On Thu, 22 May 2025 at 16:44, Jonathan Wakely <jwak...@redhat.com> wrote:
>
> On Thu, 22 May 2025 at 16:25, Tomasz Kaminski <tkami...@redhat.com> wrote:
> >
> >
> >
> > On Thu, May 22, 2025 at 5:15 PM Tomasz Kaminski <tkami...@redhat.com> wrote:
> >>
> >>
> >>
> >> On Thu, May 22, 2025 at 5:04 PM Jonathan Wakely <jwak...@redhat.com> wrote:
> >>>
> >>> On Thu, 22 May 2025 at 15:50, Tomasz Kaminski <tkami...@redhat.com> wrote:
> >>> >
> >>> >
> >>> >
> >>> > On Thu, May 22, 2025 at 1:42 PM Jonathan Wakely <jwak...@redhat.com>
> >>> > wrote:
> >>> >>
> >>> >> The current overload set for __unique_copy handles three cases:
> >>> >>
> >>> >> - The input range uses forward iterators, the output range does not.
> >>> >> This is the simplest case, and can just compare adjacent elements of
> >>> >> the input range.
> >>> >>
> >>> >> - Neither the input range nor output range use forward iterators.
> >>> >> This requires a local variable copied from the input range and
> >>> >> updated
> >>> >> by assigning each element to the local variable.
> >>> >>
> >>> >> - The output range uses forward iterators.
> >>> >> For this case we compare the current element from the input range
> >>> >> with
> >>> >> the element just written to the output range.
> >>> >>
> >>> >> There are two problems with this implementation. Firstly, the third
> >>> >> case
> >>> >> assumes that the value type of the output range can be compared to the
> >>> >> value type of the input range, which might not be possible at all, or
> >>> >> might be possible but give different results to comparing elements of
> >>> >> the input range. This is the problem identified in LWG 2439.
> >>> >>
> >>> >> Secondly, the third case is used when both ranges use forward
> >>> >> iterators,
> >>> >> even though the first case could (and should) be used. This means that
> >>> >> we compare elements from the output range instead of the input range,
> >>> >> with the problems described above (either not well-formed, or might
> >>> >> give
> >>> >> the wrong results).
> >>> >>
> >>> >> The cause of the second problem is that the overload for the first case
> >>> >> looks like:
> >>> >>
> >>> >> OutputIterator
> >>> >> __unique_copy(ForwardIter, ForwardIter, OutputIterator, BinaryPred,
> >>> >> forward_iterator_tag, output_iterator_tag);
> >>> >>
> >>> >> When the output range uses forward iterators this overload cannot be
> >>> >> used, because forward_iterator_tag does not inherit from
> >>> >> output_iterator_tag, so is not convertible to it.
> >>> >>
> >>> >> To fix these problems we need to implement the resolution of LWG 2439
> >>> >> so
> >>> >> that the third case is only used when the value types of the two ranges
> >>> >> are the same. This ensures that the comparisons are well behaved. We
> >>> >> also need to ensure that the first case is used when both ranges use
> >>> >> forward iterators.
> >>> >>
> >>> >> This change replaces a single step of tag dispatching to choose between
> >>> >> three overloads with two step of tag dispatching, choosing between two
> >>> >> overloads at each step. The first step dispatches based on the iterator
> >>> >> category of the input range, ignoring the category of the output range.
> >>> >> The second step only happens when the input range uses non-forward
> >>> >> iterators, and dispatches based on the category of the output range and
> >>> >> whether the value type of the two ranges is the same. So now the cases
> >>> >> that are handled are:
> >>> >>
> >>> >> - The input range uses forward iterators.
> >>> >> - The output range uses non-forward iterators or a different value
> >>> >> type.
> >>> >> - The output range uses forward iterators and has the same value type.
> >>> >>
> >>> >> For the second case, the old code used
> >>> >> __gnu_cxx::__ops::__iter_comp_val
> >>> >> to wrap the predicate in another level of indirection. That seems
> >>> >> unnecessary, as we can just use a pointer to the local variable instead
> >>> >> of an iterator referring to it.
> >>> >>
> >>> >> libstdc++-v3/ChangeLog:
> >>> >>
> >>> >> PR libstdc++/120386
> >>> >> * include/bits/stl_algo.h (__unique_copy_1): New overloads for
> >>> >> the case where the input range uses non-forward iterators.
> >>> >> (__unique_copy): Replace three overloads with two, depending
> >>> >> only on the iterator category of the input range. Dispatch to
> >>> >> __unique_copy_1 for the non-forward case.
> >>> >> (unique_copy): Only pass the input range category to
> >>> >> __unique_copy.
> >>> >> ---
> >>> >>
> >>> >> Tested x86_64-linux.
> >>> >
> >>> > LGTM. Only small suggestion, regarding the change of order of arguments.
> >>>
> >>> I forgot to say that I need to add tests for each of the cases,
> >>> especially the case that fails with the existing code!
> >>>
> >>> >>
> >>> >>
> >>> >> libstdc++-v3/include/bits/stl_algo.h | 80 +++++++++++++++-------------
> >>> >> 1 file changed, 44 insertions(+), 36 deletions(-)
> >>> >>
> >>> >> diff --git a/libstdc++-v3/include/bits/stl_algo.h
> >>> >> b/libstdc++-v3/include/bits/stl_algo.h
> >>> >> index f5361aeab7e2..c0bb17f9c8b2 100644
> >>> >> --- a/libstdc++-v3/include/bits/stl_algo.h
> >>> >> +++ b/libstdc++-v3/include/bits/stl_algo.h
> >>> >> @@ -918,24 +918,20 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >>> >>
> >>> >> __gnu_cxx::__ops::__iter_comp_iter(__binary_pred));
> >>> >> }
> >>> >>
> >>> >> - /**
> >>> >> - * This is an uglified
> >>> >> - * unique_copy(_InputIterator, _InputIterator, _OutputIterator,
> >>> >> - * _BinaryPredicate)
> >>> >> - * overloaded for forward iterators and output iterator as result.
> >>> >> - */
> >>> >> + // Implementation of std::unique_copy for forward iterators.
> >>> >> + // This case is easy, just compare *i with *(i-1).
> >>> >> template<typename _ForwardIterator, typename _OutputIterator,
> >>> >> typename _BinaryPredicate>
> >>> >> _GLIBCXX20_CONSTEXPR
> >>> >> _OutputIterator
> >>> >> __unique_copy(_ForwardIterator __first, _ForwardIterator __last,
> >>> >> _OutputIterator __result, _BinaryPredicate
> >>> >> __binary_pred,
> >>> >> - forward_iterator_tag, output_iterator_tag)
> >>> >> + forward_iterator_tag)
> >>> >> {
> >>> >> _ForwardIterator __next = __first;
> >>> >> *__result = *__first;
> >>> >> while (++__next != __last)
> >>> >> - if (!__binary_pred(__first, __next))
> >>> >> + if (!__binary_pred(__next, __first))
> >>> >
> >>> > I would prefer if you will not do this change, and pass iterators that
> >>> > were already seen as the first argument.
> >>>
> >>> The standard seems clear that it should be bool(pred(*i, *(i - 1)))
> >>> In theory a predicate could depend on that.
> >>
> >> Oh, indeed.
> >>>
> >>>
> >>> > Note that the forward-output overload, preserves this order:
> >>> > *__result = *__first;
> >>> > while (++__first != __last)
> >>> > if (!__binary_pred(__result, __first))
> >>>
> >>> Ah yes, well I should have changed that too ;-)
> >>>
> >>> What's your reason for preferring the current order?
> >>
> >> That what I would intuitively expect, that left argument is an element
> >> that is left to right argument.
> >> And if you sorted range with predicate lt, then passing not_fn(lt) is
> >> equivalent to checking equality.
> >
> > It seems that I have built my expectation based on unique, that check i-1,
> > i.
> > https://eel.is/c++draft/alg.unique#1
>
> The standard seems clear, however, all three of libstdc++, libc++ and
> MSVC use the "wrong" order in unique_copy.
>
> So I think the standard should be changed.
The docs for the SGI STL matched the standard:
https://web.archive.org/web/20161012091520/http://www.sgi.com/tech/stl/unique_copy.html
But the actual SGI STL implementation (which libstdc++ is based on)
does not match its own docs:
https://web.archive.org/web/20170429054511/http://www.sgi.com/tech/stl/stl_algo.h
My copy of "The C++ Standard Template Library" by Plauger, Stepanov,
Lee and Musser is in a box somewhere in storage, but what it describes
probably matches the MSVC STL.
