time: optimize Sub

This is primarily achieved by checking for arithmetic overflow
instead of using Add and Equal.

It's a decent performance improvement even though
the function still isn't inlined.

name   old time/op  new time/op  delta
Sub-6   242ns ± 0%   122ns ± 0%  -49.59%  (p=0.002 n=8+10)

Updates #17858.

Change-Id: I1469b618183c83ea8ea54d5ce277eb15f2ec0f11
Reviewed-on: https://go-review.googlesource.com/c/go/+/131196
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

2 files changed, 34 insertions, 9 deletions

diff --git a/src/time/time.go b/src/time/time.go
index aacde0db2c..d9938861ac 100644
--- a/src/time/time.go
+++ b/src/time/time.go
@@ -906,16 +906,33 @@ func (t Time) Sub(u Time) Duration {
 		}
 		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):
+
+	ts, us := t.sec(), u.sec()
+
+	var sec, nsec, d int64
+
+	ssub := ts - us
+	if (ssub < ts) != (us > 0) {
+		goto overflow
+	}
+
+	if ssub < int64(minDuration/Second) || ssub > int64(maxDuration/Second) {
+		goto overflow
+	}
+	sec = ssub * int64(Second)
+
+	nsec = int64(t.nsec() - u.nsec())
+	d = sec + nsec
+	if (d > sec) != (nsec > 0) {
+		goto overflow
+	}
+	return Duration(d)
+
+overflow:
+	if t.Before(u) {
 		return minDuration // t - u is negative out of range
-	default:
-		return maxDuration // t - u is positive out of range
 	}
+	return maxDuration // t - u is positive out of range
 }
 
 // Since returns the time elapsed since t.
diff --git a/src/time/time_test.go b/src/time/time_test.go
index 0ac3c3a27f..dd3a8160cd 100644
--- a/src/time/time_test.go
+++ b/src/time/time_test.go
@@ -690,7 +690,7 @@ var gobTests = []Time{
 	Date(0, 1, 2, 3, 4, 5, 6, UTC),
 	Date(7, 8, 9, 10, 11, 12, 13, FixedZone("", 0)),
 	Unix(81985467080890095, 0x76543210), // Time.sec: 0x0123456789ABCDEF
-	{}, // nil location
+	{},                                  // nil location
 	Date(1, 2, 3, 4, 5, 6, 7, FixedZone("", 32767*60)),
 	Date(1, 2, 3, 4, 5, 6, 7, FixedZone("", -32768*60)),
 }
@@ -1008,6 +1008,14 @@ func TestSub(t *testing.T) {
 	}
 }
 
+func BenchmarkSub(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		for _, st := range subTests {
+			st.t.Sub(st.u)
+		}
+	}
+}
+
 var nsDurationTests = []struct {
 	d    Duration
 	want int64