func gcallers(gp *g, skip int, pcbuf []uintptr) int {
	var u unwinder
	u.init(gp, unwindSilentErrors)
	return tracebackPCs(&u, skip, pcbuf)
}

// showframe reports whether the frame with the given characteristics should
// be printed during a traceback.
func showframe(sf srcFunc, gp *g, firstFrame bool, calleeID abi.FuncID) bool {
	mp := getg().m