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From: "amacleod at redhat dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/101335] [12 Regression] wrong code at -O2 and
 above (casts between signed and unsigned) by r12-1796
Date: Mon, 12 Jul 2021 15:35:21 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101335

--- Comment #2 from Andrew Macleod <amacleod at redhat dot com> ---
yeah, it because we have been treating casts to objects of the same precisi=
on
as equivalences.    This normally works fine, but in this case we have
c_9 =3D (int)_2          c_9 =3D=3D _2
_3 =3D c_9 - 10      so  _3 is < c_9
d_10 =3D (unsigned)_3    d_10 =3D=3D _3=20

if (_2 < d_10)
with the equivalences, it now thinks that d_10 < _2.

So I have to either not allow compounding relations thru casting equivalenc=
es
(painful), or something else.

-fwrapv make its work because we don't register the relation _3 < c_9 if
wrapping is on.


I'm running an experiment... When the recomputation code was introduced, it
completely eliminated the need to track downcasts. It seems that removing a=
ll
equivalences between casts doesn't miss anything either, so perhaps this ol=
der
bit isn't needed anymore.

I'm running it thru the testsuite now...=