if traceEnabled() {
		throw("traceInitReadCPU called with trace enabled")
	}
	// Create new profBuf for CPU samples that will be emitted as events.
	// Format: after the timestamp, header is [pp.id, gp.goid, mp.procid].
	trace.cpuLogRead[0] = newProfBuf(3, profBufWordCount, profBufTagCount)
	trace.cpuLogRead[1] = newProfBuf(3, profBufWordCount, profBufTagCount)
	// We must not acquire trace.signalLock outside of a signal handler: a

	if gp.lockedm == 0 {
		return nil, plainError("goroutine not locked to thread")
	}

	tid := int(gp.lockedm.ptr().procid)
	if tid == 0 {
		return nil, plainError("missing tid")
	}

	f := efaceOf(&fn)
	if f._type == nil || f._type.Kind_&abi.KindMask != abi.Func {
		return nil, plainError("fn must be a function")
	}

		ts = w.traceBuf.lastTime + 1
	}
	w.traceBuf.lastTime = ts
	w.traceBuf.link = nil
	w.traceBuf.pos = 0

	// Tolerate a nil mp.
	mID := ^uint64(0)
	if w.mp != nil {
		mID = uint64(w.mp.procid)
	}

	// Write the buffer's header.
	if exp == traceNoExperiment {
		w.byte(byte(traceEvEventBatch))
	} else {
		w.byte(byte(traceEvExperimentalBatch))
		w.byte(byte(exp))
	}

			go121Events: convertOldFormat(tr),
		}, nil
	case version.Go122, version.Go123:
		return &Reader{
			r: br,
			order: ordering{
				mStates:     make(map[ThreadID]*mState),
				pStates:     make(map[ProcID]*pState),
				gStates:     make(map[GoID]*gState),
				activeTasks: make(map[TaskID]taskState),
			},
			// Don't emit a sync event when we first go to emit events.
			emittedSync: true,
		}, nil
	default:

		w = w.writeProcStatus(uint64(pp.id), procStatus, pp.trace.inSweep)
	}
	if gp := tl.mp.curg; gp != nil && !gp.trace.statusWasTraced(tl.gen) && gp.trace.acquireStatus(tl.gen) {
		w = w.writeGoStatus(uint64(gp.goid), int64(tl.mp.procid), goStatus, gp.inMarkAssist, 0 /* no stack */)
	}
	return traceEventWriter{w}
}

// commit writes out a trace event and calls end. It's a helper to make the

	case "gstatus":
		u = uint64(arg.(go122.GoStatus))
	case "g":
		u = uint64(arg.(trace.GoID))
	case "m":
		u = uint64(arg.(trace.ThreadID))
	case "p":
		u = uint64(arg.(trace.ProcID))
	case "string":
		u = b.gen.String(arg.(string))
	case "task":
		u = uint64(arg.(trace.TaskID))
	default:
		panic(fmt.Sprintf("unsupported arg type %q for spec %q", typStr, argSpec))
	}
	return u
}