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.
> > *++__result = *__first;
> >
> >> {
> >> __first = __next;
> >> *++__result = *__first;
> >> @@ -943,27 +939,21 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >> return ++__result;
> >> }
> >>
> >> - /**
> >> - * This is an uglified
> >> - * unique_copy(_InputIterator, _InputIterator, _OutputIterator,
> >> - * _BinaryPredicate)
> >> - * overloaded for input iterators and output iterator as result.
> >> - */
> >> + // Implementation of std::unique_copy for non-forward iterators,
> >> + // where we cannot compare with elements written to the output.
> >> template<typename _InputIterator, typename _OutputIterator,
> >> typename _BinaryPredicate>
> >> _GLIBCXX20_CONSTEXPR
> >> _OutputIterator
> >> - __unique_copy(_InputIterator __first, _InputIterator __last,
> >> - _OutputIterator __result, _BinaryPredicate
> __binary_pred,
> >> - input_iterator_tag, output_iterator_tag)
> >> + __unique_copy_1(_InputIterator __first, _InputIterator __last,
> >> + _OutputIterator __result, _BinaryPredicate
> __binary_pred,
> >> + __false_type)
> >> {
> >> - typename iterator_traits<_InputIterator>::value_type __value =
> *__first;
> >> - __decltype(__gnu_cxx::__ops::__iter_comp_val(__binary_pred))
> >> - __rebound_pred
> >> - = __gnu_cxx::__ops::__iter_comp_val(__binary_pred);
> >> + typedef typename iterator_traits<_InputIterator>::value_type
> _Val;
> >> + _Val __value = *__first;
> >> *__result = __value;
> >> while (++__first != __last)
> >> - if (!__rebound_pred(__first, __value))
> >> + if (!__binary_pred(__first, std::__addressof(__value)))
> >
> > I would instead change the order here to std::__addressof(__value),
> __first.
> >>
> >> {
> >> __value = *__first;
> >> *++__result = __value;
> >> @@ -971,19 +961,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >> return ++__result;
> >> }
> >>
> >> - /**
> >> - * This is an uglified
> >> - * unique_copy(_InputIterator, _InputIterator, _OutputIterator,
> >> - * _BinaryPredicate)
> >> - * overloaded for input iterators and forward iterator as result.
> >> - */
> >> + // Implementation of std::unique_copy for non-forward iterators,
> >> + // where we can compare with the last element written to the output.
> >> template<typename _InputIterator, typename _ForwardIterator,
> >> typename _BinaryPredicate>
> >> - _GLIBCXX20_CONSTEXPR
> >> _ForwardIterator
> >> - __unique_copy(_InputIterator __first, _InputIterator __last,
> >> - _ForwardIterator __result, _BinaryPredicate
> __binary_pred,
> >> - input_iterator_tag, forward_iterator_tag)
> >> + __unique_copy_1(_InputIterator __first, _InputIterator __last,
> >> + _ForwardIterator __result, _BinaryPredicate
> __binary_pred,
> >> + __true_type)
> >> {
> >> *__result = *__first;
> >> while (++__first != __last)
> >> @@ -992,6 +977,31 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >> return ++__result;
> >> }
> >>
> >> + // Implementation of std::unique_copy for non-forward iterators.
> >> + // We cannot compare *i to *(i-1) so we need to either make a copy
> >> + // or compare with the last element written to the output range.
> >> + template<typename _InputIterator, typename _OutputIterator,
> >> + typename _BinaryPredicate>
> >> + _GLIBCXX20_CONSTEXPR
> >> + _OutputIterator
> >> + __unique_copy(_InputIterator __first, _InputIterator __last,
> >> + _OutputIterator __result, _BinaryPredicate
> __binary_pred,
> >> + input_iterator_tag)
> >> + {
> >> + // _GLIBCXX_RESOLVE_LIB_DEFECTS
> >> + // 2439. unique_copy() sometimes can't fall back to reading its
> output
> >> + typedef iterator_traits<_InputIterator> _InItTraits;
> >> + typedef iterator_traits<_OutputIterator> _OutItTraits;
> >> + typedef typename _OutItTraits::iterator_category _Cat;
> >> + const bool __output_is_fwd = __is_base_of(forward_iterator_tag,
> _Cat);
> >> + const bool __same_type = __is_same(typename
> _OutItTraits::value_type,
> >> + typename
> _InItTraits::value_type);
> >>
> >> + typedef __truth_type<__output_is_fwd && __same_type>
> __cmp_with_output;
> >
> > No change needed, but I was wondering of output only iterator, can have
> non-void value type,
> > but making sure that they are also forwards mitigates this question.
> >>
> >> + return std::__unique_copy_1(__first, __last, __result,
> __binary_pred,
> >> + typename __cmp_with_output::__type());
> >> + }
> >> +
> >> +
> >> /**
> >> * This is an uglified reverse(_BidirectionalIterator,
> >> * _BidirectionalIterator)
> >> @@ -4456,8 +4466,7 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
> >> return __result;
> >> return std::__unique_copy(__first, __last, __result,
> >>
> __gnu_cxx::__ops::__iter_equal_to_iter(),
> >> - std::__iterator_category(__first),
> >> - std::__iterator_category(__result));
> >> + std::__iterator_category(__first));
> >> }
> >>
> >> /**
> >> @@ -4499,8 +4508,7 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
> >> return __result;
> >> return std::__unique_copy(__first, __last, __result,
> >>
> __gnu_cxx::__ops::__iter_comp_iter(__binary_pred),
> >> - std::__iterator_category(__first),
> >> - std::__iterator_category(__result));
> >> + std::__iterator_category(__first));
> >> }
> >>
> >> #if __cplusplus <= 201103L || _GLIBCXX_USE_DEPRECATED
> >> --
> >> 2.49.0
> >>
>
>
