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From: "theodort at inf dot ethz.ch" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/109893] New: [14 Regression]  Missed Dead
 Code Elimination when using __builtin_unreachable since 
 r14-160-gf828503eeb79ad1f1ada6db7deccc5abcc2f3ca3
Date: Wed, 17 May 2023 15:15:58 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 14.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109893

            Bug ID: 109893
           Summary: [14 Regression]  Missed Dead Code Elimination when
                    using __builtin_unreachable since
                    r14-160-gf828503eeb79ad1f1ada6db7deccc5abcc2f3ca3
           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: ---

void foo(void);
void bar(void);
static char a;
static int b, e, f;
static int *c =3D &b, *g;
int main() {
    int *j =3D 0;
    if (a) {
        g =3D 0;
        if (c)
            bar();
    } else {
        j =3D &e;
        c =3D 0;
    }
    if (c =3D=3D &f =3D=3D b || c =3D=3D &e)
        ;
    else
        __builtin_unreachable();
    if (g || e) {
        if (j =3D=3D &e || j =3D=3D 0)
            ;
        else
            foo();
    }
    a =3D 4;
}

gcc -O3:=20

main:
        cmpb    $0, a(%rip)
        je      .L2
        xorl    %esi, %esi
        cmpq    $0, c(%rip)
        movq    %rsi, g(%rip)
        je      .L7
        pushq   %rdx
        call    bar
        movb    $4, a(%rip)
        xorl    %eax, %eax
        popq    %rcx
        ret
.L2:
        xorl    %eax, %eax
        movq    %rax, c(%rip)
.L7:
        movb    $4, a(%rip)
        xorl    %eax, %eax
        ret
c:
        .quad   b

gcc-trunk -O3=20

main:
        subq    $8, %rsp
        cmpb    $0, a(%rip)
        je      .L2
        xorl    %edx, %edx
        cmpq    $0, c(%rip)
        movq    %rdx, g(%rip)
        je      .L6
        call    bar
        xorl    %eax, %eax
.L4:
        cmpq    $0, g(%rip)
        je      .L9
.L6:
        movb    $4, a(%rip)
        xorl    %eax, %eax
        addq    $8, %rsp
        ret
.L2:
        xorl    %eax, %eax
        movq    %rax, c(%rip)
        movl    $e, %eax
        jmp     .L4
.L9:
        cmpl    $0, e(%rip)
        je      .L6
        testq   %rax, %rax
        je      .L6
        cmpq    $e, %rax
        je      .L6
        call    foo
        jmp     .L6
c:
        .quad   b

Bisects to:
https://gcc.gnu.org/git/gitweb.cgi?p=3Dgcc.git;h=3Df828503eeb79ad1f1ada6db7=
deccc5abcc2f3ca3=