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.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.
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 >
