encoding/gob: speedup decoding of maps by zeroing values

Instead of allocating a new reflect.Value object on every loop we zero it.

DecodeComplex128Slice-8  13.1µs ± 7%  13.2µs ± 8%     ~     (p=0.347 n=18+19)
DecodeFloat64Slice-8     8.13µs ± 5%  8.00µs ± 3%     ~     (p=0.168 n=20+19)
DecodeInt32Slice-8       8.27µs ± 5%  8.08µs ± 5%   -2.27%  (p=0.001 n=19+18)
DecodeStringSlice-8      17.9µs ±12%  17.8µs ±11%     ~     (p=0.989 n=20+19)
DecodeInterfaceSlice-8    163µs ±10%   159µs ± 4%     ~     (p=0.057 n=19+19)
DecodeMap-8               220µs ± 2%   183µs ± 1%  -17.07%  (p=0.000 n=19+18)

Updates #19525

Change-Id: I27f8edd4761787f6b9928d34cefa08a34a6e25b2
Reviewed-on: https://go-review.googlesource.com/39203Reviewed-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>

src/encoding/gob/codec_test.go

@@ -548,12 +548,14 @@ func TestEndToEnd(t *testing.T) {
 		X float64
 		Z *int
 	}
-	s1 := "string1"
-	s2 := "string2"
 	type T1 struct {
 		A, B, C  int
 		M        map[string]*float64
 		M2       map[int]T3
+		Mstring  map[string]string
+		Mintptr  map[int]*int
+		Mcomp    map[complex128]complex128
+		Marr     map[[2]string][2]*float64
 		EmptyMap map[string]int // to check that we receive a non-nil map.
 		N        *[3]float64
 		Strs     *[2]string
@@ -565,14 +567,35 @@ func TestEndToEnd(t *testing.T) {
 	}
 	pi := 3.14159
 	e := 2.71828
+	two := 2.0
 	meaning := 42
 	fingers := 5
+	s1 := "string1"
+	s2 := "string2"
+	var comp1 complex128 = complex(1.0, 1.0)
+	var comp2 complex128 = complex(1.0, 1.0)
+	var arr1 [2]string
+	arr1[0] = s1
+	arr1[1] = s2
+	var arr2 [2]string
+	arr2[0] = s2
+	arr2[1] = s1
+	var floatArr1 [2]*float64
+	floatArr1[0] = &pi
+	floatArr1[1] = &e
+	var floatArr2 [2]*float64
+	floatArr2[0] = &e
+	floatArr2[1] = &two
 	t1 := &T1{
 		A:        17,
 		B:        18,
 		C:        -5,
 		M:        map[string]*float64{"pi": &pi, "e": &e},
 		M2:       map[int]T3{4: T3{X: pi, Z: &meaning}, 10: T3{X: e, Z: &fingers}},
+		Mstring:  map[string]string{"pi": "3.14", "e": "2.71"},
+		Mintptr:  map[int]*int{meaning: &fingers, fingers: &meaning},
+		Mcomp:    map[complex128]complex128{comp1: comp2, comp2: comp1},
+		Marr:     map[[2]string][2]*float64{arr1: floatArr1, arr2: floatArr2},
 		EmptyMap: make(map[string]int),
 		N:        &[3]float64{1.5, 2.5, 3.5},
 		Strs:     &[2]string{s1, s2},

src/encoding/gob/decode.go

@@ -565,10 +565,16 @@ func (dec *Decoder) decodeMap(mtyp reflect.Type, state *decoderState, value refl
 	elemIsPtr := mtyp.Elem().Kind() == reflect.Ptr
 	keyInstr := &decInstr{keyOp, 0, nil, ovfl}
 	elemInstr := &decInstr{elemOp, 0, nil, ovfl}
+	keyP := reflect.New(mtyp.Key())
+	keyZ := reflect.Zero(mtyp.Key())
+	elemP := reflect.New(mtyp.Elem())
+	elemZ := reflect.Zero(mtyp.Elem())
 	for i := 0; i < n; i++ {
-		key := decodeIntoValue(state, keyOp, keyIsPtr, allocValue(mtyp.Key()), keyInstr)
-		elem := decodeIntoValue(state, elemOp, elemIsPtr, allocValue(mtyp.Elem()), elemInstr)
+		key := decodeIntoValue(state, keyOp, keyIsPtr, keyP.Elem(), keyInstr)
+		elem := decodeIntoValue(state, elemOp, elemIsPtr, elemP.Elem(), elemInstr)
 		value.SetMapIndex(key, elem)
+		keyP.Elem().Set(keyZ)
+		elemP.Elem().Set(elemZ)
 	}
 }