[Bug c/14319] incorrect optimization of union of structs with common initial sequences

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

------- Additional Comments From jsturm at gcc dot gnu dot org  2004-02-27 19:07 -------
(In reply to comment #2)
> I can't believe the wording means what you imply. Assume that caller 
> and callee are in different translation units, then there is no 
> way for the compiler to see that the two arguments to the called function 
> may in fact be members of the same union. 

Isn't that the reason for the visibility rule?  Read the committee's response to
DR 257.

> So the only way to assume that they could alias each other is to 
> search the universe for a union type in which both of them are members. 
> That certainly can't be the intent of the standard or DR. It only makes 
> sense, if as RTH says the access is through such a union. 

Yet it intrigues me then that the committee would have a need to create a
special exception for this one case.  In all other situations (type-punning) the
behavior seems to be implementation defined.

The 2nd example in 6.5.2.3#8 is equivalent to mine, except for the scope of the
union declaration.

Despite RTH's comment I don't see any mention of this exception for common
initial sequences in the -fstrict-aliasing docs.  There are general examples of
type-punning though.

Anyhow... folks, I don't really care one way or other.  I wasn't even aware of
DR 257 prior to Dan Nicolaescu's message.  If this is considered settled for GCC
pending any clarification from the committee, then the PR might as well be closed.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14319