[Bug middle-end/38851] [4.4 regression] Compiler warns about uninitialized variable that is an object with a constructor

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

------- Comment #14 from mmitchel at gcc dot gnu dot org  2009-01-25 19:45 -------
Richard --

I don't agree that it's necessarily the FE's job to omit all stores to such
types.  Our general theory is that FEs get to emit dumb code and the optimizers
get to fix it up.  Of course, I don't object to making the FE generate simpler
code, if that's easy to do; just that if our design relies on that, I think
that's a mistake.

I can imagine ways this could come up in other languages as well.  For example,
copying a C structure with an anonymous bit-field, but no other content, or an
Ada record that uses Ada's layout directives to control size.

Therefore, I don't think that the key here is "zero-size".  Instead, it's the
fact that structure cannot be initialized.  That's useful both for warnings and
for optimization; it can't be initialized, so there's no point about warning
about uninitialized uses, and there's no reason to actually generate code for
the copies.

That leads to something I do think is something that the FEs could be asked to
do: set a bit on the type to indicate that it is uninitializable or, if you
like, logically empty.

I also don't see this as a P1 defect.  It's certainly annoying, but,
fundamentally, it limits the utility of a warning which has been known to give
false positives for a long time.  Important to fix, yes -- but as important as
generating wrong code?

-- Mark


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38851