[Bug c++/111019] [12/13/14 Regression] Optimizer incorrectly assumes variable is not changed while change happens through another pointer

["rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111019

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |hubicka at gcc dot gnu.org

--- Comment #7 from Richard Biener <rguenth at gcc dot gnu.org> ---
And what in the end disambiguates this in LIM is aliasing_component_refs_p.
Note that LIM feeds it canonicalized __MEM <struct Target> ((struct Target
*)_26).D_43475.next, it instead uses MEM[(struct Base * *)_26 + 8B].

I think it goes wrong because LIM canonicalizes _26->...next which is a
ref at offset 8 to MEM[_26 + 8] _at offset 8_, the base is actually
recorded as MEM[_26] but get_ref_base_and_extent wouldn't get 8 out of
MEM[_26 + 8] and aliasing_matching_component_refs_p re-does that to adjust
the offset which in the end leads to some inconsistency.

What LIM tries to do is find a canonical representation for the ref which
is MEM[_26 + 8B], it also tries to preserve the base which is MEM[_26],
but that's somewhat in conflict here.  It works for LIMs purposes but as
can be seen it doesn't work here.

The bad thing is that there isn't a consistent representation for this,
but also aliasing_matching_component_refs_p relying on this is bad?