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 7941F3858D39; Fri, 29 Jul 2022 08:24:57 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 7941F3858D39
From: "tmayerl at student dot ethz.ch" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/106474] New: DCE depends on how if expressions are nested
Date: Fri, 29 Jul 2022 08:24:57 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: c
X-Bugzilla-Version: 12.1.0
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: tmayerl at student dot ethz.ch
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
 bug_severity priority component assigned_to reporter target_milestone
Message-ID: <bug-106474-4@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, 29 Jul 2022 08:24:57 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106474

            Bug ID: 106474
           Summary: DCE depends on how if expressions are nested
           Product: gcc
           Version: 12.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tmayerl at student dot ethz.ch
  Target Milestone: ---

In some cases, the compiler's ability to eliminate dead code depends on how=
 if
expressions are nested.

GCC detects that the if expressions in the following code snippet evaluate =
to
false and thus removes the dead code:

#include <stdio.h>
#include <stdbool.h>

static void __attribute__ ((noinline)) DCEMarker0_() {printf("DCE2.0");}

void f(bool s, bool c) {
    if (!c =3D=3D !s) {
        if (!c && s) {
            DCEMarker0_();
        }
    }
}

In the following snippet the !c term is moved into the outer if expression.
However, GCC cannot eliminate the dead code anymore:

#include <stdio.h>
#include <stdbool.h>

static void __attribute__ ((noinline)) DCEMarker0_() {printf("DCE2.0");}

void f(bool s, bool c) {
    if ((!c =3D=3D !s) && !c) {
        if (s) {
            DCEMarker0_();
        }
    }
}

In the following snippet, only the outer if statement is kept and all terms=
 are
moved there. GCC is now able to eliminate the code again:

#include <stdio.h>
#include <stdbool.h>

static void __attribute__ ((noinline)) DCEMarker0_() {printf("DCE2.0");}

void f(bool s, bool c) {
    if (((!c =3D=3D !s) && !c) && s) {
        DCEMarker0_();
    }
}

This can also be seen via the following Compiler Explorer link:
https://godbolt.org/z/9WPn5cG1W=