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.
> 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?
> *++__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
>>
