Comment by highfrequency

Could someone clarify the example where an unlikely fallback path is wrapped in a musttail?

My understanding is that musttail is useful because it prevents stack frame construction and register spilling; the calling function basically says "you can re-use the existing stack and not worry about my local variables."

But doesn't the stack frame construction overhead / register spilling only occur when the fallback path is actually invoked; therefore if it is unlikely and you care less about performance in this slow path it doesn't matter if you wrap the fallback path in musttail?

(Is this purely a branch prediction effect, where the possibility of extra work needing to be done slows down the fast path even when the work does not need to be done?)