encoding/gob: Speedup map decoding by reducing the allocations.

The improvementis achieved in encoding/gob/decode.go decodeMap by
allocate keyInstr and elemInstr only once and pass it to
decodeIntoValue, instead of allocating a new instance on every loop
cycle.

name                     old time/op  new time/op  delta
DecodeComplex128Slice-8  64.2µs ±10%  62.2µs ± 8%     ~     (p=0.686 n=4+4)
DecodeFloat64Slice-8     37.1µs ± 3%  36.5µs ± 5%     ~     (p=0.343 n=4+4)
DecodeInt32Slice-8       33.7µs ± 3%  32.7µs ± 4%     ~     (p=0.200 n=4+4)
DecodeStringSlice-8      59.7µs ± 5%  57.3µs ± 1%     ~     (p=0.114 n=4+4)
DecodeInterfaceSlice-8    543µs ± 7%   497µs ± 3%     ~     (p=0.057 n=4+4)
DecodeMap-8              3.78ms ± 8%  2.66ms ± 2%  -29.69%  (p=0.029 n=4+4)

Updates #19525

Change-Id: Iec5fa4530de76f0a70da5de8a129a567b4aa096e
Reviewed-on: https://go-review.googlesource.com/38317
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

1 files changed, 25 insertions, 0 deletions

diff --git a/src/encoding/gob/timing_test.go b/src/encoding/gob/timing_test.go
index a7e7e683cc..e3578992fc 100644
--- a/src/encoding/gob/timing_test.go
+++ b/src/encoding/gob/timing_test.go
@@ -364,3 +364,28 @@ func BenchmarkDecodeInterfaceSlice(b *testing.B) {
 		}
 	}
 }
+
+func BenchmarkDecodeMap(b *testing.B) {
+	count := 10000
+	m := make(map[int]int, count)
+	for i := 0; i < count; i++ {
+		m[i] = i
+	}
+	var buf bytes.Buffer
+	enc := NewEncoder(&buf)
+	err := enc.Encode(m)
+	if err != nil {
+		b.Fatal(err)
+	}
+	bbuf := benchmarkBuf{data: buf.Bytes()}
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		rm := make(map[int]int, 0)
+		bbuf.reset()
+		dec := NewDecoder(&bbuf)
+		err := dec.Decode(&rm)
+		if err != nil {
+			b.Fatal(i, err)
+		}
+	}
+}