encoding/json: improve performance of Unmarshal on primitive types

Attempt 2. The first fix was much faster but ignored syntax errors.

benchmark                      old ns/op    new ns/op    delta
BenchmarkCodeEncoder            74993543     72982390   -2.68%
BenchmarkCodeMarshal            77811181     75610637   -2.83%
BenchmarkCodeDecoder           213337123    190144982  -10.87%
BenchmarkCodeUnmarshal         212180972    190394852  -10.27%
BenchmarkCodeUnmarshalReuse    202113428    182106660   -9.90%
BenchmarkUnmarshalString            1343          919  -31.57%
BenchmarkUnmarshalFloat64           1149          908  -20.97%
BenchmarkUnmarshalInt64              967          778  -19.54%
BenchmarkSkipValue              28851581     28414125   -1.52%

benchmark                       old MB/s     new MB/s  speedup
BenchmarkCodeEncoder               25.88        26.59    1.03x
BenchmarkCodeMarshal               24.94        25.66    1.03x
BenchmarkCodeDecoder                9.10        10.21    1.12x
BenchmarkCodeUnmarshal              9.15        10.19    1.11x
BenchmarkSkipValue                 69.05        70.11    1.02x

Fixes #3949.

R=rsc
CC=golang-dev
https://golang.org/cl/7231058

src/pkg/encoding/json/decode.go

@@ -52,16 +52,34 @@ import (
 // an UnmarshalTypeError describing the earliest such error.
 //
 func Unmarshal(data []byte, v interface{}) error {
-	d := new(decodeState).init(data)
 
 	// Quick check for well-formedness.
 	// Avoids filling out half a data structure
 	// before discovering a JSON syntax error.
+	var d decodeState
 	err := checkValid(data, &d.scan)
 	if err != nil {
 		return err
 	}
 
+	// skip heavy processing for primitive values
+	var first byte
+	var i int
+	for i, first = range data {
+		if first > ' ' || !isSpace(rune(first)) {
+			break
+		}
+	}
+	if first != '{' && first != '[' {
+		rv := reflect.ValueOf(v)
+		if rv.Kind() != reflect.Ptr || rv.IsNil() {
+			return &InvalidUnmarshalError{reflect.TypeOf(v)}
+		}
+		d.literalStore(data[i:], rv.Elem(), false)
+		return d.savedError
+	}
+
+	d.init(data)
 	return d.unmarshal(v)
 }