Comment by JonChesterfield

If those functions are external you've lost that optimisation anyway. If they're not, the compiler chooses whether to ignore your annotation or not as usual. As is always the answer, the compiler doesn't get to make observable changes (unless you ask it to, fwrong-math style).

I'd like to specify things like extra live out registers, reduced clobber lists, pass everything on the stack - but on the function declaration or implementation, not having to special case it in the compiler itself.

Sufficiently smart programmers beat ahead of time compilers. Sufficiently smart ahead of time compilers beat programmers. If they're both sufficiently smart you get a common fix point. I claim that holds for a jit too, but note that it's just far more common for a compiler to rewrite the code at runtime than for a programmer to do so.

I'd say that assembly programmers are rather likely to cut out parts of the program that are redundant, and they do so with domain knowledge and guesswork that is difficult to encode in the compiler. Both sides are prone to error, with the classes of error somewhat overlapping.

I think compilers could be a lot better at codegen than they presently are, but the whole "programmers can't beat gcc anymore" idea isn't desperately true even with the current state of the art.

Mostly though I want control over calling conventions in the language instead of in compiler magic because it scales much better than teaching the compiler about properties of known functions. E.g. if I've written memcpy in asm, it shouldn't be stuck with the C caller save list, and avoiding that shouldn't involve a special case branch in the compiler backend.