[Bug c/33823] bitfields on packed struct aligns by a few bits if the types differ

["alexandre dot nunes at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #8 from alexandre dot nunes at gmail dot com  2008-02-14 22:06 -------
(In reply to comment #7)
> Yes, so for packed structs (which are a GCCism), GCC sets the rule.  Better
> documentation is certainly appreciated, but - what is the bug here?  Did
> the behavior change (I think it did for some 3.x releases) recently?
> 

I'm not sure if it's a bug, call it a clarification request, that may or may
not involve a bug: It's stated somewhere in gcc docs that gcc tries to do
things uniformly, only diverging between targets where there's a compelling
reason to do so. So what I'm asking is:

- Is the attribute((packed)) behaviour consistent between gcc ports regarding
bitfield alignments? If not, why?

- If attribute((packed)) is not consistent between gcc ports, could it be made
so? At least sysv abi (as far as I've read) does not mandate anything about the
bitfields within a word. I've not pointed to any ABI that does, however, that
was the explanation I've got. That's where I said the abi could change (for
several arches). You may say it's not worthy, and I would agree, but here is
where the "bug" would (or would not) lie.

- If not and there's no point changing it, could there be another attribute, or
anything, that would have that effect (uniformity between ABIs)?

I understand perfectly that using structures like that is meant to be
non-portable, but it's quite handy to have something like that without too much
worry about platforms, since altough I develop for several arches, almost all
of them today have a gcc port, and I know there are others with similar issues.

If this revert to be a non-bug and I can still hold a feature request on it, I
would be satisfied.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33823