[PATCH] Improve TBAA with unions

[Eric Botcazou]

> We have a good place in the middle-end to apply such rules which
> is component_uses_parent_alias_set_from - this is where I move
> the logic that is duplicated in various frontends.
> 
> The Java and Ada frontends do not allow union type punning (LTO does),
> so this patch may eventually pessimize them.  I don't care anything
> about Java but Ada folks might want to chime in.

The role of UNION_TYPE is negligible in Ada, it's only used for unchecked 
unions, which are quite rare.  Moreover, they are explicitly designed to be 
compatible with C unions so behaving like them kind of makes sense I think.

-- 
Eric Botcazou