when i explore the mutex lock function implemtion in go .
func (m *Mutex) Lock()
I find some function called in this method which without function body .
func runtime_canSpin(i int) bool // in sync/runtime.go
func runtime_doSpin()
I want to find the code and login with these method . But i only can find something like this in https://golang.org/ref/spec#Function_declarations
A function declaration may omit the body. Such a declaration provides the signature for a function implemented outside Go, such as an assembly routine.
what can i do to find the code hidden?