encoding/json: optimize Marshal for maps

Optimize marshaling of maps by using slices.SortFunc.

This drops an unnecessary field from reflectWithString,
which also reduces the cost of each swap operation.

benchmark                  old ns/op     new ns/op     delta
BenchmarkMarshalMap-10     228           139           -39.24%

benchmark                  old allocs     new allocs     delta
BenchmarkMarshalMap-10     11             8              -27.27%

benchmark                  old bytes     new bytes     delta
BenchmarkMarshalMap-10     432           232           -46.30%

Change-Id: Ic2ba7a1590863c7536305c6f6536372b26ec9b0c
Reviewed-on: https://go-review.googlesource.com/c/go/+/515176
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: qiulaidongfeng <2645477756@qq.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

1 files changed, 16 insertions, 0 deletions

diff --git a/src/encoding/json/bench_test.go b/src/encoding/json/bench_test.go
index b7e2b6974a..0f080acdbf 100644
--- a/src/encoding/json/bench_test.go
+++ b/src/encoding/json/bench_test.go
@@ -246,6 +246,22 @@ func BenchmarkMarshalBytesError(b *testing.B) {
 	b.Run("4096", benchMarshalBytesError(4096))
 }
 
+func BenchmarkMarshalMap(b *testing.B) {
+	b.ReportAllocs()
+	m := map[string]int{
+		"key3": 3,
+		"key2": 2,
+		"key1": 1,
+	}
+	b.RunParallel(func(pb *testing.PB) {
+		for pb.Next() {
+			if _, err := Marshal(m); err != nil {
+				b.Fatal("Marshal:", err)
+			}
+		}
+	})
+}
+
 func BenchmarkCodeDecoder(b *testing.B) {
 	b.ReportAllocs()
 	if codeJSON == nil {