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 CC0DF385840A; Fri, 22 Oct 2021 09:40:37 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org CC0DF385840A
From: "tnfchris at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/102893] New: [8/9/10/11/12 Regression] CDDCE
 does not detect empty infinite nested loops
Date: Fri, 22 Oct 2021 09:40:37 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 8.5.0
X-Bugzilla-Keywords: missed-optimization
X-Bugzilla-Severity: normal
X-Bugzilla-Who: tnfchris at gcc dot gnu.org
X-Bugzilla-Status: UNCONFIRMED
X-Bugzilla-Resolution: 
X-Bugzilla-Priority: P3
X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org
X-Bugzilla-Target-Milestone: ---
X-Bugzilla-Flags: 
X-Bugzilla-Changed-Fields: bug_id short_desc product version bug_status
 keywords bug_severity priority component assigned_to reporter
 target_milestone
Message-ID: <bug-102893-4@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
X-BeenThere: gcc-bugs@gcc.gnu.org
X-Mailman-Version: 2.1.29
Precedence: list
List-Id: Gcc-bugs mailing list <gcc-bugs.gcc.gnu.org>
List-Unsubscribe: <https://gcc.gnu.org/mailman/options/gcc-bugs>,
 <mailto:gcc-bugs-request@gcc.gnu.org?subject=unsubscribe>
List-Archive: <https://gcc.gnu.org/pipermail/gcc-bugs/>
List-Post: <mailto:gcc-bugs@gcc.gnu.org>
List-Help: <mailto:gcc-bugs-request@gcc.gnu.org?subject=help>
List-Subscribe: <https://gcc.gnu.org/mailman/listinfo/gcc-bugs>,
 <mailto:gcc-bugs-request@gcc.gnu.org?subject=subscribe>
X-List-Received-Date: Fri, 22 Oct 2021 09:40:37 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102893

            Bug ID: 102893
           Summary: [8/9/10/11/12 Regression] CDDCE does not detect empty
                    infinite nested loops
           Product: gcc
           Version: 8.5.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tnfchris at gcc dot gnu.org
  Target Milestone: ---

The following example

int main() {
  while(1) {
    for(int i=3D0; i<9000000; i++){}
  }
}

In GCC 7 compiled to

main:
.L2:
        b       .L2

but since GCC 8 compiles to

main:
.L3:
        mov     w0, 21568
        movk    w0, 0x89, lsl 16
.L2:
        subs    w0, w0, #1
        bne     .L2
        b       .L3

The difference seems to be in cddce which previously was able to detect the
code as dead:

;; 3 loops found
;;
;; Loop 0
;;  header 0, latch 1
;;  depth 0, outer -1
;;  nodes: 0 1 2 3 4 5 6
;;
;; Loop 1
;;  header 3, latch 6
;;  depth 1, outer 0
;;  nodes: 3 6 5 4
;;
;; Loop 2
;;  header 5, latch 4
;;  depth 2, outer 1
;;  nodes: 5 4
;; 2 succs { 3 }
;; 3 succs { 5 }
;; 4 succs { 5 }
;; 5 succs { 4 6 }
;; 6 succs { 3 }
can not prove finiteness of loop 1
Removing basic block 4
Merging blocks 3 and 5
Removing basic block 6

but since GCC 8 does

;; 3 loops found
;;
;; Loop 0
;;  header 0, latch 1
;;  depth 0, outer -1
;;  nodes: 0 1 2 3 4 5 6
;;
;; Loop 1
;;  header 3, latch 6
;;  depth 1, outer 0
;;  nodes: 3 6 5 4
;;
;; Loop 2
;;  header 5, latch 4
;;  depth 2, outer 1
;;  nodes: 5 4
;; 2 succs { 3 }
;; 3 succs { 5 }
;; 4 succs { 5 }
;; 5 succs { 4 6 }
;; 6 succs { 3 }
cannot prove finiteness of loop 1=