIBCPP_ABI_TRIVIALLY_COPYABLE_PAIR
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static_assert(std::is_trivially_copyable<std::pair<trivially_copyable_no_construction, int> >::value, "");
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#else
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static_assert(!std::is_trivially_copyable<std::pair<trivially_copyable_no_construction, int> >::value, "");
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#endif
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static_assert(std::is_trivially_copy_constructible<std::pair<int, int> >::value, "");
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static_assert(std::is_trivially_move_constructible<std::pair<int, int> >::value, "");
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static_assert(std::is_trivially_destructible<std::pair<int, int> >::value, "");
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#ifdef _LIBCPP_ABI_TRIVIALLY_COPYABLE_PAIR
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static_assert(std::is_trivially_copy_assignable<std::pair<int, int> >::value, "");
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static_assert(std::is_trivially_move_assignable<std::pair<int, int> >::value, "");
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#else
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static_assert(!std::is_trivially_copy_assignable<std::pair<int, int> >::value, "");
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static_assert(!std::is_trivially_move_assignable<std::pair<int, int> >::value, "");
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static_assert(std::is_trivially_destructible<std::pair<int, int> >::value, "");
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#endif // _LIBCPP_ABI_TRIVIALLY_COPYABLE_PAIR

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