Ah, that's because cmp_once doesn't yield a constant even when the inputs are constant. The reason that matters is there are some arrays in the kernel declared like

  int things[max(size1, size2)];

and there's been recently a push to remove all VLAs (arrays with non constant size) from the kernel. The -Wvla flag was warning about these arrays even though they're actually constant size, so something needed to be done.