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) 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::_M_range_initialize(FwIt, FwIt, forward_iterator_tag)):
	Use _M_range_initialize_n.
	(vector::_M_range_initialize_n): New function.
	* testsuite/23_containers/vector/cons/108487.cc: New test.

gcc/testsuite/ChangeLog:

	* g++.dg/tree-ssa/initlist-opt1.C: Match _M_range_initialize_n
	instead of _M_range_initialize.
	* g++.dg/tree-ssa/initlist-opt2.C: Likewise.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
(cherry picked from commit e200f53a5556516ec831e6b7a34aaa0f10a4ab0a)

diff --git a/gcc/testsuite/g++.dg/tree-ssa/initlist-opt1.C b/gcc/testsuite/g++.dg/tree-ssa/initlist-opt1.C
index b1d2d25faf4c06acc59b5f0f6d256554f9a2fdb5..f4f09117e65e9093bb462c5c82be3a9994a0286d 100644 (file)
--- a/gcc/testsuite/g++.dg/tree-ssa/initlist-opt1.C
+++ b/gcc/testsuite/g++.dg/tree-ssa/initlist-opt1.C
@@ -3,7 +3,7 @@
 // { dg-do compile { target c++11 } }
 
 // Test that we do range-initialization from const char *.
-// { dg-final { scan-tree-dump {_M_range_initialize<const char\* const\*>} "gimple" } }
+// { dg-final { scan-tree-dump {_M_range_initialize_n<const char\* const\*>} "gimple" } }
 // { dg-final { scan-tree-dump {static const char.*72} "gimple" } }
 
 #include <string>

diff --git a/gcc/testsuite/g++.dg/tree-ssa/initlist-opt2.C b/gcc/testsuite/g++.dg/tree-ssa/initlist-opt2.C
index 1e9ac739b2d79f4f7fa9ca2c195977a46b11039b..63f1feb30381db75b0665a0dd95e3afc0ed64408 100644 (file)
--- a/gcc/testsuite/g++.dg/tree-ssa/initlist-opt2.C
+++ b/gcc/testsuite/g++.dg/tree-ssa/initlist-opt2.C
@@ -3,7 +3,7 @@
 // { dg-do compile { target c++11 } }
 
 // Test that we do range-initialization from const char *.
-// { dg-final { scan-tree-dump {_M_range_initialize<const char\* const\*>} "gimple" } }
+// { dg-final { scan-tree-dump {_M_range_initialize_n<const char\* const\*} "gimple" } }
 // And that the backing array is static.
 // { dg-final { scan-tree-dump {static const char.*72} "gimple" } }
 

diff --git a/libstdc++-v3/include/bits/stl_vector.h b/libstdc++-v3/include/bits/stl_vector.h
index b7ee62025251782f1006055354e183e3ea7440fe..6b4147e77312fcee5f5b00a7a0a4fd7ce699ae18 100644 (file)
--- 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
 
 #include <debug/assertions.h>
 
@@ -679,8 +682,7 @@ _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
 
@@ -708,6 +710,17 @@ _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));
        }
@@ -1689,14 +1702,22 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
        _M_range_initialize(_ForwardIterator __first, _ForwardIterator __last,
                            std::forward_iterator_tag)
        {
-         const size_type __n = std::distance(__first, __last);
-         this->_M_impl._M_start
-           = this->_M_allocate(_S_check_init_len(__n, _M_get_Tp_allocator()));
-         this->_M_impl._M_end_of_storage = this->_M_impl._M_start + __n;
-         this->_M_impl._M_finish =
-           std::__uninitialized_copy_a(__first, __last,
-                                       this->_M_impl._M_start,
-                                       _M_get_Tp_allocator());
+         _M_range_initialize_n(__first, __last,
+                               std::distance(__first, __last));
+       }
+
+      template<typename _Iterator>
+       _GLIBCXX20_CONSTEXPR
+       void
+       _M_range_initialize_n(_Iterator __first, _Iterator __last,
+                             size_type __n)
+       {
+         pointer __start = this->_M_impl._M_start =
+           this->_M_allocate(_S_check_init_len(__n, _M_get_Tp_allocator()));
+         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 (file)
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();
+}