Revert "time: optimize Sub"

This reverts commit CL 131196 because there is a bug
in the calculation of nanoseconds.

Fixes #33677

Change-Id: Ic8e94c547ee29b8aeda1b9a5cb9764dbf47b14b4
Reviewed-on: https://go-review.googlesource.com/c/go/+/190497
Run-TryBot: Andrew Bonventre <andybons@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Andrew Bonventre <andybons@golang.org>

src/time/time.go

@@ -906,33 +906,16 @@ func (t Time) Sub(u Time) Duration {
 		}
 		return d
 	}
-
-	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) {
+	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
 	}
-	return maxDuration // t - u is positive out of range
 }
 
 // Since returns the time elapsed since t.

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)),
 }
@@ -1008,14 +1008,6 @@ 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