Re: _LIBCXX_DEBUG value initialized singular iterators assert failures in std algorithms [PR104316]

[Jonathan Wakely <jwakely.gcc@gmail.com>]

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On Sat, 16 Mar 2024, 12:16 François Dumont, <frs.dumont@gmail.com> wrote:

> With the patch, sorry.
>
> On 14/03/2024 22:49, François Dumont wrote:
> > Hi
> >
> > This is what I started to do.
> >
> > For now I haven't touch to __cpp_lib_null_iterators definition as
> > _Safe_local_iterator still need some work.
> >
> > libstdc++: Implement N3644 on _Safe_iterator<> [PR114316]
> >
> > Consider range of value-initialized iterators as valid and empty.
> >
> > libstdc++-v3/ChangeLog:
> >
> >         PR libstdc++/114316
> >         * include/debug/safe_iterator.tcc
> > (_Safe_iterator<>::_M_valid_range):
> >         First check if both iterators are value-initialized before
> > checking if
> >         singular.
> >         * testsuite/23_containers/set/debug/114316.cc: New test case.
> >         * testsuite/23_containers/vector/debug/114316.cc: New test case.
> >
> > Tested under Linux x86_64, ok to commit ?
>

OK for trunk, thanks!

I think this is OK to backport to 13 too.

Maybe after this we can define the __cpp_lib_null_itetators macro for debug
mode?


>
> > François
> >
> >
> > On 12/03/2024 10:52, Jonathan Wakely wrote:
> >> On Tue, 12 Mar 2024 at 01:03, Jonathan Wakely <jwakely@redhat.com>
> >> wrote:
> >>> On Tue, 12 Mar 2024 at 00:55, Maciej Miera <maciej.miera@gmail.com>
> >>> wrote:
> >>>>
> >>>>
> >>>> Wiadomość napisana przez Jonathan Wakely <jwakely@redhat.com> w
> >>>> dniu 11.03.2024, o godz. 21:40:
> >>>>
> >>>> On Mon, 11 Mar 2024 at 20:07, Maciej Miera <maciej.miera@gmail.com>
> >>>> wrote:
> >>>>
> >>>>
> >>>> Hello,
> >>>>
> >>>> I have tried to introduce an extra level of safety to my codebase
> >>>> and utilize _GLIBCXX_DEBUG in my test builds in order to catch
> >>>> faulty iterators.
> >>>> However, I have encountered the following problem: I would like to
> >>>> utilize singular, value-initialized iterators as an arbitrary "null
> >>>> range”.
> >>>> However, this leads to failed assertions in std:: algorithms taking
> >>>> such range.
> >>>>
> >>>> Consider the following code sample with find_if:
> >>>>
> >>>> #include <map>
> >>>> #include <algorithm>
> >>>> #include <iterator>
> >>>>
> >>>> #ifndef __cpp_lib_null_iterators
> >>>> #warning "Not standard compliant"
> >>>> #endif
> >>>>
> >>>> int main()
> >>>> {
> >>>>     std::multimap<char, int>::iterator it1{};
> >>>>     std::multimap<char, int>::iterator it2{};
> >>>>
> >>>>     (void) (it1==it2); // OK
> >>>>     (void) std::find_if(
> >>>>         it1, it2, [](const auto& el) { return el.second == 8;});
> >>>> }
> >>>>
> >>>> Compiled with -std=c++20 and -D_GLIBCXX_DEBUG it produces the
> >>>> warning "Not standard compliant"
> >>>> and the execution results in the following assert failure:
> >>>>
> >>>>
> /opt/compiler-explorer/gcc-12.2.0/include/c++/12.2.0/bits/stl_algo.h:3875:
> >>>>
> >>>> In function:
> >>>>     constexpr _IIter std::find_if(_IIter, _IIter, _Predicate) [with
> >>>> _IIter =
> >>>> gnu_debug::_Safe_iterator<_Rb_tree_iterator<pair<const char, int> >,
> >>>>     debug::multimap<char, int>, bidirectional_iterator_tag>;
> >>>> _Predicate =
> >>>>     main()::<lambda(const auto:16&)>]
> >>>>
> >>>> The question is though: is it by design, or is it just a mere
> >>>> oversight? The warning actually suggest the first option.
> >>>> If it is an intentional design choice, could you provide some
> >>>> rationale behind it, please?
> >>>>
> >>>>
> >>>> The macro was not defined because the C++14 rule wasn't implemented
> >>>> for debug mode, but that should have been fixed for GCC 11, according
> >>>> to https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98466 and
> >>>> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70303
> >>>> So we should be able to define macro now, except maybe it wasn't fixed
> >>>> for the RB tree containers.
> >>>>
> >>>>
> >>>>
> >>>> Just to make sure there are no misunderstandings: comparison via ==
> >>>> works fine. The feature check macro without _GLIBCXX_DEBUG and with
> >>>> <iterator> included is also fine. Maybe the need to include a
> >>>> header is the issue, but that’s not the core of the problem anyway.
> >>> No, it has nothing to do with the headers included. The feature test
> >>> macro is defined like so:
> >>>
> >>> # if (__cplusplus >= 201402L) && (!defined(_GLIBCXX_DEBUG))
> >>> #  define __glibcxx_null_iterators 201304L
> >>>
> >>> It's a very deliberate choice to not define it when _GLIBCXX_DEBUG is
> >>> defined. But as I said, I think we should have changed that.
> >>>
> >>>> The actual question is though, whether passing singular iterators
> >>>> to std algorithms (like find_if) should make the asserts at the
> >>>> beginning of the algo function fail when compiled with
> >>>> _GLIBCXX_DEBUG. IMHO, intuitively it should not, as comparing
> >>>> iterators equal would just ensure early exit and return of the same
> >>>> singular iterator.
> >>>> This seems not to be the case though. The actual message is this:
> >>>> Error: the function requires a valid iterator range [first, last).
> >>>> What bothers me is whether the empty virtual range limited by two
> >>>> same singular iterators is actually valid or not.
> >>> Yes, it's valid. So the bug is in the __glibcxx_requires_valid_range
> >>> macro.
> >> Thanks for the bugzilla report:
> >> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114316
> >> We'll get it fixed!
> >>