Function instack

lib/lua/src/ldebug.c:681–689

** Check whether pointer 'o' points to some value in the stack frame of ** the current function and, if so, returns its index. Because 'o' may ** not point to a value in this stack, we cannot compare it with the ** region boundaries (undefined behavior in ISO C). */

** region boundaries (undefined behavior in ISO C).
*/
static int instack (CallInfo *ci, const TValue *o) {
  int pos;
  StkId base = ci->func.p + 1;
  for (pos = 0; base + pos < ci->top.p; pos++) {
    if (o == s2v(base + pos))
      return pos;
  }
  return -1;  /* not found */
}


/*