On Tue, 25 Mar 2025 at 16:49, Tomasz Kaminski <tkami...@redhat.com> wrote:
>
>
>
> On Tue, Mar 25, 2025 at 5:30 PM Jonathan Wakely <jwak...@redhat.com> wrote:
>>
>> 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.
>>
>> 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.
>> ---
>>
>> Tests running for x86_64-linux.
>>
>> This gives a 10x speed up for the PR108487 testcase using iota_view.
>>
>> I don't see why doing this wouldn't be allowed by the standard, so it
>> seems worth doing.
>>
>> libstdc++-v3/include/bits/stl_vector.h | 48 ++++++++++++-------
>> .../23_containers/vector/cons/108487.cc | 24 ++++++++++
>> 2 files changed, 56 insertions(+), 16 deletions(-)
>> create mode 100644
>> libstdc++-v3/testsuite/23_containers/vector/cons/108487.cc
>>
>> diff --git a/libstdc++-v3/include/bits/stl_vector.h
>> b/libstdc++-v3/include/bits/stl_vector.h
>> index 21f6cd04f49..458adc987da 100644
>> --- a/libstdc++-v3/include/bits/stl_vector.h
>> +++ b/libstdc++-v3/include/bits/stl_vector.h
>> @@ -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());
>
> Should we guard the allocation using _Guard_alloc here, as it was done
> previously for _M_range_initialize?
I forgot to mention that in the commit message. I don't think the
_Guard_alloc was ever needed.
The ~_Vector_base destructor does
_GLIBCXX20_CONSTEXPR
~_Vector_base() _GLIBCXX_NOEXCEPT
{
_M_deallocate(_M_impl._M_start,
_M_impl._M_end_of_storage - _M_impl._M_start);
}
So if we just assign the allocation to _M_impl._M_start immediately,
and set _M_impl._M_end_of_storage, then the base class guards it for
us. There is no need for a local _Guard_alloc.
>>
>> }
>>
>> // Called by the first initialize_dispatch above and by the
>> diff --git a/libstdc++-v3/testsuite/23_containers/vector/cons/108487.cc
>> b/libstdc++-v3/testsuite/23_containers/vector/cons/108487.cc
>> new file mode 100644
>> index 00000000000..13f2c478e79
>> --- /dev/null
>> +++ b/libstdc++-v3/testsuite/23_containers/vector/cons/108487.cc
>> @@ -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();
>> +}
>> --
>> 2.49.0
>>
