// stack ID for it.
//
// It then returns a traceArg representing that stack which may be
// passed to write.
func (tl traceLocker) startPC(pc uintptr) traceArg {
	// +PCQuantum because makeTraceFrame expects return PCs and subtracts PCQuantum.
	return traceArg(trace.stackTab[tl.gen%2].put([]uintptr{
		logicalStackSentinel,
		startPCForTrace(pc) + sys.PCQuantum,
	}))
}