time: optimize GoString

Optimize Time.GoString by avoiding multiple calls to absDate.

	name        old time/op    new time/op    delta
	GoString-8     313ns ± 2%     197ns ± 1%  -37.08%  (p=0.008 n=5+5)

	name        old alloc/op   new alloc/op   delta
	GoString-8     80.0B ± 0%     80.0B ± 0%     ~     (all equal)

	name        old allocs/op  new allocs/op  delta
	GoString-8      1.00 ± 0%      1.00 ± 0%     ~     (all equal)

Fixes #54436

Change-Id: I8e6f8e7bbb9857b4bc0cdf6ed29a6b2415775db7
Reviewed-on: https://go-review.googlesource.com/c/go/+/423634
Run-TryBot: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Joseph Tsai <joetsai@digital-static.net>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>

2 files changed, 19 insertions, 9 deletions

diff --git a/src/time/format.go b/src/time/format.go
index 721a207c0e..80495c0266 100644
--- a/src/time/format.go
+++ b/src/time/format.go
@@ -540,26 +540,29 @@ func (t Time) String() string {
 // GoString implements fmt.GoStringer and formats t to be printed in Go source
 // code.
 func (t Time) GoString() string {
-	buf := make([]byte, 0, 70)
+	abs := t.abs()
+	year, month, day, _ := absDate(abs, true)
+	hour, minute, second := absClock(abs)
+
+	buf := make([]byte, 0, len("time.Date(9999, time.September, 31, 23, 59, 59, 999999999, time.Local)"))
 	buf = append(buf, "time.Date("...)
-	buf = appendInt(buf, t.Year(), 0)
-	month := t.Month()
+	buf = appendInt(buf, year, 0)
 	if January <= month && month <= December {
 		buf = append(buf, ", time."...)
-		buf = append(buf, t.Month().String()...)
+		buf = append(buf, longMonthNames[month-1]...)
 	} else {
 		// It's difficult to construct a time.Time with a date outside the
 		// standard range but we might as well try to handle the case.
 		buf = appendInt(buf, int(month), 0)
 	}
 	buf = append(buf, ", "...)
-	buf = appendInt(buf, t.Day(), 0)
+	buf = appendInt(buf, day, 0)
 	buf = append(buf, ", "...)
-	buf = appendInt(buf, t.Hour(), 0)
+	buf = appendInt(buf, hour, 0)
 	buf = append(buf, ", "...)
-	buf = appendInt(buf, t.Minute(), 0)
+	buf = appendInt(buf, minute, 0)
 	buf = append(buf, ", "...)
-	buf = appendInt(buf, t.Second(), 0)
+	buf = appendInt(buf, second, 0)
 	buf = append(buf, ", "...)
 	buf = appendInt(buf, t.Nanosecond(), 0)
 	buf = append(buf, ", "...)
@@ -586,7 +589,7 @@ func (t Time) GoString() string {
 		// case we hope not to hit too often.
 		buf = append(buf, `time.Location(`...)
 		buf = append(buf, []byte(quote(loc.name))...)
-		buf = append(buf, `)`...)
+		buf = append(buf, ')')
 	}
 	buf = append(buf, ')')
 	return string(buf)
diff --git a/src/time/time_test.go b/src/time/time_test.go
index 059c71bb8a..8ab79d3801 100644
--- a/src/time/time_test.go
+++ b/src/time/time_test.go
@@ -1487,6 +1487,13 @@ func BenchmarkISOWeek(b *testing.B) {
 	}
 }
 
+func BenchmarkGoString(b *testing.B) {
+	t := Now()
+	for i := 0; i < b.N; i++ {
+		_ = t.GoString()
+	}
+}
+
 func TestMarshalBinaryZeroTime(t *testing.T) {
 	t0 := Time{}
 	enc, err := t0.MarshalBinary()