Back when I was employed to work on a compiler, our approach to such things was to implement our run-time libraries in C and fix the compiler so it was able to

Back when I was employed to work on a compiler, our approach to such things was to implement our run-time libraries in C and fix the compiler so it was able to produce the desired output. In this case, though, the biggest differences aren't coming from C vs assembler but from walking away from platform conventions -- ABI, memory layout, CPU compatibility. It might be easier to write assembly than to teach the compiler how to generate code that does this, but that's largely because you don't need these optimisations in most cases. If the methods used provided a general speedup, it would be worth implementing them in compilers.