[committed] libstdc++: Reduce header dependencies for C++20 std::erase [PR92546]

[Jonathan Wakely]

On Thu, 7 Oct 2021, 07:35 François Dumont, <frs.dumont@gmail.com> wrote:

> On 02/10/21 2:47 pm, Jonathan Wakely wrote:
>
>
>
> On Sat, 2 Oct 2021, 13:02 François Dumont via Libstdc++, <
> libstdc++@gcc.gnu.org> wrote:
>
>> On 01/10/21 9:38 pm, Jonathan Wakely via Libstdc++ wrote:
>> > This reduces the preprocessed size of <deque>, <string> and <vector> by
>> > not including <bits/stl_algo.h> for std::remove and std::remove_if.
>> >
>> > Also unwrap iterators using __niter_base, to avoid redundant debug mode
>> > checks.
>>
>> I don't know if you noticed but the __niter_base is a no-op here.
>>
>
> Oh, I didn't notice.
>
>
>> __niter_base unwrap only random access iterators because it is the only
>> category for which we know that we have been able to confirm validity or
>> not.
>>
>
> But these are all random access. I must be missing something.
>
> It is in a method called '__erases_node_if', I'm not aware of any
> node-based container providing random access iterators.
>

Ah, I thought you meant the deque, string and vector part of the patch,
sorry.

Moreover I just noticed that if it was really doing anything then you would
> be missing the std::__niter_wrap in the __cont.erase(__iter), it just
> wouldn't compile.
>
>
>> We would need to introduce another function to do this or specialize in
>> some way erase_if for debug containers. I'll try to find a solution.
>>
>
> OK, thanks. Maybe we can just leave the checking there. I wanted to avoid
> the overhead because we know that the iterator range is valid. Any checks
> done on each increment and equality comparison are wasteful, as they will
> never fail.
>
> Yes, that would be better indeed.
>
> But doing it this way you still have the overhead of the _Safe_iterator
> addition to the list of the safe container iterators, so a mutex
> lock/unlock.
>
> I'll try to find out how to get a normal iterator from a safe container
> even if in this case we will have to support operations on safe container
> with normal iterators.
>
When we know the type, we could use __cont._GLIBCXX_STD_C::vector<T,
Alloc>::begin() to access the base version directly. But for the node
containers we have a generic function where we don't know the exact type.
Is the _Base typedef accessible? __cont.decltype(__cont)::_Base::begin()
would work, but it's ugly.