[Bug c/67999] Wrong optimization of pointer comparisons

["danielmicay at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

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

Daniel Micay <danielmicay at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |danielmicay at gmail dot com

--- Comment #5 from Daniel Micay <danielmicay at gmail dot com> ---
An implementation can have object size limits, but I think it's incorrect if
those limits are not enforced for all standard ways of allocating objects.
Objects larger than PTRDIFF_MAX are forbidden with musl (malloc, mmap and
friends report ENOMEM and it's explicitly undefined to create them in another
way and use them with libc), and it would make a lot of sense for glibc to do
the same thing. I recently landed the same feature in Android's Bionic libc for
mmap.

Since glibc's implementations of standard library functions don't handle
objects larger than PTRDIFF_MAX, this can definitely be considered as a buggy
area in glibc. The typical issue is `end - start` but compilers considering
addition to be undefined in this case isn't surprising either.

FWIW, Clang also treats `ptr + size` with `size > PTRDIFF_MAX` as undefined for
standard C pointer arithmetic because the underlying getelementptr instruction
in LLVM is inherently a signed arithmetic operation. Clang marks standard C
pointer arithmetic operations as "inbounds", which among other things makes use
of the value returned from wrapping into undefined behavior. Last time I
checked, the non-standard GNU C `void *` arithmetic doesn't get tagged as
"inbounds" by Clang, so wrapping is well-defined for that.