Use mapassign_faststr to decode map whose key type is string

internal/decoder/map.go

@@ -1,6 +1,7 @@
 package decoder
 
 import (
+	"reflect"
 	"unsafe"
 
 	"github.com/goccy/go-json/internal/errors"
@@ -11,6 +12,7 @@ type mapDecoder struct {
 	mapType       *runtime.Type
 	keyType       *runtime.Type
 	valueType     *runtime.Type
+	stringKeyType bool
 	keyDecoder    Decoder
 	valueDecoder  Decoder
 	structName    string
@@ -22,6 +24,7 @@ func newMapDecoder(mapType *runtime.Type, keyType *runtime.Type, keyDec Decoder,
 		mapType:       mapType,
 		keyDecoder:    keyDec,
 		keyType:       keyType,
+		stringKeyType: keyType.Kind() == reflect.String,
 		valueType:     valueType,
 		valueDecoder:  valueDec,
 		structName:    structName,
@@ -32,9 +35,22 @@ func newMapDecoder(mapType *runtime.Type, keyType *runtime.Type, keyDec Decoder,
 //go:linkname makemap reflect.makemap
 func makemap(*runtime.Type, int) unsafe.Pointer
 
+//go:linkname mapassign_faststr runtime.mapassign_faststr
+//go:noescape
+func mapassign_faststr(t *runtime.Type, m unsafe.Pointer, s string) unsafe.Pointer
+
 //go:linkname mapassign reflect.mapassign
 //go:noescape
-func mapassign(t *runtime.Type, m unsafe.Pointer, key, val unsafe.Pointer)
+func mapassign(t *runtime.Type, m unsafe.Pointer, k, v unsafe.Pointer)
+
+func (d *mapDecoder) mapassign(t *runtime.Type, m, k, v unsafe.Pointer) {
+	if d.stringKeyType {
+		mapV := mapassign_faststr(d.mapType, m, *(*string)(k))
+		typedmemmove(d.valueType, mapV, v)
+	} else {
+		mapassign(t, m, k, v)
+	}
+}
 
 func (d *mapDecoder) DecodeStream(s *Stream, depth int64, p unsafe.Pointer) error {
 	depth++
@@ -77,7 +93,7 @@ func (d *mapDecoder) DecodeStream(s *Stream, depth int64, p unsafe.Pointer) erro
 		if err := d.valueDecoder.DecodeStream(s, depth, v); err != nil {
 			return err
 		}
-		mapassign(d.mapType, mapValue, k, v)
+		d.mapassign(d.mapType, mapValue, k, v)
 		s.skipWhiteSpace()
 		if s.equalChar('}') {
 			**(**unsafe.Pointer)(unsafe.Pointer(&p)) = mapValue
@@ -141,7 +157,7 @@ func (d *mapDecoder) Decode(ctx *RuntimeContext, cursor, depth int64, p unsafe.P
 		if err != nil {
 			return 0, err
 		}
-		mapassign(d.mapType, mapValue, k, v)
+		d.mapassign(d.mapType, mapValue, k, v)
 		cursor = skipWhiteSpace(buf, valueCursor)
 		if buf[cursor] == '}' {
 			**(**unsafe.Pointer)(unsafe.Pointer(&p)) = mapValue