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From: "amacleod at redhat dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/101511] [12 Regression] ice in
 query_relation, at value-relation.cc:879
Date: Mon, 19 Jul 2021 20:59:40 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101511

Andrew Macleod <amacleod at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |aldyh at gcc dot gnu.org

--- Comment #3 from Andrew Macleod <amacleod at redhat dot com> ---
The new relation code is dominance based,  in the interest of efficiency, it
can currently miss the odd equivalence if things are processed in a
non-dominance order.

Its not incorrect, it just may lead to a missed opportunity.

This testcase triggers an assert which checks for this situation.  There are
numerous UNDEFINED values, and we end up register an equivalences:

 Registering value_relation (Error_18 =3D=3D _4) (bb4) at Error_18 =3D _4 +=
 Error_13;
 Registering value_relation (Error_13 =3D=3D _4) (bb2) at Error_13 =3D _4 +=
 Error_9;

Not we register the bb4 equivalence first,  and then alter register the one=
 in
bb2.

As a result, we don't currently update post dominated blocks, so we dont
combine these into Error_18, Error_13 and _4 in bb4.
When we later check for an equivalence between we detect this anomaly.
There is nothing incorrect in allowing it, and we'll do so for now.=20=20
Eventually, when its flushed out better, we'll change this back to an asser=
t.=