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

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>

2 files changed, 56 insertions, 16 deletions

diff --git a/libstdc++-v3/include/bits/stl_vector.h b/libstdc++-v3/include/bits/stl_vector.h
index 21f6cd0..458adc9 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());
 	}
 
       // 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 0000000..13f2c47
--- /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();
+}