encoding/json: reduce unmarshal mallocs for unmapped fields

JSON decoding performs poorly for unmapped and ignored fields. We noticed better
performance when unmarshalling unused fields. The loss comes mostly from calls
to scanner.error as described at #17914.

benchmark                 old ns/op     new ns/op     delta
BenchmarkIssue10335-8     431           408           -5.34%
BenchmarkUnmapped-8       1744          1314          -24.66%

benchmark                 old allocs     new allocs     delta
BenchmarkIssue10335-8     4              3              -25.00%
BenchmarkUnmapped-8       18             4              -77.78%

benchmark                 old bytes     new bytes     delta
BenchmarkIssue10335-8     320           312           -2.50%
BenchmarkUnmapped-8       568           344           -39.44%

Fixes #17914, improves #10335

Change-Id: I7d4258a94eb287c0fe49e7334795209b90434cd0
Reviewed-on: https://go-review.googlesource.com/33276
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

1 files changed, 11 insertions, 0 deletions

diff --git a/src/encoding/json/bench_test.go b/src/encoding/json/bench_test.go
index cd7380b1ef..ec5a88a4e2 100644
--- a/src/encoding/json/bench_test.go
+++ b/src/encoding/json/bench_test.go
@@ -221,3 +221,14 @@ func BenchmarkIssue10335(b *testing.B) {
 		}
 	}
 }
+
+func BenchmarkUnmapped(b *testing.B) {
+	b.ReportAllocs()
+	var s struct{}
+	j := []byte(`{"s": "hello", "y": 2, "o": {"x": 0}, "a": [1, 99, {"x": 1}]}`)
+	for n := 0; n < b.N; n++ {
+		if err := Unmarshal(j, &s); err != nil {
+			b.Fatal(err)
+		}
+	}
+}