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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/111293] New: [14 Regression] Missed Dead Code
 Elimination since r14-3414-g0cfc9c953d0
Date: Tue, 05 Sep 2023 15:01:10 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D111293

            Bug ID: 111293
           Summary: [14 Regression] Missed Dead Code Elimination since
                    r14-3414-g0cfc9c953d0
           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/9aj7sr5d6

Given the following code:

void foo(void);
static int a, d;
static short e;
static int(f)(int b, int c) { return c =3D=3D 0 ? b : b / c; }
static char g(char h) {
    char i =3D 1;
    int j =3D 8;
    e =3D 3;
    for (; d; ++d) {
        if (f(0 =3D=3D j, e)) return a;
        for (; i; i--) {
            if (j) {
                if (e)
                    ;
                else if (a)
                    continue;
                else
                    foo();
                j =3D -h;
                break;
            }
            __builtin_unreachable();
            break;
        }
    }
    return 1;
}
int main() {
    g(-1);
    a =3D e;
}

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

main:
        subq    $8, %rsp
        movl    d(%rip), %ecx
        movl    $3, %edx
        movw    %dx, e(%rip)
        testl   %ecx, %ecx
        je      .L6
        movl    $3, %edx
        .p2align 4,,10
        .p2align 3
.L2:
        testw   %dx, %dx
        jne     .L3
        movl    a(%rip), %eax
        testl   %eax, %eax
        je      .L18
.L5:
        addl    $1, d(%rip)
        jne     .L5
.L6:
        movswl  e(%rip), %eax
        movl    %eax, a(%rip)
        xorl    %eax, %eax
        addq    $8, %rsp
        ret
        .p2align 4,,10
        .p2align 3
.L18:
        call    foo
.L3:
        addl    $1, d(%rip)
        movzwl  e(%rip), %edx
        jne     .L2
        jmp     .L6

gcc-13.2.0 -O2 eliminates the call to foo:

main:
        movl    d(%rip), %edx
        movl    $3, %eax
        movw    %ax, e(%rip)
        testl   %edx, %edx
        je      .L3
        movl    $0, d(%rip)
.L3:
        movl    $3, a(%rip)
        xorl    %eax, %eax
        ret

Bisects to r14-3414-g0cfc9c953d0=