Let's submit a proper patch proposal then.
The occasion for me to ask if there is any reason for cow string not
being C++11 allocator compliant ? Just lack of interest ?
I wanted to consider it to get rid of the __gnu_debug::_Safe_container
_IsCxx11AllocatorAware template parameter.
libstdc++: Optimize basic_string move assignment
Since resolution of Issue 2593 [1] we can consider that equal
allocators
before the propagate-on-move-assignment operations will still be equal
afterward.
So we can extend the optimization of transfering the storage of the
move-to
instance to the move-from one that is currently limited to always equal
allocators.
[1] https://cplusplus.github.io/LWG/issue2593
libstdc++-v3/ChangeLog:
* include/bits/basic_string.h (operator=(basic_string&&)):
Transfer move-to
storage to the move-from instance when allocators are equal.
*
testsuite/21_strings/basic_string/allocator/char/move_assign.cc (test04):
New test case.
Tested under linux x86_64, ok to commit ?
François
On 17/01/23 20:18, Jonathan Wakely wrote:
On Wed, 4 Jan 2023 at 18:21, François Dumont via Libstdc++
<libstd...@gcc.gnu.org> wrote:
On 04/01/23 00:11, waffl3x via Libstdc++ wrote:
Example: https://godbolt.org/z/sKhGqG1qK
https://gcc.gnu.org/git/?p=gcc.git;a=blob;f=libstdc%2B%2B-v3/include/bits/basic_string.h;hb=HEAD#l880
When move assigning to a basic_string, the allocated memory of the moved into
string is stored into the source string instead of deallocating it, a good
optimization when everything is compatible. However in the case of a stateful
allocator (is_always_true() evaluating as false) this optimization is never
taken. Unless there is some reason I can't think of that makes equal stateful
allocators incompatible here, I believe the if statement on line 880 of
basic_string.h should also compare the equality of each strings allocator. The
first condition in the function seems to indicate to me that this scenario was
being considered and just forgotten about, as the memory doesn't get
deallocated immediately if the two allocators are equal. I'll note that because
of how everything is handled, this doesn't result in a leak so this bug is
still only a minor missed optimization.
mailto:libstd...@gcc.gnu.org
Hmmm, I don't know, at least it is not as simple as you present it.
You cannot add a check on allocator equality as you are proposing
because it is too late. __str allocator might have already been
propagated to *this on the previous call to std::__alloc_on_move. Note
that current check is done only if
!_Alloc_traits::_S_propagate_on_move_assign().
This patch might do the job but I wonder if equal allocators can become
un-equal after the propagate-on-move-assignment ?
Since https://cplusplus.github.io/LWG/issue2593 they can't. But I
think when I wrote that code, they could do, which is probably why the
optimization wasn't done.
diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
index aa018262c98..c81dc0d425a 100644
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -844,9 +844,10 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
operator=(basic_string&& __str)
noexcept(_Alloc_traits::_S_nothrow_move())
{
+ const bool __equal_allocs = _Alloc_traits::_S_always_equal()
+ || _M_get_allocator() == __str._M_get_allocator();
if (!_M_is_local() && _Alloc_traits::_S_propagate_on_move_assign()
- && !_Alloc_traits::_S_always_equal()
- && _M_get_allocator() != __str._M_get_allocator())
+ && !__equal_allocs)
{
// Destroy existing storage before replacing allocator.
_M_destroy(_M_allocated_capacity);
@@ -868,16 +869,14 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
_M_set_length(__str.size());
}
}
- else if (_Alloc_traits::_S_propagate_on_move_assign()
- || _Alloc_traits::_S_always_equal()
- || _M_get_allocator() == __str._M_get_allocator())
+ else if (_Alloc_traits::_S_propagate_on_move_assign() || __equal_allocs)
{
// Just move the allocated pointer, our allocator can free it.
pointer __data = nullptr;
size_type __capacity;
if (!_M_is_local())
{
- if (_Alloc_traits::_S_always_equal())
+ if (__equal_allocs)
{
// __str can reuse our existing storage.
__data = _M_data();
diff --git a/libstdc++-v3/testsuite/21_strings/basic_string/allocator/char/move_assign.cc b/libstdc++-v3/testsuite/21_strings/basic_string/allocator/char/move_assign.cc
index cc58348e116..21e0b1cb4f4 100644
--- a/libstdc++-v3/testsuite/21_strings/basic_string/allocator/char/move_assign.cc
+++ b/libstdc++-v3/testsuite/21_strings/basic_string/allocator/char/move_assign.cc
@@ -28,6 +28,8 @@ const C c = 'a';
using traits = std::char_traits<C>;
using __gnu_test::propagating_allocator;
+using __gnu_test::tracker_allocator_counter;
+using __gnu_test::tracker_allocator;
void test01()
{
@@ -146,10 +148,60 @@ void test03()
VERIFY(7 == v8.get_allocator().get_personality());
}
+void test04()
+{
+ typedef propagating_allocator<C, true, tracker_allocator<C>> alloc_type;
+ typedef std::basic_string<C, traits, alloc_type> test_type;
+
+ {
+ tracker_allocator_counter::reset();
+ test_type v1(alloc_type(1));
+ v1 = "abcdefghijklmnopqr10";
+ auto ref_alloc_count = tracker_allocator_counter::get_allocation_count();
+
+ test_type v2(alloc_type(2));
+ v2 = "abcdefghijklmnopqr20";
+ v2 = std::move(v1);
+ VERIFY(1 == v1.get_allocator().get_personality());
+ VERIFY(1 == v2.get_allocator().get_personality());
+
+ VERIFY( tracker_allocator_counter::get_allocation_count() == 2 * ref_alloc_count );
+ VERIFY( tracker_allocator_counter::get_deallocation_count() == ref_alloc_count );
+
+ v1 = "abcdefghijklmnopqr11";
+
+ VERIFY( tracker_allocator_counter::get_allocation_count() == 3 * ref_alloc_count );
+ }
+
+ {
+ tracker_allocator_counter::reset();
+ test_type v1(alloc_type(1));
+ v1 = "abcdefghijklmnopqr10";
+ auto ref_alloc_count = tracker_allocator_counter::get_allocation_count();
+
+ test_type v2(alloc_type(1));
+ v2 = "abcdefghijklmnopqr20";
+ v2 = std::move(v1);
+ VERIFY(1 == v1.get_allocator().get_personality());
+ VERIFY(1 == v2.get_allocator().get_personality());
+
+ VERIFY( tracker_allocator_counter::get_allocation_count() == 2 * ref_alloc_count );
+ VERIFY( tracker_allocator_counter::get_deallocation_count() == 0 );
+
+ v1 = "abcdefghijklmnopqr11";
+
+ VERIFY( tracker_allocator_counter::get_allocation_count() == 2 * ref_alloc_count );
+ }
+
+ VERIFY( tracker_allocator_counter::get_allocation_count() ==
+ tracker_allocator_counter::get_deallocation_count() );
+}
+
int main()
{
test01();
test02();
test03();
+ test04();
return 0;
}
