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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/110269] New: 13/14 Regression] Missed Dead
 Code Elimination when using __builtin_unreachable since r13-4607-g2dc5d6b1e7e
Date: Thu, 15 Jun 2023 16:22:17 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110269

            Bug ID: 110269
           Summary: 13/14 Regression] Missed Dead Code Elimination when
                    using __builtin_unreachable since
                    r13-4607-g2dc5d6b1e7e
           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: ---

https://godbolt.org/z/5qqPMsxEh

Given the following code:

void foo(void);
static int a =3D 1, c;
static int *b =3D &a;
static int **d =3D &b;
static int ***e =3D &d;
void __assert_fail() __attribute__((__noreturn__));
static int f() {
    if (a) return a;
    for (; c;) *e =3D 0;
    if (b) __assert_fail();
    return 6;
}
int main() {
    if (f()) {
        *d =3D 0;
        if (b =3D=3D 0)
            ;
        else {
            __builtin_unreachable();
            __assert_fail();
        }
    }
    if (b =3D=3D 0)
        ;
    else
        foo();
    ;
}

gcc-trunk -O3 generates:

main:
        subq    $8, %rsp
        movl    a(%rip), %eax
        testl   %eax, %eax
        jne     .L2
        cmpq    $0, b(%rip)
        jne     .L6
.L2:
        movq    d(%rip), %rax
        movq    $0, (%rax)
        cmpq    $0, b(%rip)
        je      .L3
        call    foo
.L3:
        xorl    %eax, %eax
        addq    $8, %rsp
        ret
.L6:
        xorl    %eax, %eax
        call    __assert_fail

gcc-12.3.0 -O3 generates:

main:
        movl    a(%rip), %edx
        testl   %edx, %edx
        jne     .L2
        cmpq    $0, b(%rip)
        jne     .L7
.L2:
        movq    d(%rip), %rax
        movq    $0, (%rax)
        xorl    %eax, %eax
        ret
.L7:
        pushq   %rax
        xorl    %eax, %eax
        call    __assert_fail

Bisects to r13-4607-g2dc5d6b1e7e=