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 C4AC83858401; Thu,  1 Jun 2023 13:54:43 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org C4AC83858401
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org;
	s=default; t=1685627683;
	bh=gMSRu2nYqpJZFqDvI5X0kcBMWsIlUGW739+N1Z+/wtk=;
	h=From:To:Subject:Date:From;
	b=Vq361uzec5HRAGnW/ON4JZWwhvTBTVzM7hYjpTf1Wy/u9ZpSVToUEyyqfB/bWdqpP
	 KIfSv4xA9jqmHKvTs7gxhoZHrIRCj1b0byMcgcEJPl9Qesr9lSGfl/C1AimY08IvxC
	 QljI9qJ+rfjvLjBrO9ngaqfmBi8zo3ENk9cqJTVk=
From: "theodort at inf dot ethz.ch" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/110080] New: [13/14 Regression] Missed Dead
 Code Elimination at -Os when using __builtin_unreachable since
 r13-6945-g429a7a88438
Date: Thu, 01 Jun 2023 13:54:43 +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: 14.0
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: theodort at inf 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-110080-4@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=3D110080

            Bug ID: 110080
           Summary: [13/14 Regression] Missed Dead Code Elimination at -Os
                    when using __builtin_unreachable since
                    r13-6945-g429a7a88438
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: theodort at inf dot ethz.ch
  Target Milestone: ---

Given:=20

void foo(void);
static unsigned char a =3D 131;
static int *b;
static int **c =3D &b;
static void d(int e, unsigned f) {
    int *g;
    if (e){
        for (; a; ++a)
            for (e =3D 0; 0;)
                ;
        g =3D &e;
        int **h =3D &g;
        if (**h) {
            foo();
        }
    }
    *c =3D &e;
}
int main() { d(4 & a, a); }


gcc trunk/13.1/12.3 at -Os generate:

main:
        testb   $4, a(%rip)
        je      .L3
        movb    $0, a(%rip)
.L3:
        leaq    -4(%rsp), %rax
        movq    %rax, b(%rip)
        xorl    %eax, %eax
        ret
a:
        .byte   -125

If I include a __builtin_unreachable() to help the compiler:

void foo(void);
static unsigned char a =3D 131;
static int *b;
static int **c =3D &b;
static void d(int e, unsigned f) {
    int *g;
    if (f !=3D 131) {
        __builtin_unreachable(); // <- THIS
    }
    if (!e){
        for (; a; ++a)
            for (e =3D 0; 0;)
                ;
        g =3D &e;
        int **h =3D &g;
        if (**h) {
            foo();
        }
    }
    *c =3D &e;
}
int main() { d(4 & a, a); }

gcc-12.3 at -Os generates better code:

main:
        leaq    -4(%rsp), %rax
        movb    $0, a(%rip)
        movq    %rax, b(%rip)
        xorl    %eax, %eax
        ret
a:
        .byte   -125

But gcc-13.1/trunk at -Os generate worse code:

main:
        subq    $24, %rsp
        movb    a(%rip), %al
        movl    %eax, %edx
        andl    $4, %edx
        movl    %edx, 12(%rsp)
        xorl    %edx, %edx
.L2:
        testb   %al, %al
        je      .L8
        incl    %eax
        movb    $1, %dl
        jmp     .L2
.L8:
        testb   %dl, %dl
        je      .L4
        movb    $0, a(%rip)
        jmp     .L5
.L4:
        cmpl    $0, 12(%rsp)
        je      .L5
        call    foo
.L5:
        leaq    12(%rsp), %rax
        movq    %rax, b(%rip)
        xorl    %eax, %eax
        addq    $24, %rsp
        ret
a:
        .byte   -125

https://godbolt.org/z/zvqshjEj3


Started with r13-6945-g429a7a88438=