consistent unspecified pointer comparison

[Andrew Pinski]

On Thu, Jun 27, 2024 at 11:57 AM Jason Merrill via Gcc <gcc@gcc.gnu.org> wrote:
>
> On Thu, Jun 27, 2024 at 2:38 PM Richard Biener
> <richard.guenther@gmail.com> wrote:
> > > Am 27.06.2024 um 19:04 schrieb Jason Merrill via Gcc <gcc@gcc.gnu.org>:
> > >
> > > https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2024/p2434r1.html
> > > proposes to require that repeated unspecified comparisons be
> > > self-consistent, which does not match current behavior in either GCC
> > > or Clang.  The argument is that the current allowance to be
> > > inconsistent is user-unfriendly and does not enable significant
> > > optimizations.  Any feedback about this?
> >
> > Can you give an example of an unspecified comparison?  I think the only way to do what the paper wants is for the implementation to make the comparison specified (without the need to document it).  Is the self-consistency required only within some specified scope (a single expression?) or even across TUs (which might be compiled by different compilers or compiler versions)?
> >
> > So my feedback would be to make the comparison well-defined.
> >
> > I’m still curious about which ones are unspecified now.
>
> https://eel.is/c++draft/expr#eq-3.1
> "If one pointer represents the address of a complete object, and
> another pointer represents the address one past the last element of a
> different complete object, the result of the comparison is
> unspecified."
>
> This is historically unspecified primarily because we don't want to
> force a particular layout of multiple variables.
>
> See the example under "consequences for implementations" in the paper.

There is instability due to floating point too;
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93681
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93806

and uninitialized variables:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93301
(but that might be fixed via https://wg21.link/P2795R5).

>
> Jason
>