[libstdc++] Improve M_check_len

[Jonathan Wakely]

On Tue, 20 Jun 2023 at 09:21, Andreas Schwab wrote:

> On Jun 20 2023, Jakub Jelinek via Gcc-patches wrote:
>
> > Is it safe even on 64bit targets?  I mean, doesn't say PowerPC already
> allow
> > full 64-bit virtual address space?  The assumption that one can't have
> > more than half of virtual address space allocations is true right now at
> > least on x86-64, aarch64 and others, but isn't that something that can
> > change with newer versions of CPUs without the need to recompile
> > applications (add another level or two of page tables)?
>
> At least s390 can allocate more than half the address space.  That
> triggered a failure in gawk.
>

Is PTRDIFF_MAX large enough to represent the difference between any two
pointers?

What we're considering for libstdc++ is treating PTRDIFF_MAX as an upper
limit on allocation size. If there are targets that can really allocate a
2^63 byte array, they won't be able to represent the difference between the
first element and the last element unless ptrdiff_t is wider than 64 bits.