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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/110361] New: [13/14 Regression] Missed Dead
 Code Elimination when using __builtin_unreachable since r13-2020-g16b013c9d9b
Date: Thu, 22 Jun 2023 11:40:40 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110361

            Bug ID: 110361
           Summary: [13/14 Regression] Missed Dead Code Elimination when
                    using __builtin_unreachable since
                    r13-2020-g16b013c9d9b
           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/qab3679sq

Given the following code:

void foo(void);
static int a, b, d, p, k;
static int *c, *e, *g;
static short f;
static unsigned h;
static int **i =3D &c, **o =3D &g;
void __assert_fail() __attribute__((__noreturn__));
static void j(char, unsigned r) {
    h =3D r =3D=3D 0 ?: 2 % r;
    int l =3D h;
    if (!(((l) >=3D 1) && ((l) <=3D 1))) {
        __builtin_unreachable();
    }
    int m;
    *i =3D &m;
}
static int *n();
static short q(short t) {
    *o =3D &p;
    return t;
}
static unsigned s() {
    int *j =3D n();
    if (e =3D=3D 0 || e =3D=3D &a || e =3D=3D &d)
        ;
    else
        __assert_fail();
    return *j;
}
static int *n() {
    if (b) {
        if (c) __assert_fail();
        e =3D 0;
    }
    return &k;
}
int main() {
    j(q(s()), f);
    if (e =3D=3D 0 || e =3D=3D &a || e =3D=3D &d)
        ;
    else
        foo();
    f =3D 0;
}

gcc-trunk -O2 does not eliminate the call to foo:

main:
        subq    $24, %rsp
        movq    e(%rip), %rax
        movswl  f(%rip), %edx
        testq   %rax, %rax
        je      .L2
        cmpq    $a, %rax
        je      .L4
        cmpq    $d, %rax
        jne     .L17
.L4:
        movq    $p, g(%rip)
        testl   %edx, %edx
        je      .L18
        leaq    12(%rsp), %rdx
        movq    %rdx, c(%rip)
        cmpq    $a, %rax
        je      .L5
        cmpq    $d, %rax
        je      .L5
        call    foo
.L5:
        xorl    %eax, %eax
        movw    %ax, f(%rip)
        xorl    %eax, %eax
        addq    $24, %rsp
        ret
.L18:
        leaq    12(%rsp), %rax
        movq    %rax, c(%rip)
        jmp     .L5
.L2:
        movq    $p, g(%rip)
        leaq    12(%rsp), %rax
        movq    %rax, c(%rip)
        jmp     .L5
.L17:
        xorl    %eax, %eax
        call    __assert_fail

gcc-12.3.0 -O2 eliminates the call to foo:

main:
        subq    $24, %rsp
        movq    e(%rip), %rax
        testq   %rax, %rax
        je      .L2
        cmpq    $a, %rax
        je      .L2
        cmpq    $d, %rax
        jne     .L14
.L2:
        movq    $p, g(%rip)
        leaq    12(%rsp), %rax
        movq    %rax, c(%rip)
        xorl    %eax, %eax
        movw    %ax, f(%rip)
        xorl    %eax, %eax
        addq    $24, %rsp
        ret
.L14:
        xorl    %eax, %eax
        call    __assert_fail

Bisects to r13-2020-g16b013c9d9b=