[Bug c/67999] Wrong optimization of pointer comparisons

["ch3root at openwall dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67999

--- Comment #18 from Alexander Cherepanov <ch3root at openwall dot com> ---
I guess nobody doubts that the current situation in gcc+glibc (and clang+glibc)
should be fixed as valid programs are miscompiled. And it's easy to imagine
security consequences of this when buffers have sizes controlled by attackers.
The problem is not limited to the comparisons of the form 'p + a < p', all
comparison of the form 'p + a < p + b' are probably miscompiled. And
subtraction of pointers is problematic too:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=45779

The issue affects not only all 32-bit platforms (i386, x32, arm, etc.) but also
16-bit ones, right? Or all of them are dead? Recently even 18-bit one was
mentioned...

Whether gcc violates C11 or not is not clear. The standard mostly speaks about
compiler+library. OTOH gcc can be used as a freestanding implementation and
even in a hosted environment, in practice, AIUI there could be external
objects, not from compiler or libc.

Hence IMHO this limitation should at least be documented in a user-visible
place. (The same for libc's: if they cannot deal with huge objects it should be
documented even if they cannot create them.)