[Bug tree-optimization/109943] New: [13/14 Regression] Missed Dead Code Elimination when using __builtin_unreachable since r13-6834-g41ade3399bd

["theodort at inf dot ethz.ch" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109943

            Bug ID: 109943
           Summary: [13/14 Regression] Missed Dead Code Elimination when
                    using __builtin_unreachable since
                    r13-6834-g41ade3399bd
           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);
static int b = 5;
static int *c = &b;
static char(a)(char d, char g) {
    ;
    return d % g;
}
static void e(unsigned char d, unsigned g) {
    char h = a(g, d);
    if (h)
        if ((d) >= 0) {
            foo();
        };
}
static void f(int d) { e(b, d); }
int main() {
    f(*c);
    *c = 0;
}

gcc-trunk -O3 generates:

main:
        movl    $0, b(%rip)
        xorl    %eax, %eax
        ret

However, including additional info via __builtin_unreachable results to worse
code:

void foo(void);
static int b = 5;
static int *c = &b;
static char(a)(char d, char g) {
    if ((g) <= 0) {
        __builtin_unreachable();
    }
    return d % g;
}
static void e(unsigned char d, unsigned g) {
    char h = a(g, d);
    if (h)
        if ((d) >= 0) {
            foo();
        };
}
static void f(int d) { e(b, d); }
int main() {
    f(*c);
    *c = 0;
}

gcc-trunk (and 13) -O3 generates:

main:
        movl    b(%rip), %ecx
        movsbl  %cl, %eax
        movzbl  %cl, %ecx
        cltd
        idivl   %ecx
        testl   %edx, %edx
        jne     .L11
        movl    $0, b(%rip)
        xorl    %eax, %eax
        ret
.L11:
        pushq   %rax
        call    foo
        xorl    %edx, %edx
        xorl    %eax, %eax
        movl    %edx, b(%rip)
        popq    %rcx
        ret

https://godbolt.org/z/n34Tsr3vq

gcc-12 can generate optimized code for both cases

Started with r13-6834-g41ade3399bd