runtime, time: don't use monotonic clock inside synctest bubbles

Don't include a monotonic time in time.Times created inside
a bubble, to avoid the confusion of different Times using
different monotonic clock epochs.

For #67434

goos: darwin
goarch: arm64
pkg: time
cpu: Apple M1 Pro
         │ /tmp/bench.0 │            /tmp/bench.1            │
         │    sec/op    │   sec/op     vs base               │
Since-10    18.42n ± 2%   18.68n ± 1%       ~ (p=0.101 n=10)
Until-10    18.28n ± 2%   18.46n ± 2%  +0.98% (p=0.009 n=10)
geomean     18.35n        18.57n       +1.20%

Change-Id: Iaf1b80d0a4df52139c5b80d4bde4410ef8a49f2f
Reviewed-on: https://go-review.googlesource.com/c/go/+/657415
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

2 files changed, 19 insertions, 2 deletions

diff --git a/src/time/time.go b/src/time/time.go
index 14e79672ca..bcaeee407e 100644
--- a/src/time/time.go
+++ b/src/time/time.go
@@ -1221,7 +1221,7 @@ func subMono(t, u int64) Duration {
 // Since returns the time elapsed since t.
 // It is shorthand for time.Now().Sub(t).
 func Since(t Time) Duration {
-	if t.wall&hasMonotonic != 0 {
+	if t.wall&hasMonotonic != 0 && !runtimeIsBubbled() {
 		// Common case optimization: if t has monotonic time, then Sub will use only it.
 		return subMono(runtimeNano()-startNano, t.ext)
 	}
@@ -1231,7 +1231,7 @@ func Since(t Time) Duration {
 // Until returns the duration until t.
 // It is shorthand for t.Sub(time.Now()).
 func Until(t Time) Duration {
-	if t.wall&hasMonotonic != 0 {
+	if t.wall&hasMonotonic != 0 && !runtimeIsBubbled() {
 		// Common case optimization: if t has monotonic time, then Sub will use only it.
 		return subMono(t.ext, runtimeNano()-startNano)
 	}
@@ -1325,6 +1325,9 @@ func runtimeNow() (sec int64, nsec int32, mono int64)
 //go:linkname runtimeNano
 func runtimeNano() int64
 
+//go:linkname runtimeIsBubbled
+func runtimeIsBubbled() bool
+
 // 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
diff --git a/src/time/time_test.go b/src/time/time_test.go
index ff253be46b..dcb477b658 100644
--- a/src/time/time_test.go
+++ b/src/time/time_test.go
@@ -1497,6 +1497,20 @@ func BenchmarkNowUnixMicro(b *testing.B) {
 	}
 }
 
+func BenchmarkSince(b *testing.B) {
+	start := Now()
+	for b.Loop() {
+		u = int64(Since(start))
+	}
+}
+
+func BenchmarkUntil(b *testing.B) {
+	end := Now().Add(1 * Hour)
+	for b.Loop() {
+		u = int64(Until(end))
+	}
+}
+
 func BenchmarkFormat(b *testing.B) {
 	t := Unix(1265346057, 0)
 	for i := 0; i < b.N; i++ {