If go chooses to inline a function in general, then it doesn’t need to add the prologue to the inlined code, no. The prologue applies to all functions that remain after the inlining is done.
There’s also functions that can be marked as “nosplit” that skip the prologue as well.
But otherwise, it has to be in every function because you might be 1 byte away from the top of go’s (small) stack size, then you call that simple add function, and if the prologue isn’t run the stack will overflow. Go has tiny stacks by default that grow if they need to, with this prologue functioning as the “do I need to split/grow the stack?” check, so it needs to be every function that does it. The scheduler hook is just a single branch that’s part of the prologue, so it’s not that much more expensive if you’re doing the prologue anyway.
