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 02BAA3858C1F; Thu, 30 Mar 2023 12:53:11 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 02BAA3858C1F
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org;
	s=default; t=1680180791;
	bh=WQ8iCZA4X46LcfGa+7TDFPJB5L/67oRi4+p9r3+4GMs=;
	h=From:To:Subject:Date:In-Reply-To:References:From;
	b=PkaDnKOxO9XQyI5kOpRT32po+nTFWy0bkORYpngBVsFPSQLue0xaWUB/liy61MG9d
	 jbKTUmg+AbSmuzTo+RUR0ZFSkItwxBGDxZQ6AmgyT1wvAAoMtPCCdPaiPb7IsyH6BZ
	 YR9WVtkxrFmOrSoOoHHy1I3wdzU5B0uuyltqy4K4=
From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/81958] [10/11/12/13 Regression] spurious
 -Wmaybe-uninitialized warning in gcc-8, or with -O1
Date: Thu, 30 Mar 2023 12:53:09 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: changed
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 8.0
X-Bugzilla-Keywords: diagnostic, missed-optimization
X-Bugzilla-Severity: normal
X-Bugzilla-Who: rguenth at gcc dot gnu.org
X-Bugzilla-Status: NEW
X-Bugzilla-Resolution: 
X-Bugzilla-Priority: P2
X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org
X-Bugzilla-Target-Milestone: 10.5
X-Bugzilla-Flags: 
X-Bugzilla-Changed-Fields: keywords
Message-ID: <bug-81958-4-RoMgoSNqgi@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-81958-4@http.gcc.gnu.org/bugzilla/>
References: <bug-81958-4@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=3D81958

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization

--- Comment #8 from Richard Biener <rguenth at gcc dot gnu.org> ---
We see

<bb 4> [count: 0]:
# eq_7 =3D PHI <eq_14(D)(2), eq_16(8)>
# ivtmp.7_23 =3D PHI <0(2), ivtmp.7_24(8)>
_25 =3D (unsigned int) ivtmp.7_23;
if (maxidx_15(D) > _25)
  goto <bb 3>; [0.00%]
else
  goto <bb 5>; [0.00%] // loop exit

<bb 5> [count: 0]:
if (maxidx_15(D) =3D=3D _25)
  goto <bb 6>; [0.00%]
else
  goto <bb 10>; [0.00%]

<bb 7> [count: 0]:
# eq_9 =3D PHI <eq_7(10), eq_18(6), eq_16(9)>
printk (eq_9); [tail call]
return;

and the appearant uninit use is on the 10->7 edge, when maxidx !=3D _25
but at the same time maxidx > _25.  We don't realize that the
maxidx =3D=3D _25 condition is always true.  The possible easiest way
is probably to rewrite the in-loop condition to maxid !=3D _25 somewhere
(ivcanon?).

uninit analysis itself just sees maxid <=3D _25 && maxid =3D=3D_25 as the
initialized cases - it could possibly analyze how _25 evolves
when defined in a loop to further optimize this to true.=