From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugzilla@gcc.gnu.org>
Received: by sourceware.org (Postfix, from userid 48)
	id 474A33858C39; Thu, 13 Oct 2022 16:54:58 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 474A33858C39
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org;
	s=default; t=1665680098;
	bh=1o311OC8jtSqxJ66A2SQ11XaYx9jtDItKD3fVqKJJ/s=;
	h=From:To:Subject:Date:In-Reply-To:References:From;
	b=ipq4pUpULHCm4wVOAcCWz9Lt55AZylCAge/+sB76u+TY6NlRALRutP3CkFsyLrlhl
	 Ert96bAORgVGT+El6bHuxP7Fc4PF7WBGSgr3IngpBXcPv1nYLBvxJxS1NihG5RMYYe
	 aGRHosxUDWRjSFh+OwU0EdvGv8h++JAGNhKMfLh0=
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 16:54:47 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: changed
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: target
X-Bugzilla-Version: 13.0
X-Bugzilla-Keywords: wrong-code
X-Bugzilla-Severity: normal
X-Bugzilla-Who: hjl.tools at gmail dot com
X-Bugzilla-Status: NEW
X-Bugzilla-Resolution: 
X-Bugzilla-Priority: P1
X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org
X-Bugzilla-Target-Milestone: 13.0
X-Bugzilla-Flags: 
X-Bugzilla-Changed-Fields: 
Message-ID: <bug-107172-4-0rlKyIwaH8@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-107172-4@http.gcc.gnu.org/bugzilla/>
References: <bug-107172-4@http.gcc.gnu.org/bugzilla/>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/
Auto-Submitted: auto-generated
MIME-Version: 1.0
List-Id: <gcc-bugs.sourceware.org>

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107172

--- Comment #26 from H.J. Lu <hjl.tools at gmail dot com> ---
(In reply to Uro=C5=A1 Bizjak from comment #24)
> (In reply to Hongtao.liu from comment #23)
> > looking at i386.c put_condition_code used by *setcc_qi, it looks like (=
EQ
> > (reg:CCCmode FLAG_REG) (const_int 0)) means get carry flag.
> > Not (LTU: (REG:CCCmode FLAGS_REG) (const_int 0)).
> > Now I got more confused.
>=20
> CCCmode means that single flag is tested, it uses EQ and NE, so "c" and "=
nc"
> suffix is emitted. When CCmode is used, LTU/GEU operation on CCmode flags
> reg produces "b" and "nb" suffix, which decodes to exactly the same assem=
bly
> as "c" and "nc" suffixes.
>=20
> However, it looks that somewhere LTU/GEU is also generated with CCCmode
> flags reg, and some fixup was introduced to put_condition_code to "fix" t=
his
> inconsistency. If LTU/GEU is valid only for CCmode, then the producers of
> invalid RTX should be fixed.

EQ and NE are only used to set FLAGS_REG in CCCmode from 2 integer operands.
LTU/GEU are only used to check FLAGS_REG against constant 0.  This is very
consistent.  simplify_const_relational_operation has

 /* 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;

In case of the comparison in CCCmode with constant op0 and op1, it should
let the backend perform the actual comparison.=