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

["vincent-gcc at vinc17 dot net" <gcc-bugzilla@gcc.gnu.org>]

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

Vincent Lefèvre <vincent-gcc at vinc17 dot net> changed:

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                 CC|                            |vincent-gcc at vinc17 dot net

--- Comment #17 from Vincent Lefèvre <vincent-gcc at vinc17 dot net> ---
(In reply to Tim Rentsch from comment #10)
> Four:  Despite the last observation, the "one special guarantee" clause
> (and hence also DR 257) is clearly not germane to this problem.  The
> reason for this is that the "one special guarantee" clause is concerned
> with read access ("inspect" is the word used in the Standard),
[...]

In any case, the aliasing rule is only about read access: "An object shall have
its stored value accessed only by [...]" (6.5#7). If you are accessing a stored
value, this means that you are reading it. Thus the "yp->y = 1;" does not imply
undefined behavior, in case this is what you were thinking about.

Note: Forbidding write accesses with a different effective type would make "for
subsequent accesses that do not modify the value" from 6.5#6 pointless.