time: optimize Format for RFC3339 and RFC3339Nano

Optimise Format for the most frequently used RFC3339
and RFC3339Nano layouts by avoiding parsing of layout.

> benchstat oldBench.txt newBench.txt
name                 old time/op    new time/op    delta
FormatRFC3339-8         302ns ± 1%     203ns ± 0%  -32.89%  (p=0.016 n=5+4)
FormatRFC3339Nano-8     337ns ± 1%     219ns ± 1%  -34.91%  (p=0.008 n=5+5)

name                 old alloc/op   new alloc/op   delta
FormatRFC3339-8         32.0B ± 0%     32.0B ± 0%     ~     (all equal)
FormatRFC3339Nano-8     32.0B ± 0%     32.0B ± 0%     ~     (all equal)

name                 old allocs/op  new allocs/op  delta
FormatRFC3339-8          1.00 ± 0%      1.00 ± 0%     ~     (all equal)
FormatRFC3339Nano-8      1.00 ± 0%      1.00 ± 0%     ~     (all equal)

Fixes #54093

Change-Id: Ifc84fce6078e24514ecbcd234875bca4aaab5e0e
Reviewed-on: https://go-review.googlesource.com/c/go/+/421877
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Joseph Tsai <joetsai@digital-static.net>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Joseph Tsai <joetsai@digital-static.net>
Run-TryBot: Joseph Tsai <joetsai@digital-static.net>

src/time/format.go

@@ -626,6 +626,15 @@ func (t Time) AppendFormat(b []byte, layout string) []byte {
 		min   int
 		sec   int
 	)
+
+	// Handle most frequent layouts separately.
+	switch layout {
+	case RFC3339:
+		return t.appendFormatRFC3339(b, abs, offset, false)
+	case RFC3339Nano:
+		return t.appendFormatRFC3339(b, abs, offset, true)
+	}
+
 	// Each iteration generates one std value.
 	for layout != "" {
 		prefix, std, suffix := nextStdChunk(layout)
@@ -781,6 +790,48 @@ func (t Time) AppendFormat(b []byte, layout string) []byte {
 	return b
 }
 
+func (t Time) appendFormatRFC3339(b []byte, abs uint64, offset int, nanos bool) []byte {
+	// Format date.
+	year, month, day, _ := absDate(abs, true)
+	b = appendInt(b, year, 4)
+	b = append(b, '-')
+	b = appendInt(b, int(month), 2)
+	b = append(b, '-')
+	b = appendInt(b, day, 2)
+
+	b = append(b, 'T')
+
+	// Format time.
+	hour, min, sec := absClock(abs)
+	b = appendInt(b, hour, 2)
+	b = append(b, ':')
+	b = appendInt(b, min, 2)
+	b = append(b, ':')
+	b = appendInt(b, sec, 2)
+
+	if nanos {
+		std := stdFracSecond(stdFracSecond9, 9, '.')
+		b = formatNano(b, uint(t.Nanosecond()), std)
+	}
+
+	if offset == 0 {
+		return append(b, 'Z')
+	}
+
+	// Format zone.
+	zone := offset / 60 // convert to minutes
+	if zone < 0 {
+		b = append(b, '-')
+		zone = -zone
+	} else {
+		b = append(b, '+')
+	}
+	b = appendInt(b, zone/60, 2)
+	b = append(b, ':')
+	b = appendInt(b, zone%60, 2)
+	return b
+}
+
 var errBad = errors.New("bad value for field") // placeholder not passed to user
 
 // ParseError describes a problem parsing a time string.

src/time/time_test.go

@@ -1402,6 +1402,20 @@ func BenchmarkFormat(b *testing.B) {
 	}
 }
 
+func BenchmarkFormatRFC3339(b *testing.B) {
+	t := Unix(1265346057, 0)
+	for i := 0; i < b.N; i++ {
+		t.Format("2006-01-02T15:04:05Z07:00")
+	}
+}
+
+func BenchmarkFormatRFC3339Nano(b *testing.B) {
+	t := Unix(1265346057, 0)
+	for i := 0; i < b.N; i++ {
+		t.Format("2006-01-02T15:04:05.999999999Z07:00")
+	}
+}
+
 func BenchmarkFormatNow(b *testing.B) {
 	// Like BenchmarkFormat, but easier, because the time zone
 	// lookup cache is optimized for the present.