Golang标准库time（1） - 程序员需要相信的关于时间的谎言 时间可以倒流

func Now() Time {
	sec, nsec, mono := now()
	mono -= startNano
	sec += unixToInternal - minWall
	if uint64(sec)>>33 != 0 {
		return Time{uint64(nsec), sec + minWall, Local}
	}
	return Time{hasMonotonic | uint64(sec)<<nsecShift | uint64(nsec), mono, Local}
}

//go:linkname time_now time.now
func time_now() (sec int64, nsec int32, mono int64) {
	sec, nsec = walltime()
	return sec, nsec, nanotime()
}

// func walltime() (sec int64, nsec int32)
TEXT runtime·walltime(SB),NOSPLIT,$24-12
	MOVD	RSP, R20	// R20 is unchanged by C code
	MOVD	RSP, R1

	MOVD	g_m(g), R21	// R21 = m

	// Set vdsoPC and vdsoSP for SIGPROF traceback.
	// Save the old values on stack and restore them on exit,
	// so this function is reentrant.
	MOVD	m_vdsoPC(R21), R2
	MOVD	m_vdsoSP(R21), R3
	MOVD	R2, 8(RSP)
	MOVD	R3, 16(RSP)

	MOVD	$ret-8(FP), R2 // caller's SP
	MOVD	LR, m_vdsoPC(R21)
	MOVD	R2, m_vdsoSP(R21)

	MOVD	m_curg(R21), R0
	CMP	g, R0
	BNE	noswitch

	MOVD	m_g0(R21), R3
	MOVD	(g_sched+gobuf_sp)(R3), R1	// Set RSP to g0 stack

noswitch:
	SUB	$16, R1
	BIC	$15, R1	// Align for C code
	MOVD	R1, RSP

	MOVW	$CLOCK_REALTIME, R0
	MOVD	runtime·vdsoClockgettimeSym(SB), R2
	CBZ	R2, fallback

	// Store g on gsignal's stack, so if we receive a signal
	// during VDSO code we can find the g.
	// If we don't have a signal stack, we won't receive signal,
	// so don't bother saving g.
	// When using cgo, we already saved g on TLS, also don't save
	// g here.
	// Also don't save g if we are already on the signal stack.
	// We won't get a nested signal.
	MOVBU	runtime·iscgo(SB), R22
	CBNZ	R22, nosaveg
	MOVD	m_gsignal(R21), R22          // g.m.gsignal
	CBZ	R22, nosaveg
	CMP	g, R22
	BEQ	nosaveg
	MOVD	(g_stack+stack_lo)(R22), R22 // g.m.gsignal.stack.lo
	MOVD	g, (R22)

	BL	(R2)

	MOVD	ZR, (R22)  // clear g slot, R22 is unchanged by C code

	B	finish

nosaveg:
	BL	(R2)
	B	finish

fallback:
	MOVD	$SYS_clock_gettime, R8
	SVC

finish:
	MOVD	0(RSP), R3	// sec
	MOVD	8(RSP), R5	// nsec

	MOVD	R20, RSP	// restore SP
	// Restore vdsoPC, vdsoSP
	// We don't worry about being signaled between the two stores.
	// If we are not in a signal handler, we'll restore vdsoSP to 0,
	// and no one will care about vdsoPC. If we are in a signal handler,
	// we cannot receive another signal.
	MOVD	16(RSP), R1
	MOVD	R1, m_vdsoSP(R21)
	MOVD	8(RSP), R1
	MOVD	R1, m_vdsoPC(R21)

	MOVD	R3, sec+0(FP)
	MOVW	R5, nsec+8(FP)
	RET

const (
	hasMonotonic = 1 << 63
	maxWall      = wallToInternal + (1<<33 - 1) // year 2157
	minWall      = wallToInternal               // year 1885
	nsecMask     = 1<<30 - 1
	nsecShift    = 30
)

if uint64(sec)>>33 != 0 {
	return Time{uint64(nsec), sec + minWall, Local}
}

type Time struct {
	wall uint64
	ext  int64
	loc *Location
}

return Time{hasMonotonic | uint64(sec)<<nsecShift | uint64(nsec), mono, Local}

func (t Time) Sub(u Time) Duration {
	if t.wall&u.wall&hasMonotonic != 0 {
		te := t.ext
		ue := u.ext
		d := Duration(te - ue)
		if d < 0 && te > ue {
			return maxDuration // t - u is positive out of range
		}
		if d > 0 && te < ue {
			return minDuration // t - u is negative out of range
		}
		return d
	}
	d := Duration(t.sec()-u.sec())*Second + Duration(t.nsec()-u.nsec())
	// Check for overflow or underflow.
	switch {
	case u.Add(d).Equal(t):
		return d // d is correct
	case t.Before(u):
		return minDuration // t - u is negative out of range
	default:
		return maxDuration // t - u is positive out of range
	}
}

func (t *Time) addSec(d int64) {
	if t.wall&hasMonotonic != 0 {
		sec := int64(t.wall << 1 >> (nsecShift + 1))
		dsec := sec + d
		if 0 <= dsec && dsec <= 1<<33-1 {
			t.wall = t.wall&nsecMask | uint64(dsec)<<nsecShift | hasMonotonic
			return
		}
		// Wall second now out of range for packed field.
		// Move to ext.
		t.stripMono()
	}

	// Check if the sum of t.ext and d overflows and handle it properly.
	sum := t.ext + d
	if (sum > t.ext) == (d > 0) {
		t.ext = sum
	} else if d > 0 {
		t.ext = 1<<63 - 1
	} else {
		t.ext = -(1<<63 - 1)
	}
}

func (t Time) After(u Time) bool {
	if t.wall&u.wall&hasMonotonic != 0 {
		return t.ext > u.ext
	}
	ts := t.sec()
	us := u.sec()
	return ts > us || ts == us && t.nsec() > u.nsec()
}

func (t Time) Before(u Time) bool {
	if t.wall&u.wall&hasMonotonic != 0 {
		return t.ext < u.ext
	}
	ts := t.sec()
	us := u.sec()
	return ts < us || ts == us && t.nsec() < u.nsec()
}

func (t Time) Equal(u Time) bool {
	if t.wall&u.wall&hasMonotonic != 0 {
		return t.ext == u.ext
	}
	return t.sec() == u.sec() && t.nsec() == u.nsec()
}