Allowing developer control over calling conventions is also simultaneous with disallowing optimization in the case that Function A calls Function B calls Function C calls Function D etc. but along the way one or more of those functions could have their arguments swapped around to a different convention to reduce overhead. What semantics would preserve such an optimization but allow control? Would it just be illusory?
And in practice assembly has the performance disadvantage of not being subject to most compiler optimizations, often including "introspecting on its operation, determining it is fully redundant, and eliminating it entirely". It's not the 1990s anymore.
In the cases where that kind of optimization is not even possible to consider, though, the only place I'd expect inline assembly to be decisively beaten is using profile-guided optimization. That's the only way to extract more information than "perfect awareness of how the application code works", which the app dev has and the compiler dev does not. The call overhead can be eliminated by simply writing more assembly until you've covered the relevant hot boundaries.
And in practice assembly has the performance disadvantage of not being subject to most compiler optimizations, often including "introspecting on its operation, determining it is fully redundant, and eliminating it entirely". It's not the 1990s anymore.
In the cases where that kind of optimization is not even possible to consider, though, the only place I'd expect inline assembly to be decisively beaten is using profile-guided optimization. That's the only way to extract more information than "perfect awareness of how the application code works", which the app dev has and the compiler dev does not. The call overhead can be eliminated by simply writing more assembly until you've covered the relevant hot boundaries.