encoding/gob: speed up encoding of arrays and slices

We borrow a trick from the fmt package and avoid reflection
to walk the elements when possible. We could push further with
unsafe (and we may) but this is a good start.
Decode can benefit similarly; it will be done separately.

Use go generate (engen.go) to produce the helper functions
(enc_helpers.go).

benchmark                            old ns/op     new ns/op     delta
BenchmarkEndToEndPipe                6593          6482          -1.68%
BenchmarkEndToEndByteBuffer          3662          3684          +0.60%
BenchmarkEndToEndSliceByteBuffer     350306        351693        +0.40%
BenchmarkComplex128Slice             96347         80045         -16.92%
BenchmarkInt32Slice                  42484         26008         -38.78%
BenchmarkFloat64Slice                51143         36265         -29.09%
BenchmarkStringSlice                 53402         35077         -34.32%

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/156310043

1 files changed, 64 insertions, 0 deletions

diff --git a/src/encoding/gob/timing_test.go b/src/encoding/gob/timing_test.go
index ec55c4d63d..abfe936e83 100644
--- a/src/encoding/gob/timing_test.go
+++ b/src/encoding/gob/timing_test.go
@@ -131,3 +131,67 @@ func TestCountDecodeMallocs(t *testing.T) {
 		t.Fatalf("mallocs per decode of type Bench: %v; wanted 4\n", allocs)
 	}
 }
+
+func BenchmarkComplex128Slice(b *testing.B) {
+	var buf bytes.Buffer
+	enc := NewEncoder(&buf)
+	a := make([]complex128, 1000)
+	for i := range a {
+		a[i] = 1.2 + 3.4i
+	}
+	for i := 0; i < b.N; i++ {
+		buf.Reset()
+		err := enc.Encode(a)
+		if err != nil {
+			b.Fatal(err)
+		}
+	}
+}
+
+func BenchmarkInt32Slice(b *testing.B) {
+	var buf bytes.Buffer
+	enc := NewEncoder(&buf)
+	a := make([]int32, 1000)
+	for i := range a {
+		a[i] = 1234
+	}
+	for i := 0; i < b.N; i++ {
+		buf.Reset()
+		err := enc.Encode(a)
+		if err != nil {
+			b.Fatal(err)
+		}
+	}
+}
+
+func BenchmarkFloat64Slice(b *testing.B) {
+	var buf bytes.Buffer
+	enc := NewEncoder(&buf)
+	a := make([]float64, 1000)
+	for i := range a {
+		a[i] = 1.23e4
+	}
+	for i := 0; i < b.N; i++ {
+		buf.Reset()
+		err := enc.Encode(a)
+		if err != nil {
+			b.Fatal(err)
+		}
+	}
+}
+
+func BenchmarkStringSlice(b *testing.B) {
+	var buf bytes.Buffer
+	enc := NewEncoder(&buf)
+	a := make([]string, 1000)
+	for i := range a {
+		a[i] = "now is the time"
+	}
+	for i := 0; i < b.N; i++ {
+		buf.Reset()
+		err := enc.Encode(a)
+		if err != nil {
+			b.Fatal(err)
+		}
+	}
+}