为什么这个程序中的argc变量不能赋值给stack_top？

Question

我在 i386 中实现了一个模拟 load_arg 的函数。用VSCode调试的时候，发现在函数的倒数第三行：

*(size_t*)stack_top = argc;

这一行的最后，argc没有存入(size_t)stack_top，VSCode显示“optimized out”（如图）。

我该如何解决这个问题？或者如何将 argc 的值放入 (size_t)stack_top？

uint32_t load_arg(PD *pgdir, char *const argv[]) {
  char *stack_top = (char*)vm_walk(pgdir, USR_MEM - PGSIZE, 7) + PGSIZE;

  size_t argv_va[MAX_ARGS_NUM + 1];
  int argc;
  for (argc = 0; argv[argc]; ++argc) {
    assert(argc < MAX_ARGS_NUM);
    // push the string of argv[argc] to stack, record its va to argv_va[argc]
    stack_top -= (strlen(argv[argc])+1);
    strcpy(stack_top, argv[argc]);
    argv_va[argc] = USR_MEM - PGSIZE + ADDR2OFF(stack_top);
  }
  argv_va[argc] = 0; // set last argv NULL
  stack_top -= ADDR2OFF(stack_top) % 4; // align to 4 bytes
  for (int i = argc; i >= 0; --i) {
    // push the address of argv_va[argc] to stack to make argv array
    stack_top -= sizeof(size_t);
    *(size_t*)stack_top = argv_va[i];
  }

  // push the address of the argv array as argument for _start
  stack_top -= sizeof(size_t);
  *(size_t*)stack_top = USR_MEM - PGSIZE + ADDR2OFF(stack_top);

  //push argc as argument for _start
  stack_top -= sizeof(size_t);
  *(size_t*)stack_top = argc;
  stack_top -= sizeof(size_t); // a hole for return value (useless but necessary)
  return USR_MEM - PGSIZE + ADDR2OFF(stack_top);
}