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libstdc++: Optimize std::vector construction from input iterators [PR108487]

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LWG 3291 make std::ranges::iota_view's iterator have input_iterator_tag
as its iterator_category, even though it satisfies the C++20
std::forward_iterator concept. This means that the traditional
std::vector::vector(InputIterator, InputIterator) constructor treats
iota_view iterators as input iterators, because it only understands the
C++17 iterator requirements, not the C++20 iterator concepts. This
results in a loop that calls emplace_back for each individual element of
the iota_view, requiring the vector to reallocate repeatedly as the
values are inserted. This makes it unnecessarily slow to construct a
vector from an iota_view.

This change adds a new _M_range_initialize_n function for initializing a
vector from a range (which doesn't have to be common) and a size. This
new function can be used by vector(InputIterator, InputIterator) and
vector(from_range_t, R&&) when std::ranges::distance can be used to get
the size. It can also be used by the _M_range_initialize overload that
gets the size for a Cpp17ForwardIterator pair using std::distance, and
by the vector(initializer_list) constructor.

With this new function constructing a std::vector from iota_view does
a single allocation of the correct size and so doesn't need to
reallocate in a loop.

Previously the _M_range_initialize overload for Cpp17ForwardIterator was
using a local RAII _Guard_alloc object to own the storage, but that was
redundant. The _Vector_base can own the storage right away, and its
destructor will deallocate it if _M_range_initialize exits via an
exception.

libstdc++-v3/ChangeLog:

	PR libstdc++/108487
	* include/bits/stl_vector.h (vector(initializer_list)): Call
	_M_range_initialize_n instead of _M_range_initialize.
	(vector(InputIterator, InputIterator)): Use _M_range_initialize_n
	for C++20 sized sentinels and forward iterators.
	(vector(from_range_t, R&&)): Use _M_range_initialize_n for sized
	ranges and forward ranges.
	(vector::_M_range_initialize(FwIt, FwIt, forward_iterator_tag)):
	Likewise.
	(vector::_M_range_initialize_n): New function.
	* testsuite/23_containers/vector/cons/108487.cc: New test.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
This commit is contained in:
Jonathan Wakely 2025-03-25 13:24:08 +00:00 • committed by Jonathan Wakely
parent 2848b8dabc
commit e200f53a55
No known key found for this signature in database
2 changed files with 56 additions and 16 deletions
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48
libstdc++-v3/include/bits/stl_vector.h
View file

@ -65,6 +65,9 @@
#if __cplusplus >= 202002L
# include <compare>
#endif
#if __glibcxx_concepts // C++ >= C++20
# include <bits/ranges_base.h> // ranges::distance
#endif
#if __glibcxx_ranges_to_container // C++ >= 23
# include <bits/ranges_algobase.h> // ranges::copy
# include <bits/ranges_util.h> // ranges::subrange
@ -706,8 +709,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
const allocator_type& __a = allocator_type())
: _Base(__a)
{
_M_range_initialize(__l.begin(), __l.end(),
random_access_iterator_tag());
_M_range_initialize_n(__l.begin(), __l.end(), __l.size());
}
#endif
@ -735,6 +737,17 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
const allocator_type& __a = allocator_type())
: _Base(__a)
{
#if __glibcxx_concepts // C++ >= C++20
if constexpr (sized_sentinel_for<_InputIterator, _InputIterator>
|| forward_iterator<_InputIterator>)
{
const auto __n
= static_cast<size_type>(ranges::distance(__first, __last));
_M_range_initialize_n(__first, __last, __n);
return;
}
else
#endif
_M_range_initialize(__first, __last,
std::__iterator_category(__first));
}
@ -763,13 +776,9 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
{
if constexpr (ranges::forward_range<_Rg> || ranges::sized_range<_Rg>)
{
const auto __n = size_type(ranges::distance(__rg));
pointer __start =
this->_M_allocate(_S_check_init_len(__n,
_M_get_Tp_allocator()));
this->_M_impl._M_finish = this->_M_impl._M_start = __start;
this->_M_impl._M_end_of_storage = __start + __n;
_Base::_M_append_range(__rg);
const auto __n = static_cast<size_type>(ranges::distance(__rg));
_M_range_initialize_n(ranges::begin(__rg), ranges::end(__rg),
__n);
}
else
{
@ -1962,15 +1971,22 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
_M_range_initialize(_ForwardIterator __first, _ForwardIterator __last,
std::forward_iterator_tag)
{
const size_type __n = std::distance(__first, __last);
pointer __start =
_M_range_initialize_n(__first, __last,
std::distance(__first, __last));
}
template<typename _Iterator, typename _Sentinel>
_GLIBCXX20_CONSTEXPR
void
_M_range_initialize_n(_Iterator __first, _Sentinel __last,
size_type __n)
{
pointer __start = this->_M_impl._M_start =
this->_M_allocate(_S_check_init_len(__n, _M_get_Tp_allocator()));
_Guard_alloc __guard(__start, __n, *this);
this->_M_impl._M_finish = std::__uninitialized_copy_a
(__first, __last, __start, _M_get_Tp_allocator());
this->_M_impl._M_start = __start;
(void) __guard._M_release();
this->_M_impl._M_end_of_storage = __start + __n;
this->_M_impl._M_finish
= std::__uninitialized_copy_a(_GLIBCXX_MOVE(__first), __last,
__start, _M_get_Tp_allocator());
}
// Called by the first initialize_dispatch above and by the

24
libstdc++-v3/testsuite/23_containers/vector/cons/108487.cc Normal file
View file

@ -0,0 +1,24 @@
// { dg-do run { target c++20 } }
// Bug libstdc++/108487
// ~20-30x slowdown in populating std::vector from std::ranges::iota_view
#include <ranges>
#include <testsuite_hooks.h>
#include <testsuite_allocator.h>
void
test_pr108487()
{
using __gnu_test::tracker_allocator;
using __gnu_test::tracker_allocator_counter;
auto r = std::ranges::iota_view{0, 20};
tracker_allocator_counter::reset();
std::vector<int, tracker_allocator<int>> v{r.begin(), r.end()};
const std::size_t bytes = v.capacity() * sizeof(v.front());
VERIFY( tracker_allocator_counter::get_allocation_count() == bytes );
}
int main()
{
test_pr108487();
}