[PATCH] minor optimization bug in basic_string move assignment

[Jonathan Wakely]

On Wed, 25 Jan 2023 at 18:38, François Dumont <frs.dumont@gmail.com> wrote:
>
> 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 ?

Mostly lack of interest, but also I don't really want to "encourage"
the use of the old string by investing lots of maintenance effort into
it. If you want new features like C++11 Allocators and
resize_and_overwrite etc then you should use the new type.

I don't remember if there were any actual blockers that made it
difficult to support stateful allocators in the COW string. I might
have written something about it in mails to the list when I was adding
the SSO string, but I don't remember now.

Anyway, for this patch ...

>
> 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 ?

OK for trunk, thanks!

+Reviewed-by: Jonathan Wakely <jwakely@redhat.com>


> > On Wed, 4 Jan 2023 at 18:21, François Dumont via Libstdc++
> > <libstdc++@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:libstdc++@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.
> >