On Thu, Jun 27, 2024 at 2:38 PM Richard Biener <richard.guent...@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. Jason
