[libstdc++] Improve M_check_len

[Jan Hubicka]

> > >
> > >       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.

Perhaps something like:
  size > std::min ((uint64_t)__max_size, ((uint64_t)1 << 62) / sizeof (_Tp))
is safe for all allocators and 32bit, so we won't need __is_same test
and test for 64bit?

Honza
> 
> Thanks!
> Honza