https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94747
Nathan Sidwell <nathan at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Last reconfirmed| |2020-04-24
Summary|Undefined behavior: integer |Confusing code in
|overflow in |libsupc++/dyncast.cc
|libsupc++/dyncast.cc |
CC| |nathan at gcc dot gnu.org
Status|RESOLVED |ASSIGNED
Assignee|unassigned at gcc dot gnu.org |nathan at gcc dot
gnu.org
Resolution|INVALID |---
Ever confirmed|0 |1
--- Comment #3 from Nathan Sidwell <nathan at gcc dot gnu.org> ---
While the code is well formed, it does not convey intent well. I wrote that
code, and probably even considered spelling as -ptrdiff_t (offsetof ...). but
didn't because it causes a line wrap and has no semantic difference.
negating unsigned numbers often shows programmer confusion, and can cause bugs
on I32LP64 systems, where the negation happens at a lower precision than
expected.
That's not the case here. If size_t has fewer bits than a pointer, there cannot
be overflow from the top size_t bit to the pointer arithmetic -- that breaks C
& C++ semantics.
Something for stage 1
