[Bug ipa/80899] [6/7/8 Regression] Devirtualization causes incorrect code generation with placement new in some cases

["jason at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80899

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jason at gcc dot gnu.org

--- Comment #5 from Jason Merrill <jason at gcc dot gnu.org> ---
(In reply to Jan Hubicka from comment #4)
> Is this valid C++? bar.mem is non-POD and is already constructed and it
> seems fishy to placement new it to something different.

It's very fishy; the code should use aligned_storage rather a different
non-trivial type.  My old proposed resolution for wg21.link/cwg1116 would have
clarified that this is undefined, and might still happen to resolve
wg21.link/cwg1027 , but it hasn't been accepted yet.

On the other hand, it is careful to use the pointer returned from placement new
rather than a pointer derived from mem, and it seems like we ought to use the
information from placement new to guide devirtualization.

So...sketchy code in a volatile area of semantics, but it seems like we could
get it right without breaking important optimizations.