runtime, time: refactor startNano handling

Move startNano from runtime to time package.
In preparation for a subsequent change that speeds up Since and Until.
This also makes code simpler as we have less assembly as the result,
monotonic time handling is better localized in time package.
This changes values returned from nanotime on windows
(it does not account for startNano anymore), current comments state
that it's important, but it's unclear how it can be important
since no other OS does this.

Update #25729

Change-Id: I2275d57b7b5ed8fd0d53eb0f19d55a86136cc555
Reviewed-on: https://go-review.googlesource.com/c/146340Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/proc.go

@@ -157,8 +157,7 @@ func main() {
 		}
 	}()
 
-	// Record when the world started. Must be after runtime_init
-	// because nanotime on some platforms depends on startNano.
+	// Record when the world started.
 	runtimeInitTime = nanotime()
 
 	gcenable()

src/runtime/sys_windows_386.s

@@ -455,9 +455,7 @@ loop:
 	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
+	// wintime*100 = DX:AX
 	MOVL	AX, ret_lo+0(FP)
 	MOVL	DX, ret_hi+4(FP)
 	RET
@@ -482,9 +480,6 @@ loop:
 	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)
 

src/runtime/sys_windows_amd64.s

@@ -486,7 +486,6 @@ loop:
 	SHLQ	$32, CX
 	ORQ	BX, CX
 	IMULQ	$100, CX
-	SUBQ	runtime·startNano(SB), CX
 	MOVQ	CX, ret+0(FP)
 	RET
 useQPC:
@@ -506,7 +505,6 @@ loop:
 	SHLQ	$32, AX
 	ORQ	BX, AX
 	IMULQ	$100, AX
-	SUBQ	runtime·startNano(SB), AX
 	MOVQ	AX, mono+16(FP)
 
 	MOVQ	$_SYSTEM_TIME, DI

src/runtime/sys_windows_arm.s

@@ -510,11 +510,7 @@ loop:
 	MULLU	R0, R2, (R4, R3)    // R4:R3 = R1:R0 * R2
 	MULA	R1, R2, R4, R4
 
-	// wintime*100 = R4:R3, subtract startNano and return
-	MOVW    runtime·startNano+0(SB), R0
-	MOVW    runtime·startNano+4(SB), R1
-	SUB.S   R0, R3
-	SBC	R1, R4
+	// wintime*100 = R4:R3
 	MOVW	R3, ret_lo+0(FP)
 	MOVW	R4, ret_hi+4(FP)
 	RET
@@ -540,11 +536,7 @@ loop:
 	MULLU	R0, R2, (R4, R3)    // R4:R3 = R1:R0 * R2
 	MULA	R1, R2, R4, R4
 
-	// wintime*100 = R4:R3, subtract startNano and return
-	MOVW    runtime·startNano+0(SB), R0
-	MOVW    runtime·startNano+4(SB), R1
-	SUB.S   R0, R3
-	SBC	R1, R4
+	// wintime*100 = R4:R3
 	MOVW	R3, mono+12(FP)
 	MOVW	R4, mono+16(FP)
 

src/runtime/time.go

@@ -470,11 +470,3 @@ func poll_runtimeNano() int64 {
 func time_runtimeNano() int64 {
 	return nanotime()
 }
-
-// Monotonic times are reported as offsets from startNano.
-// We initialize startNano to nanotime() - 1 so that on systems where
-// monotonic time resolution is fairly low (e.g. Windows 2008
-// which appears to have a default resolution of 15ms),
-// we avoid ever reporting a nanotime of 0.
-// (Callers may want to use 0 as "time not set".)
-var startNano int64 = nanotime() - 1

src/runtime/timeasm.go

@@ -3,8 +3,6 @@
 // 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 windows
 

src/runtime/timestub.go

@@ -14,5 +14,5 @@ import _ "unsafe" // for go:linkname
 //go:linkname time_now time.now
 func time_now() (sec int64, nsec int32, mono int64) {
 	sec, nsec = walltime()
-	return sec, nsec, nanotime() - startNano
+	return sec, nsec, nanotime()
 }

src/time/sleep.go

@@ -8,9 +8,6 @@ package time
 // A negative or zero duration causes Sleep to return immediately.
 func Sleep(d Duration)
 
-// runtimeNano returns the current value of the runtime clock in nanoseconds.
-func runtimeNano() int64
-
 // Interface to timers implemented in package runtime.
 // Must be in sync with ../runtime/time.go:/^type timer
 type runtimeTimer struct {

src/time/time.go

@@ -1050,9 +1050,21 @@ func daysIn(m Month, year int) int {
 // Provided by package runtime.
 func now() (sec int64, nsec int32, mono int64)
 
+// runtimeNano returns the current value of the runtime clock in nanoseconds.
+func runtimeNano() int64
+
+// Monotonic times are reported as offsets from startNano.
+// We initialize startNano to runtimeNano() - 1 so that on systems where
+// monotonic time resolution is fairly low (e.g. Windows 2008
+// which appears to have a default resolution of 15ms),
+// we avoid ever reporting a monotonic time of 0.
+// (Callers may want to use 0 as "time not set".)
+var startNano int64 = runtimeNano() - 1
+
 // Now returns the current local time.
 func Now() Time {
 	sec, nsec, mono := now()
+	mono -= startNano
 	sec += unixToInternal - minWall
 	if uint64(sec)>>33 != 0 {
 		return Time{uint64(nsec), sec + minWall, Local}