[gcc r15-5720] libstdc++: Simplify std::forward_list assignment using 'if constexpr'
Jonathan Wakely
redi@gcc.gnu.org
Wed Nov 27 12:35:35 GMT 2024
https://gcc.gnu.org/g:751f91be72d8b73a9dc71456f475268397330100
commit r15-5720-g751f91be72d8b73a9dc71456f475268397330100
Author: Jonathan Wakely <jwakely@redhat.com>
Date: Mon Nov 25 21:57:57 2024 +0000
libstdc++: Simplify std::forward_list assignment using 'if constexpr'
Use diagnostic pragmas to allow using `if constexpr` in C++11 mode, so
that we don't need to use tag dispatching.
The unused member functions are preserved for the purposes of explicit
instantiations. The _M_assign function template can be removed, because
member function templates aren't instantiated by explicit instantiations
anyway.
libstdc++-v3/ChangeLog:
* include/bits/forward_list.h (operator=(forward_list&&)): Use
if constexpr instead of dispatching to _M_move_assign.
(assign(InputIterator, InputIterator)): Use if constexpr instead
of dispatching to _M_assign.
(assign(size_type, const T&)): Use if constexpr instead of
dispatching to _M_assign_n.
(_M_move_assign, _M_assign_n): Do not define for versioned
namespace.
(_M_assign): Remove.
Diff:
---
libstdc++-v3/include/bits/forward_list.h | 119 ++++++++++++++++++++-----------
1 file changed, 79 insertions(+), 40 deletions(-)
diff --git a/libstdc++-v3/include/bits/forward_list.h b/libstdc++-v3/include/bits/forward_list.h
index 663ed0c46aff..ac1b3593c794 100644
--- a/libstdc++-v3/include/bits/forward_list.h
+++ b/libstdc++-v3/include/bits/forward_list.h
@@ -644,6 +644,8 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
forward_list&
operator=(const forward_list& __list);
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wc++17-extensions" // if constexpr
/**
* @brief The %forward_list move assignment operator.
* @param __list A %forward_list of identical element and allocator
@@ -663,7 +665,24 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
constexpr bool __move_storage =
_Node_alloc_traits::_S_propagate_on_move_assign()
|| _Node_alloc_traits::_S_always_equal();
- _M_move_assign(std::move(__list), __bool_constant<__move_storage>());
+ if constexpr (!__move_storage)
+ {
+ if (__list._M_get_Node_allocator() != this->_M_get_Node_allocator())
+ {
+ // The rvalue's allocator cannot be moved, or is not equal,
+ // so we need to individually move each element.
+ this->assign(std::make_move_iterator(__list.begin()),
+ std::make_move_iterator(__list.end()));
+ return *this;
+ }
+ }
+
+ clear();
+ this->_M_impl._M_head._M_next = __list._M_impl._M_head._M_next;
+ __list._M_impl._M_head._M_next = nullptr;
+ if constexpr (_Node_alloc_traits::_S_propagate_on_move_assign())
+ this->_M_get_Node_allocator()
+ = std::move(__list._M_get_Node_allocator());
return *this;
}
@@ -699,9 +718,30 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
void
assign(_InputIterator __first, _InputIterator __last)
{
- typedef is_assignable<_Tp, decltype(*__first)> __assignable;
- _M_assign(__first, __last, __assignable());
+ if constexpr (is_assignable<_Tp, decltype(*__first)>::value)
+ {
+ auto __prev = before_begin();
+ auto __curr = begin();
+ auto __end = end();
+ while (__curr != __end && __first != __last)
+ {
+ *__curr = *__first;
+ ++__prev;
+ ++__curr;
+ ++__first;
+ }
+ if (__first != __last)
+ insert_after(__prev, __first, __last);
+ else if (__curr != __end)
+ erase_after(__prev, __end);
+ }
+ else
+ {
+ clear();
+ insert_after(cbefore_begin(), __first, __last);
+ }
}
+#pragma GCC diagnostic pop
#if __glibcxx_ranges_to_container // C++ >= 23
/**
@@ -736,6 +776,8 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
}
#endif // ranges_to_container
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wc++17-extensions" // if constexpr
/**
* @brief Assigns a given value to a %forward_list.
* @param __n Number of elements to be assigned.
@@ -748,7 +790,31 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
*/
void
assign(size_type __n, const _Tp& __val)
- { _M_assign_n(__n, __val, is_copy_assignable<_Tp>()); }
+ {
+ if constexpr (is_copy_assignable<_Tp>::value)
+ {
+ auto __prev = before_begin();
+ auto __curr = begin();
+ auto __end = end();
+ while (__curr != __end && __n > 0)
+ {
+ *__curr = __val;
+ ++__prev;
+ ++__curr;
+ --__n;
+ }
+ if (__n > 0)
+ insert_after(__prev, __n, __val);
+ else if (__curr != __end)
+ erase_after(__prev, __end);
+ }
+ else
+ {
+ clear();
+ insert_after(cbefore_begin(), __n, __val);
+ }
+ }
+#pragma GCC diagnostic pop
/**
* @brief Assigns an initializer_list to a %forward_list.
@@ -1460,7 +1526,14 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
void
_M_default_insert_after(const_iterator __pos, size_type __n);
- // Called by operator=(forward_list&&)
+#if ! _GLIBCXX_INLINE_VERSION
+ // XXX GLIBCXX_ABI Deprecated
+ // These members are unused by std::forward_list now, but we keep them
+ // here so that an explicit instantiation will define them.
+ // This ensures that explicit instantiations still define these symbols,
+ // so that explicit instantiation declarations of std::forward_list that
+ // were compiled with old versions of GCC can still find these symbols.
+
void
_M_move_assign(forward_list&& __list, true_type) noexcept
{
@@ -1471,7 +1544,6 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
__list._M_get_Node_allocator());
}
- // Called by operator=(forward_list&&)
void
_M_move_assign(forward_list&& __list, false_type)
{
@@ -1484,39 +1556,6 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
std::make_move_iterator(__list.end()));
}
- // Called by assign(_InputIterator, _InputIterator) if _Tp is
- // CopyAssignable.
- template<typename _InputIterator>
- void
- _M_assign(_InputIterator __first, _InputIterator __last, true_type)
- {
- auto __prev = before_begin();
- auto __curr = begin();
- auto __end = end();
- while (__curr != __end && __first != __last)
- {
- *__curr = *__first;
- ++__prev;
- ++__curr;
- ++__first;
- }
- if (__first != __last)
- insert_after(__prev, __first, __last);
- else if (__curr != __end)
- erase_after(__prev, __end);
- }
-
- // Called by assign(_InputIterator, _InputIterator) if _Tp is not
- // CopyAssignable.
- template<typename _InputIterator>
- void
- _M_assign(_InputIterator __first, _InputIterator __last, false_type)
- {
- clear();
- insert_after(cbefore_begin(), __first, __last);
- }
-
- // Called by assign(size_type, const _Tp&) if Tp is CopyAssignable
void
_M_assign_n(size_type __n, const _Tp& __val, true_type)
{
@@ -1536,13 +1575,13 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
erase_after(__prev, __end);
}
- // Called by assign(size_type, const _Tp&) if Tp is non-CopyAssignable
void
_M_assign_n(size_type __n, const _Tp& __val, false_type)
{
clear();
insert_after(cbefore_begin(), __n, __val);
}
+#endif // ! _GLIBCXX_INLINE_VERSION
};
#if __cpp_deduction_guides >= 201606
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