Why does this simple C program depend on the GCC optimization level?

Question

I came across this surprising tidbit while translating jonesforth.s from x86 ASM to C with GCC extensions. depends_on_optimization.c is a "simple" C program which depends on whether it's compiled with (any -O) or without (-O0) optimizations. It's entirely possible that the program invokes Undefined Behavior as it's using _start(void) instead of int main(int argc, char *argv[]) then poking around the C stack to find argc. That's an artefact of translating ASM to C.

#include <stdlib.h>
#include <unistd.h>

#define NEXT goto **ip++
#ifdef __OPTIMIZE__
#define OFFSET 3
#else
#define OFFSET 0
#endif

void _start()
{
    intptr_t *stack[1024];
    register intptr_t **sp = stack + (sizeof stack / sizeof *stack), *p;
    register void **ip;

    goto _start;

ADD:
    p = *sp++;
    sp[0] += (intptr_t)p;
    NEXT;
LIT:
    *--sp = (intptr_t *)*ip++;
    NEXT;
S0:
    *--sp = (intptr_t *)(stack + (sizeof stack / sizeof *stack));
    NEXT;
EXIT:
    p = *sp++;
    exit(*p);

    static void *code[] = {&&S0, &&LIT, (void *)OFFSET, &&ADD, &&EXIT};
_start:
    ip = code;
    NEXT;
}

What compiler flag should I use to make GCC produce the same stack layout regardless of optimization level? I tried various combinations of -fno-stack-check, -fno-stack-protector, and -fno-stack-clash-protection to no avail.