time: optimize Now on darwin, windows

Fetch both monotonic and wall time together when possible.
Avoids skew and is cheaper.

Also shave a few ns off in conversion in package time.

Compared to current implementation (after monotonic changes):

name   old time/op  new time/op  delta
Now    19.6ns ± 1%   9.7ns ± 1%  -50.63%  (p=0.000 n=41+49) darwin/amd64
Now    23.5ns ± 4%  10.6ns ± 5%  -54.61%  (p=0.000 n=30+28) windows/amd64
Now    54.5ns ± 5%  29.8ns ± 9%  -45.40%  (p=0.000 n=27+29) windows/386

More importantly, compared to Go 1.8:

name   old time/op  new time/op  delta
Now     9.5ns ± 1%   9.7ns ± 1%   +1.94%  (p=0.000 n=41+49) darwin/amd64
Now    12.9ns ± 5%  10.6ns ± 5%  -17.73%  (p=0.000 n=30+28) windows/amd64
Now    15.3ns ± 5%  29.8ns ± 9%  +94.36%  (p=0.000 n=30+29) windows/386

This brings time.Now back in line with Go 1.8 on darwin/amd64 and windows/amd64.

It's not obvious why windows/386 is still noticeably worse than Go 1.8,
but it's better than before this CL. The windows/386 speed is not too
important; the changes just keep the two architectures similar.

Change-Id: If69b94970c8a1a57910a371ee91e0d4e82e46c5d
Reviewed-on: https://go-review.googlesource.com/36428
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/heapdump.go

@@ -548,7 +548,7 @@ func dumpmemstats() {
 	dumpint(memstats.gc_sys)
 	dumpint(memstats.other_sys)
 	dumpint(memstats.next_gc)
-	dumpint(memstats.last_gc)
+	dumpint(memstats.last_gc_unix)
 	dumpint(memstats.pause_total_ns)
 	for i := 0; i < 256; i++ {
 		dumpint(memstats.pause_ns[i])

src/runtime/mgc.go

@@ -1291,10 +1291,13 @@ func gcMarkTermination() {
 	}
 
 	// Update timing memstats
-	now, unixNow := nanotime(), unixnanotime()
+	now := nanotime()
+	sec, nsec, _ := time_now()
+	unixNow := sec*1e9 + int64(nsec)
 	work.pauseNS += now - work.pauseStart
 	work.tEnd = now
-	atomic.Store64(&memstats.last_gc, uint64(unixNow)) // must be Unix time to make sense to user
+	atomic.Store64(&memstats.last_gc_unix, uint64(unixNow)) // must be Unix time to make sense to user
+	atomic.Store64(&memstats.last_gc_nanotime, uint64(now)) // monotonic time for us
 	memstats.pause_ns[memstats.numgc%uint32(len(memstats.pause_ns))] = uint64(work.pauseNS)
 	memstats.pause_end[memstats.numgc%uint32(len(memstats.pause_end))] = uint64(unixNow)
 	memstats.pause_total_ns += uint64(work.pauseNS)

src/runtime/mstats.go

@@ -72,7 +72,7 @@ type mstats struct {
 	// Statistics about garbage collector.
 	// Protected by mheap or stopping the world during GC.
 	next_gc         uint64 // goal heap_live for when next GC ends; ^0 if disabled
-	last_gc         uint64 // last gc (in absolute time)
+	last_gc_unix    uint64 // last gc (in unix time)
 	pause_total_ns  uint64
 	pause_ns        [256]uint64 // circular buffer of recent gc pause lengths
 	pause_end       [256]uint64 // circular buffer of recent gc end times (nanoseconds since 1970)
@@ -92,6 +92,7 @@ type mstats struct {
 
 	// Statistics below here are not exported to MemStats directly.
 
+	last_gc_nanotime uint64 // last gc (monotonic time)
 	tinyallocs       uint64 // number of tiny allocations that didn't cause actual allocation; not exported to go directly
 
 	// gc_trigger is the heap size that triggers marking.
@@ -497,7 +498,7 @@ func readGCStats_m(pauses *[]uint64) {
 		p[n+i] = memstats.pause_end[j]
 	}
 
-	p[n+n] = memstats.last_gc
+	p[n+n] = memstats.last_gc_unix
 	p[n+n+1] = uint64(memstats.numgc)
 	p[n+n+2] = memstats.pause_total_ns
 	unlock(&mheap_.lock)

src/runtime/os_windows.go

@@ -578,50 +578,7 @@ func unminit() {
 	*tp = 0
 }
 
-// Described in http://www.dcl.hpi.uni-potsdam.de/research/WRK/2007/08/getting-os-information-the-kuser_shared_data-structure/
-type _KSYSTEM_TIME struct {
-	LowPart   uint32
-	High1Time int32
-	High2Time int32
-}
-
-const (
-	_INTERRUPT_TIME = 0x7ffe0008
-	_SYSTEM_TIME    = 0x7ffe0014
-)
-
-//go:nosplit
-func systime(addr uintptr) int64 {
-	timeaddr := (*_KSYSTEM_TIME)(unsafe.Pointer(addr))
-
-	var t _KSYSTEM_TIME
-	for i := 1; i < 10000; i++ {
-		// these fields must be read in that order (see URL above)
-		t.High1Time = timeaddr.High1Time
-		t.LowPart = timeaddr.LowPart
-		t.High2Time = timeaddr.High2Time
-		if t.High1Time == t.High2Time {
-			return int64(t.High1Time)<<32 | int64(t.LowPart)
-		}
-		if (i % 100) == 0 {
-			osyield()
-		}
-	}
-	systemstack(func() {
-		throw("interrupt/system time is changing too fast")
-	})
-	return 0
-}
-
-//go:nosplit
-func unixnano() int64 {
-	return (systime(_SYSTEM_TIME) - 116444736000000000) * 100
-}
-
-//go:nosplit
-func nanotime() int64 {
-	return systime(_INTERRUPT_TIME) * 100
-}
+func nanotime() int64
 
 // Calling stdcall on os stack.
 // May run during STW, so write barriers are not allowed.

src/runtime/proc.go

@@ -3818,7 +3818,6 @@ func sysmon() {
 		// poll network if not polled for more than 10ms
 		lastpoll := int64(atomic.Load64(&sched.lastpoll))
 		now := nanotime()
-		unixnow := unixnanotime()
 		if lastpoll != 0 && lastpoll+10*1000*1000 < now {
 			atomic.Cas64(&sched.lastpoll, uint64(lastpoll), uint64(now))
 			gp := netpoll(false) // non-blocking - returns list of goroutines
@@ -3843,8 +3842,8 @@ func sysmon() {
 			idle++
 		}
 		// check if we need to force a GC
-		lastgc := int64(atomic.Load64(&memstats.last_gc))
-		if gcphase == _GCoff && lastgc != 0 && unixnow-lastgc > forcegcperiod && atomic.Load(&forcegc.idle) != 0 {
+		lastgc := int64(atomic.Load64(&memstats.last_gc_nanotime))
+		if gcphase == _GCoff && lastgc != 0 && now-lastgc > forcegcperiod && atomic.Load(&forcegc.idle) != 0 {
 			lock(&forcegc.lock)
 			forcegc.idle = 0
 			forcegc.g.schedlink = 0

src/runtime/stubs.go

@@ -241,8 +241,6 @@ func stackBarrier()
 // in asm_*.s
 func return0()
 
-func walltime() (sec int64, nsec int32)
-
 // in asm_*.s
 // not called directly; definitions here supply type information for traceback.
 func call32(typ, fn, arg unsafe.Pointer, n, retoffset uint32)
@@ -279,11 +277,6 @@ func prefetcht1(addr uintptr)
 func prefetcht2(addr uintptr)
 func prefetchnta(addr uintptr)
 
-func unixnanotime() int64 {
-	sec, nsec := walltime()
-	return sec*1e9 + int64(nsec)
-}
-
 // round n up to a multiple of a.  a must be a power of 2.
 func round(n, a uintptr) uintptr {
 	return (n + a - 1) &^ (a - 1)

src/runtime/sys_darwin_386.s

@@ -114,6 +114,16 @@ TEXT runtime·setitimer(SB),NOSPLIT,$0
 // 64-bit unix nanoseconds returned in DX:AX.
 // I'd much rather write this in C but we need
 // assembly for the 96-bit multiply and RDTSC.
+//
+// Note that we could arrange to return monotonic time here
+// as well, but we don't bother, for two reasons:
+// 1. macOS only supports 64-bit systems, so no one should
+// be using the 32-bit code in production.
+// This code is only maintained to make it easier for developers
+// using Macs to test the 32-bit compiler.
+// 2. On some (probably now unsupported) CPUs,
+// the code falls back to the system call always,
+// so it can't even use the comm page at all. 
 TEXT runtime·now(SB),NOSPLIT,$40
 	MOVL	$0xffff0000, BP /* comm page base */
 	
@@ -217,9 +227,15 @@ inreg:
 	ADCL	$0, DX
 	RET
 
-// func walltime() (sec int64, nsec int32)
-TEXT runtime·walltime(SB),NOSPLIT,$0
+// func now() (sec int64, nsec int32, mono uint64)
+TEXT time·now(SB),NOSPLIT,$0-20
 	CALL	runtime·now(SB)
+	MOVL	AX, BX
+	MOVL	DX, BP
+	SUBL	runtime·startNano(SB), BX
+	SBBL	runtime·startNano+4(SB), BP
+	MOVL	BX, mono+12(FP)
+	MOVL	BP, mono+16(FP)
 	MOVL	$1000000000, CX
 	DIVL	CX
 	MOVL	AX, sec+0(FP)
@@ -230,6 +246,8 @@ TEXT runtime·walltime(SB),NOSPLIT,$0
 // func nanotime() int64
 TEXT runtime·nanotime(SB),NOSPLIT,$0
 	CALL	runtime·now(SB)
+	SUBL	runtime·startNano(SB), AX
+	SBBL	runtime·startNano+4(SB), DX
 	MOVL	AX, ret_lo+0(FP)
 	MOVL	DX, ret_hi+4(FP)
 	RET

src/runtime/sys_darwin_amd64.s

@@ -117,35 +117,44 @@ TEXT runtime·madvise(SB), NOSPLIT, $0
 #define	gtod_ns_base	0x70
 #define	gtod_sec_base	0x78
 
-TEXT monotonictime<>(SB), NOSPLIT, $32
-	MOVQ $0x7fffffe00000, SI // comm page base
-
+TEXT runtime·nanotime(SB),NOSPLIT,$0-8
+	MOVQ	$0x7fffffe00000, BP	/* comm page base */
+	// Loop trying to take a consistent snapshot
+	// of the time parameters.
 timeloop:
-	MOVL  nt_generation(SI), R8
-	TESTL R8, R8
+	MOVL	nt_generation(BP), R9
+	TESTL	R9, R9
 	JZ	timeloop
 	RDTSC
+	MOVQ	nt_tsc_base(BP), R10
+	MOVL	nt_scale(BP), R11
+	MOVQ	nt_ns_base(BP), R12
+	CMPL	nt_generation(BP), R9
+	JNE	timeloop
+
+	// Gathered all the data we need. Compute monotonic time:
+	//	((tsc - nt_tsc_base) * nt_scale) >> 32 + nt_ns_base
+	// The multiply and shift extracts the top 64 bits of the 96-bit product.
 	SHLQ	$32, DX
-	ORQ   DX, AX
-	MOVL nt_shift(SI), CX
-	SUBQ nt_tsc_base(SI), AX
-	SHLQ CX, AX
-	MOVL nt_scale(SI), CX
-	MULQ CX
+	ADDQ	DX, AX
+	SUBQ	R10, AX
+	MULQ	R11
 	SHRQ	$32, AX:DX
-	ADDQ nt_ns_base(SI), AX
-	CMPL nt_generation(SI), R8
-	JNE  timeloop
+	ADDQ	R12, AX
+	MOVQ	runtime·startNano(SB), CX
+	SUBQ	CX, AX
+	MOVQ	AX, ret+0(FP)
 	RET
 
-TEXT nanotime<>(SB), NOSPLIT, $32
+TEXT time·now(SB), NOSPLIT, $32-24
+	// Note: The 32 bytes of stack frame requested on the TEXT line
+	// are used in the systime fallback, as the timeval address
+	// filled in by the system call.
 	MOVQ	$0x7fffffe00000, BP	/* comm page base */
 	// Loop trying to take a consistent snapshot
 	// of the time parameters.
 timeloop:
 	MOVL	gtod_generation(BP), R8
-	TESTL	R8, R8
-	JZ	systime
 	MOVL	nt_generation(BP), R9
 	TESTL	R9, R9
 	JZ	timeloop
@@ -160,8 +169,8 @@ timeloop:
 	CMPL	gtod_generation(BP), R8
 	JNE	timeloop
 
-	// Gathered all the data we need. Compute time.
-	//	((tsc - nt_tsc_base) * nt_scale) >> 32 + nt_ns_base - gtod_ns_base + gtod_sec_base*1e9
+	// Gathered all the data we need. Compute:
+	//	monotonic_time = ((tsc - nt_tsc_base) * nt_scale) >> 32 + nt_ns_base
 	// The multiply and shift extracts the top 64 bits of the 96-bit product.
 	SHLQ	$32, DX
 	ADDQ	DX, AX
@@ -169,9 +178,33 @@ timeloop:
 	MULQ	R11
 	SHRQ	$32, AX:DX
 	ADDQ	R12, AX
+	MOVQ	AX, BX
+	MOVQ	runtime·startNano(SB), CX
+	SUBQ	CX, BX
+	MOVQ	BX, monotonic+16(FP)
+
+	// Compute:
+	//	wall_time = monotonic time - gtod_ns_base + gtod_sec_base*1e9
+	// or, if gtod_generation==0, invoke the system call.
+	TESTL	R8, R8
+	JZ	systime
 	SUBQ	R13, AX
 	IMULQ	$1000000000, R14
 	ADDQ	R14, AX
+
+	// Split wall time into sec, nsec.
+	// generated code for
+	//	func f(x uint64) (uint64, uint64) { return x/1e9, x%1e9 }
+	// adapted to reduce duplication
+	MOVQ	AX, CX
+	SHRQ	$9, AX
+	MOVQ	$19342813113834067, DX
+	MULQ	DX
+	SHRQ	$11, DX
+	MOVQ	DX, sec+0(FP)
+	IMULQ	$1000000000, DX
+	SUBQ	DX, CX
+	MOVL	CX, nsec+8(FP)
 	RET
 
 systime:
@@ -187,34 +220,9 @@ systime:
 	MOVL	8(SP), DX
 inreg:
 	// sec is in AX, usec in DX
-	// return nsec in AX
-	IMULQ	$1000000000, AX
 	IMULQ	$1000, DX
-	ADDQ	DX, AX
-	RET
-
-TEXT runtime·nanotime(SB),NOSPLIT,$0-8
-	CALL	monotonictime<>(SB)
-	MOVQ	AX, ret+0(FP)
-	RET
-
-// func walltime() (sec int64, nsec int32)
-TEXT runtime·walltime(SB),NOSPLIT,$0-12
-	CALL	nanotime<>(SB)
-
-	// generated code for
-	//	func f(x uint64) (uint64, uint64) { return x/1000000000, x%100000000 }
-	// adapted to reduce duplication
-	MOVQ	AX, CX
-	MOVQ	$1360296554856532783, AX
-	MULQ	CX
-	ADDQ	CX, DX
-	RCRQ	$1, DX
-	SHRQ	$29, DX
-	MOVQ	DX, sec+0(FP)
-	IMULQ	$1000000000, DX
-	SUBQ	DX, CX
-	MOVL	CX, nsec+8(FP)
+	MOVQ	AX, sec+0(FP)
+	MOVL	DX, nsec+8(FP)
 	RET
 
 TEXT runtime·sigprocmask(SB),NOSPLIT,$0

src/runtime/sys_windows_386.s

@@ -432,15 +432,103 @@ TEXT runtime·switchtothread(SB),NOSPLIT,$0
 	MOVL	BP, SP
 	RET
 
-// func walltime() (sec int64, nsec int32)
-TEXT runtime·walltime(SB),NOSPLIT,$8-12
-	CALL	runtime·unixnano(SB)
-	MOVL	0(SP), AX
-	MOVL	4(SP), DX
+// See http://www.dcl.hpi.uni-potsdam.de/research/WRK/2007/08/getting-os-information-the-kuser_shared_data-structure/
+// Must read hi1, then lo, then hi2. The snapshot is valid if hi1 == hi2.
+#define _INTERRUPT_TIME 0x7ffe0008
+#define _SYSTEM_TIME 0x7ffe0014
+#define time_lo 0
+#define time_hi1 4
+#define time_hi2 8
+
+TEXT runtime·nanotime(SB),NOSPLIT,$0-8
+loop:
+	MOVL	(_INTERRUPT_TIME+time_hi1), AX
+	MOVL	(_INTERRUPT_TIME+time_lo), CX
+	MOVL	(_INTERRUPT_TIME+time_hi2), DI
+	CMPL	AX, DI
+	JNE	loop
+
+	// wintime = DI:CX, multiply by 100
+	MOVL	$100, AX
+	MULL	CX
+	IMULL	$100, DI
+	ADDL	DI, DX
+	// wintime*100 = DX:AX, subtract startNano and return
+	SUBL	runtime·startNano+0(SB), AX
+	SBBL runtime·startNano+4(SB), DX
+	MOVL	AX, ret+0(FP)
+	MOVL	DX, ret+4(FP)
+	RET
 
+TEXT time·now(SB),NOSPLIT,$0-20
+loop:
+	MOVL	(_INTERRUPT_TIME+time_hi1), AX
+	MOVL	(_INTERRUPT_TIME+time_lo), CX
+	MOVL	(_INTERRUPT_TIME+time_hi2), DI
+	CMPL	AX, DI
+	JNE	loop
+
+	// w = DI:CX
+	// multiply by 100
+	MOVL	$100, AX
+	MULL	CX
+	IMULL	$100, DI
+	ADDL	DI, DX
+	// w*100 = DX:AX
+	// subtract startNano and save for return
+	SUBL	runtime·startNano+0(SB), AX
+	SBBL runtime·startNano+4(SB), DX
+	MOVL	AX, mono+12(FP)
+	MOVL	DX, mono+16(FP)
+
+wall:
+	MOVL	(_SYSTEM_TIME+time_hi1), CX
+	MOVL	(_SYSTEM_TIME+time_lo), AX
+	MOVL	(_SYSTEM_TIME+time_hi2), DX
+	CMPL	CX, DX
+	JNE	wall
+	
+	// w = DX:AX
+	// convert to Unix epoch (but still 100ns units)
+	#define delta 116444736000000000
+	SUBL	$(delta & 0xFFFFFFFF), AX
+	SBBL $(delta >> 32), DX
+	
+	// nano/100 = DX:AX
+	// split into two decimal halves by div 1e9.
+	// (decimal point is two spots over from correct place,
+	// but we avoid overflow in the high word.)
 	MOVL	$1000000000, CX
 	DIVL	CX
+	MOVL	AX, DI
+	MOVL	DX, SI
+	
+	// DI = nano/100/1e9 = nano/1e11 = sec/100, DX = SI = nano/100%1e9
+	// split DX into seconds and nanoseconds by div 1e7 magic multiply.
+	MOVL	DX, AX
+	MOVL	$1801439851, CX
+	MULL	CX
+	SHRL	$22, DX
+	MOVL	DX, BX
+	IMULL	$10000000, DX
+	MOVL	SI, CX
+	SUBL	DX, CX
+	
+	// DI = sec/100 (still)
+	// BX = (nano/100%1e9)/1e7 = (nano/1e9)%100 = sec%100
+	// CX = (nano/100%1e9)%1e7 = (nano%1e9)/100 = nsec/100
+	// store nsec for return
+	IMULL	$100, CX
+	MOVL	CX, nsec+8(FP)
+
+	// DI = sec/100 (still)
+	// BX = sec%100
+	// construct DX:AX = 64-bit sec and store for return
+	MOVL	$0, DX
+	MOVL	$100, AX
+	MULL	DI
+	ADDL	BX, AX
+	ADCL	$0, DX
 	MOVL	AX, sec+0(FP)
-	MOVL	$0, sec+4(FP)
-	MOVL	DX, nsec+8(FP)
+	MOVL	DX, sec+4(FP)
 	RET

src/runtime/sys_windows_amd64.s

@@ -465,10 +465,55 @@ TEXT runtime·switchtothread(SB),NOSPLIT|NOFRAME,$0
 	MOVQ	32(SP), SP
 	RET
 
-// func walltime() (sec int64, nsec int32)
-TEXT runtime·walltime(SB),NOSPLIT,$8-12
-	CALL	runtime·unixnano(SB)
-	MOVQ	0(SP), AX
+// See http://www.dcl.hpi.uni-potsdam.de/research/WRK/2007/08/getting-os-information-the-kuser_shared_data-structure/
+// Must read hi1, then lo, then hi2. The snapshot is valid if hi1 == hi2.
+#define _INTERRUPT_TIME 0x7ffe0008
+#define _SYSTEM_TIME 0x7ffe0014
+#define time_lo 0
+#define time_hi1 4
+#define time_hi2 8
+
+TEXT runtime·nanotime(SB),NOSPLIT,$0-8
+	MOVQ	$_INTERRUPT_TIME, DI
+loop:
+	MOVL	time_hi1(DI), AX
+	MOVL	time_lo(DI), BX
+	MOVL	time_hi2(DI), CX
+	CMPL	AX, CX
+	JNE	loop
+	SHLQ	$32, CX
+	ORQ	BX, CX
+	IMULQ	$100, CX
+	SUBQ	runtime·startNano(SB), CX
+	MOVQ	CX, ret+0(FP)
+	RET
+
+TEXT time·now(SB),NOSPLIT,$0-24
+	MOVQ	$_INTERRUPT_TIME, DI
+loop:
+	MOVL	time_hi1(DI), AX
+	MOVL	time_lo(DI), BX
+	MOVL	time_hi2(DI), CX
+	CMPL	AX, CX
+	JNE	loop
+	SHLQ	$32, AX
+	ORQ	BX, AX
+	IMULQ	$100, AX
+	SUBQ	runtime·startNano(SB), AX
+	MOVQ	AX, mono+16(FP)
+
+	MOVQ	$_SYSTEM_TIME, DI
+wall:
+	MOVL	time_hi1(DI), AX
+	MOVL	time_lo(DI), BX
+	MOVL	time_hi2(DI), CX
+	CMPL	AX, CX
+	JNE	wall
+	SHLQ	$32, AX
+	ORQ	BX, AX
+	MOVQ	$116444736000000000, DI
+	SUBQ	DI, AX
+	IMULQ	$100, AX
 
 	// generated code for
 	//	func f(x uint64) (uint64, uint64) { return x/1000000000, x%100000000 }
@@ -484,4 +529,3 @@ TEXT runtime·walltime(SB),NOSPLIT,$8-12
 	SUBQ	DX, CX
 	MOVL	CX, nsec+8(FP)
 	RET
-

src/runtime/time.go

@@ -302,10 +302,4 @@ func time_runtimeNano() int64 {
 	return nanotime()
 }
 
-var startNano = nanotime()
-
-//go:linkname time_now time.now
-func time_now() (sec int64, nsec int32, mono uint64) {
-	sec, nsec = walltime()
-	return sec, nsec, uint64(nanotime() - startNano + 1)
-}
+var startNano int64 = nanotime()

src/runtime/timeasm.go

+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Declarations for operating systems implementing time.now directly in assembly.
+// Those systems are also expected to have nanotime subtract startNano,
+// so that time.now and nanotime return the same monotonic clock readings.
+
+// +build darwin,amd64 darwin,386 windows
+
+package runtime
+
+import _ "unsafe"
+
+//go:linkname time_now time.now
+func time_now() (sec int64, nsec int32, mono int64)

src/runtime/timestub.go

+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Declarations for operating systems implementing time.now
+// indirectly, in terms of walltime and nanotime assembly.
+
+// +build !darwin !amd64,!386
+// +build !windows
+
+package runtime
+
+import _ "unsafe" // for go:linkname
+
+func walltime() (sec int64, nsec int32)
+
+//go:linkname time_now time.now
+func time_now() (sec int64, nsec int32, mono int64) {
+	sec, nsec = walltime()
+	return sec, nsec, nanotime() - startNano
+}

src/time/mono_test.go

@@ -246,6 +246,9 @@ var monotonicStringTests = []struct {
 }
 
 func TestMonotonicString(t *testing.T) {
+	t1 := Now()
+	t.Logf("Now() = %v", t1)
+
 	for _, tt := range monotonicStringTests {
 		t1 := Now()
 		SetMono(&t1, tt.mono)

src/time/time.go

@@ -981,14 +981,16 @@ func daysIn(m Month, year int) int {
 }
 
 // Provided by package runtime.
-func now() (sec int64, nsec int32, mono uint64)
+func now() (sec int64, nsec int32, mono int64)
 
 // Now returns the current local time.
 func Now() Time {
 	sec, nsec, mono := now()
-	t := unixTime(sec, nsec)
-	t.setMono(int64(mono))
-	return t
+	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}
 }
 
 func unixTime(sec int64, nsec int32) Time {