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

[François Dumont]

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 <mailto:libstdc%2B%2B@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.

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.