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Re: Possible problem wiht nonoverlapping_component_refs_p
- From: Richard Henderson <rth at redhat dot com>
- To: Richard Kenner <kenner at vlsi1 dot ultra dot nyu dot edu>
- Cc: gcc-patches at gcc dot gnu dot org
- Date: Thu, 6 Dec 2001 09:27:38 -0800
- Subject: Re: Possible problem wiht nonoverlapping_component_refs_p
- References: <10112061112.AA28427@vlsi1.ultra.nyu.edu>
On Thu, Dec 06, 2001 at 06:12:39AM -0500, Richard Kenner wrote:
> I'm confused. What does this "promise" refer to? I didn't realize that C
> considers this undefined, but I can see why it might.
C considers this undefined in the "strict aliasing" paragraph, 6.5/7.
GCC explicitly allows type punning through unions. This promise
can be seen in the documentation for -fstrict-aliasing.
> So if we do have offset data all the way up the chain (which we
> should), then we should be able to handle this as with your
> overlapping fields, no?
> Actually, I don't think we do have offset data all the way up, but I don't
> think we need it for this. We don't have the intermediate offset data
> because we never form a MEM for it, looking directly as far as we can in
Err, we _should_ be able to compute the field's offset within the
union, since we have all of the intermediate fields.
> The only offsets that are meaningful are from the decl
> and from the expression. So the overlap check for the intermediate fields
> would have to be conservative and see if the fields themselves overlap.
> However, that's only slightly more conservative and I think fine.
Hmm. Perhaps we're talking about the same thing.
> One question is what to do about fields at variable positions.
Ug. I had no idea we had to worry about that. I guess we have
to give up if that's accessed from a union, but otherwise we
should be able to handle it.