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From: "segher at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/107172] [13 Regression] wrong code with "-O1 -ftree-vrp"
 on x86_64-linux-gnu since r13-1268-g8c99e307b20c502e
Date: Tue, 11 Oct 2022 18:05:46 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107172

--- Comment #14 from Segher Boessenkool <segher at gcc dot gnu.org> ---
(In reply to H.J. Lu from comment #13)
> (In reply to Segher Boessenkool from comment #12)
> >=20
> > To determine the semantics of this piece of RTL you need to see the set=
ter(s)
> > of reg 17 feeding this use.  In this case, the setter was
> >   (set (reg:CCC 17)
> >        (ne:CCC (reg:SI 82)
> >                (const_int 0 [0])))
> > which has no meaning for a use that uses "ltu".
>=20
> What should a valid setter look like?  It should set reg 17 in CCC mode if
> reg 82 in SI mode isn't 0.

CCCmode can only represent the result of a comparison, like any other MODE_=
CC
thing.  The i386 CCCmode means only the carry bit can be used for this, so =
you
beed to do an unsigned comparison against (const_int 1).  This will end up =
with
the opposite polarity of what you said I guess, you need "geu" instead?=