encoding/json: realign struct encodeState, structFields, and field

This reduce the struct encodeState size from 56 to 16 bytes (-42),
struct structFields size from 40 to 24 bytes (-16), and struct field size
from 136 to 128 bytes (-8).

Benchmark memory profiling before and after,

File: json.test
Type: alloc_space
Time: Jan 25, 2024 at 4:51am (WIB)
Showing nodes accounting for -4.01GB, 4.66% of 85.95GB total
Dropped 64 nodes (cum <= 0.43GB)
      flat  flat%   sum%        cum   cum%
   -4.25GB  4.94%  4.94%    -4.01GB  4.67%  encoding/json.Marshal
    0.19GB  0.22%  4.73%     0.24GB  0.28%  encoding/json.mapEncoder.encode
    0.05GB 0.057%  4.67%     0.05GB 0.057%  reflect.copyVal
    0.04GB 0.049%  4.62%     0.04GB 0.049%  bytes.growSlice
   -0.04GB 0.045%  4.66%    -0.04GB 0.045%  encoding/json.RawMessage.MarshalJSON
         0     0%  4.66%     0.04GB 0.046%  bytes.(*Buffer).WriteString
         0     0%  4.66%     0.04GB 0.049%  bytes.(*Buffer).grow
         0     0%  4.66%    -0.04GB 0.045%  encoding/json.(*Encoder).Encode
         0     0%  4.66%     0.20GB  0.23%  encoding/json.(*encodeState).marshal
         0     0%  4.66%     0.20GB  0.23%  encoding/json.(*encodeState).reflectValue

1 files changed, 11 insertions, 9 deletions

diff --git a/src/encoding/json/encode.go b/src/encoding/json/encode.go
index 5e47413f23..0cca15b6e0 100644
--- a/src/encoding/json/encode.go
+++ b/src/encoding/json/encode.go
@@ -296,6 +296,8 @@ const hex = "0123456789abcdef"
 
 // An encodeState encodes JSON into a bytes.Buffer.
 type encodeState struct {
+	ptrSeen map[any]struct{}
+
 	bytes.Buffer // accumulated output
 
 	// Keep track of what pointers we've seen in the current recursive call
@@ -304,7 +306,6 @@ type encodeState struct {
 	// startDetectingCyclesAfter, so that we skip the work if we're within a
 	// reasonable amount of nested pointers deep.
 	ptrLevel uint
-	ptrSeen  map[any]struct{}
 }
 
 const startDetectingCyclesAfter = 1000
@@ -710,9 +711,9 @@ type structEncoder struct {
 }
 
 type structFields struct {
-	list         []field
 	byExactName  map[string]*field
 	byFoldedName map[string]*field
+	list         []field
 }
 
 func (se structEncoder) encode(e *encodeState, v reflect.Value, opts encOpts) {
@@ -1068,21 +1069,22 @@ func appendString[Bytes []byte | string](dst []byte, src Bytes, escapeHTML bool)
 
 // A field represents a single field found in a struct.
 type field struct {
-	name      string
-	nameBytes []byte // []byte(name)
+	encoder encoderFunc
+
+	typ reflect.Type
 
+	name        string
 	nameNonEsc  string // `"` + name + `":`
 	nameEscHTML string // `"` + HTMLEscape(name) + `":`
 
-	tag       bool
 	index     []int
-	typ       reflect.Type
+	nameBytes []byte // []byte(name)
+
+	tag       bool
 	omitEmpty bool
 	omitZero  bool
 	isZero    func(reflect.Value) bool
 	quoted    bool
-
-	encoder encoderFunc
 }
 
 type isZeroer interface {
@@ -1308,7 +1310,7 @@ func typeFields(t reflect.Type) structFields {
 			foldedNameIndex[string(foldName(field.nameBytes))] = &fields[i]
 		}
 	}
-	return structFields{fields, exactNameIndex, foldedNameIndex}
+	return structFields{exactNameIndex, foldedNameIndex, fields}
 }
 
 // dominantField looks through the fields, all of which are known to