push_simd/pop_simd exist and are called kernel_fpu_begin/kernel_fpu_end. Their use is practically prohibited in most areas and iiuc not available on all archs, but it's available if needed.
It's not so much that you can't ever use it, it's more a you really shouldn't. It's more expensive, harder to use and rarely worth it. Main users currently are crypto and raid checksumming.
