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From: "hjl.tools at gmail dot com" <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: Thu, 13 Oct 2022 22:34:18 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107172

--- Comment #32 from H.J. Lu <hjl.tools at gmail dot com> ---
(In reply to Segher Boessenkool from comment #30)
> (In reply to H.J. Lu from comment #26)
> > LTU/GEU are only used to check FLAGS_REG against constant 0.
>=20
> That is not what
>   (ltu (reg 17) (const_int 0))
> means though?
>=20
> Together with a previous
>   (set (reg 17) (compare A B))
>=20
> it means the result of A <=3D B.

That is correct.

> There is no actual comparison with 0, that is just notation.

True.  But simplify-rtx.cc simplifies

(ltu (reg 17) (const_int 0))

to false when reg 17 is set.

> > simplify_const_relational_operation has
> >=20
> >  /* We can't simplify MODE_CC values since we don't know what the
> >      actual comparison is.  */
> >   if (GET_MODE_CLASS (GET_MODE (op0)) =3D=3D MODE_CC)
> >     return 0;
>=20
> And combine *does* know how to find the actual comparison, in many cases.=
=20
> Some
> other passes can as well, there is no magic involved, you just need to lo=
ok
> at
> other insns as well (just one really, the CC setter).

Use the actual comparison isn't issue.  The issue is how

(ltu (reg 17) (const_int 0))

should be simplified when reg 17 is known to be set.=