Re: [libstdc++] Improve M_check_len

[Jan Hubicka <hubicka@ucw.cz>]

> >
> >       size_type
> >       _M_check_len(size_type __n, const char* __s) const
> >       {
> >         const size_type __size = size();
> >         const size_type __max_size = max_size();
> >
> >         if (__is_same(allocator_type, allocator<_Tp>)
> >               && __size > __max_size / 2)
> >
> 
> This check is wrong for C++17 and older standards, because max_size()
> changed value in C++20.
> 
> In C++17 it was PTRDIFF_MAX / sizeof(T) but in C++20 it's SIZE_MAX /
> sizeof(T). So on 32-bit targets using C++17, it's possible a std::vector
> could use PTRDIFF_MAX/2 bytes, and then the size <= max_size/2 assumption
> would not hold.

Can we go with this perhaps only for 64bit targets?
I am not sure how completely safe this idea is in 32bit world: I guess
one can have OS that lets you to allocate half of address space as one
allocation.

Thanks!
Honza