[libcxx-commits] [libcxx] [libc++] Make std::pair trivially copyable if its members are (PR #89652)
Nikolas Klauser via libcxx-commits
libcxx-commits at lists.llvm.org
Fri Jul 19 10:18:15 PDT 2024
https://github.com/philnik777 updated https://github.com/llvm/llvm-project/pull/89652
>From 8bf83cb3d4015c1cfeb55e70f0e2aecdaf84b5d1 Mon Sep 17 00:00:00 2001
From: Nikolas Klauser <nikolasklauser at berlin.de>
Date: Mon, 22 Apr 2024 20:58:34 +0200
Subject: [PATCH] [libc++][ABI BREAK] Make std::pair trivially copyable if its
members are
---
libcxx/include/__configuration/abi.h | 4 ++
libcxx/include/__type_traits/datasizeof.h | 1 +
libcxx/include/__utility/pair.h | 44 ++++++++++++++++---
.../abi.trivially_copyable.compile.pass.cpp | 22 +++++++++-
4 files changed, 65 insertions(+), 6 deletions(-)
diff --git a/libcxx/include/__configuration/abi.h b/libcxx/include/__configuration/abi.h
index cbde7887becf1..710548d90a649 100644
--- a/libcxx/include/__configuration/abi.h
+++ b/libcxx/include/__configuration/abi.h
@@ -98,6 +98,10 @@
// and WCHAR_MAX. This ABI setting determines whether we should instead track whether the fill
// value has been initialized using a separate boolean, which changes the ABI.
# define _LIBCPP_ABI_IOS_ALLOW_ARBITRARY_FILL_VALUE
+// Make a std::pair of trivially copyable types trivially copyable.
+// While this technically doesn't change the layout of pair itself, other types may decide to programatically change
+// their representation based on whether something is trivially copyable.
+# define _LIBCPP_ABI_TRIVIALLY_COPYABLE_PAIR
#elif _LIBCPP_ABI_VERSION == 1
# if !(defined(_LIBCPP_OBJECT_FORMAT_COFF) || defined(_LIBCPP_OBJECT_FORMAT_XCOFF))
// Enable compiling copies of now inline methods into the dylib to support
diff --git a/libcxx/include/__type_traits/datasizeof.h b/libcxx/include/__type_traits/datasizeof.h
index 35c12921e8ffa..a27baf67cc2d8 100644
--- a/libcxx/include/__type_traits/datasizeof.h
+++ b/libcxx/include/__type_traits/datasizeof.h
@@ -54,6 +54,7 @@ struct _FirstPaddingByte<_Tp, true> {
// the use as an extension.
_LIBCPP_DIAGNOSTIC_PUSH
_LIBCPP_CLANG_DIAGNOSTIC_IGNORED("-Winvalid-offsetof")
+_LIBCPP_GCC_DIAGNOSTIC_IGNORED("-Winvalid-offsetof")
template <class _Tp>
inline const size_t __datasizeof_v = offsetof(_FirstPaddingByte<_Tp>, __first_padding_byte_);
_LIBCPP_DIAGNOSTIC_POP
diff --git a/libcxx/include/__utility/pair.h b/libcxx/include/__utility/pair.h
index 0afbebcdc9f2a..ec3b4c2cd8a07 100644
--- a/libcxx/include/__utility/pair.h
+++ b/libcxx/include/__utility/pair.h
@@ -32,6 +32,7 @@
#include <__type_traits/is_implicitly_default_constructible.h>
#include <__type_traits/is_nothrow_assignable.h>
#include <__type_traits/is_nothrow_constructible.h>
+#include <__type_traits/is_reference.h>
#include <__type_traits/is_same.h>
#include <__type_traits/is_swappable.h>
#include <__type_traits/is_trivially_relocatable.h>
@@ -80,6 +81,36 @@ struct _LIBCPP_TEMPLATE_VIS pair
_LIBCPP_HIDE_FROM_ABI pair(pair const&) = default;
_LIBCPP_HIDE_FROM_ABI pair(pair&&) = default;
+ // When we are requested for pair to be trivially copyable by the ABI macro, we use defaulted members
+ // if it is both legal to do it (i.e. no references) and we have a way to actually implement it, which requires
+ // the __enable_if__ attribute before C++20.
+#ifndef _LIBCPP_ABI_TRIVIALLY_COPYABLE_PAIR
+ // FIXME: This should really just be a static constexpr variable. It's in a struct to avoid gdb printing the value
+ // when printing a pair
+ struct __has_defaulted_members {
+ static const bool value = !is_reference<first_type>::value && !is_reference<second_type>::value;
+ };
+# if _LIBCPP_STD_VER >= 20
+ _LIBCPP_HIDE_FROM_ABI constexpr pair& operator=(const pair&)
+ requires __has_defaulted_members::value
+ = default;
+
+ _LIBCPP_HIDE_FROM_ABI constexpr pair& operator=(pair&&)
+ requires __has_defaulted_members::value
+ = default;
+# elif __has_attribute(__enable_if__)
+ _LIBCPP_HIDE_FROM_ABI pair& operator=(const pair&)
+ __attribute__((__enable_if__(__has_defaulted_members::value, ""))) = default;
+
+ _LIBCPP_HIDE_FROM_ABI pair& operator=(pair&&)
+ __attribute__((__enable_if__(__has_defaulted_members::value, ""))) = default;
+# else
+# error "_LIBCPP_ABI_TRIVIALLY_COPYABLE_PAIR isn't supported with this compiler"
+# endif
+#else
+ enum { __has_defaulted_members = false };
+#endif // defined(_LIBCPP_ABI_TRIVIALLY_COPYABLE_PAIR) && __has_attribute(__enable_if__)
+
#ifdef _LIBCPP_CXX03_LANG
_LIBCPP_HIDE_FROM_ABI pair() : first(), second() {}
@@ -225,7 +256,8 @@ struct _LIBCPP_TEMPLATE_VIS pair
typename __make_tuple_indices<sizeof...(_Args2) >::type()) {}
_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 pair&
- operator=(__conditional_t<is_copy_assignable<first_type>::value && is_copy_assignable<second_type>::value,
+ operator=(__conditional_t<!__has_defaulted_members::value && is_copy_assignable<first_type>::value &&
+ is_copy_assignable<second_type>::value,
pair,
__nat> const& __p) noexcept(is_nothrow_copy_assignable<first_type>::value &&
is_nothrow_copy_assignable<second_type>::value) {
@@ -234,10 +266,12 @@ struct _LIBCPP_TEMPLATE_VIS pair
return *this;
}
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 pair& operator=(
- __conditional_t<is_move_assignable<first_type>::value && is_move_assignable<second_type>::value, pair, __nat>&&
- __p) noexcept(is_nothrow_move_assignable<first_type>::value &&
- is_nothrow_move_assignable<second_type>::value) {
+ _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 pair&
+ operator=(__conditional_t<!__has_defaulted_members::value && is_move_assignable<first_type>::value &&
+ is_move_assignable<second_type>::value,
+ pair,
+ __nat>&& __p) noexcept(is_nothrow_move_assignable<first_type>::value &&
+ is_nothrow_move_assignable<second_type>::value) {
first = std::forward<first_type>(__p.first);
second = std::forward<second_type>(__p.second);
return *this;
diff --git a/libcxx/test/libcxx/utilities/utility/pairs/pairs.pair/abi.trivially_copyable.compile.pass.cpp b/libcxx/test/libcxx/utilities/utility/pairs/pairs.pair/abi.trivially_copyable.compile.pass.cpp
index 1132b3e5def18..c5f9c8d0f2559 100644
--- a/libcxx/test/libcxx/utilities/utility/pairs/pairs.pair/abi.trivially_copyable.compile.pass.cpp
+++ b/libcxx/test/libcxx/utilities/utility/pairs/pairs.pair/abi.trivially_copyable.compile.pass.cpp
@@ -47,11 +47,20 @@ static_assert(!std::is_trivially_copyable<std::pair<int&, int> >::value, "");
static_assert(!std::is_trivially_copyable<std::pair<int, int&> >::value, "");
static_assert(!std::is_trivially_copyable<std::pair<int&, int&> >::value, "");
+#ifdef _LIBCPP_ABI_TRIVIALLY_COPYABLE_PAIR
+static_assert(std::is_trivially_copyable<std::pair<int, int> >::value, "");
+static_assert(std::is_trivially_copyable<std::pair<int, char> >::value, "");
+static_assert(std::is_trivially_copyable<std::pair<char, int> >::value, "");
+static_assert(std::is_trivially_copyable<std::pair<std::pair<char, char>, int> >::value, "");
+static_assert(std::is_trivially_copyable<std::pair<trivially_copyable, int> >::value, "");
+#else
static_assert(!std::is_trivially_copyable<std::pair<int, int> >::value, "");
static_assert(!std::is_trivially_copyable<std::pair<int, char> >::value, "");
static_assert(!std::is_trivially_copyable<std::pair<char, int> >::value, "");
static_assert(!std::is_trivially_copyable<std::pair<std::pair<char, char>, int> >::value, "");
static_assert(!std::is_trivially_copyable<std::pair<trivially_copyable, int> >::value, "");
+#endif // _LIBCPP_ABI_TRIVIALLY_COPYABLE_PAIR
+
#if TEST_STD_VER == 03 // Known ABI difference
static_assert(!std::is_trivially_copyable<std::pair<trivially_copyable_no_copy_assignment, int> >::value, "");
static_assert(!std::is_trivially_copyable<std::pair<trivially_copyable_no_move_assignment, int> >::value, "");
@@ -59,10 +68,21 @@ static_assert(!std::is_trivially_copyable<std::pair<trivially_copyable_no_move_a
static_assert(std::is_trivially_copyable<std::pair<trivially_copyable_no_copy_assignment, int> >::value, "");
static_assert(std::is_trivially_copyable<std::pair<trivially_copyable_no_move_assignment, int> >::value, "");
#endif
+
+#ifdef _LIBCPP_ABI_TRIVIALLY_COPYABLE_PAIR
+static_assert(std::is_trivially_copyable<std::pair<trivially_copyable_no_construction, int> >::value, "");
+#else
static_assert(!std::is_trivially_copyable<std::pair<trivially_copyable_no_construction, int> >::value, "");
+#endif
static_assert(std::is_trivially_copy_constructible<std::pair<int, int> >::value, "");
static_assert(std::is_trivially_move_constructible<std::pair<int, int> >::value, "");
+static_assert(std::is_trivially_destructible<std::pair<int, int> >::value, "");
+
+#ifdef _LIBCPP_ABI_TRIVIALLY_COPYABLE_PAIR
+static_assert(std::is_trivially_copy_assignable<std::pair<int, int> >::value, "");
+static_assert(std::is_trivially_move_assignable<std::pair<int, int> >::value, "");
+#else
static_assert(!std::is_trivially_copy_assignable<std::pair<int, int> >::value, "");
static_assert(!std::is_trivially_move_assignable<std::pair<int, int> >::value, "");
-static_assert(std::is_trivially_destructible<std::pair<int, int> >::value, "");
+#endif // _LIBCPP_ABI_TRIVIALLY_COPYABLE_PAIR
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