[json] Use SmallVector to buffer properties and elements

This improves performance a little for especially small parsed
objects, e.g., parsing json-parse-financial data in kraken 100.000
times goes from 3.25 s to 3.1 or below.

Change-Id: Ic9b668b44fc766da9d8ad03f51924f7dd8b5cc7a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2752881Reviewed-by: Victor Gomes <victorgomes@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73360}

src/json/json-parser.cc

@@ -401,7 +401,7 @@ Handle<Map> ParentOfDescriptorOwner(Isolate* isolate, Handle<Map> maybe_root,
 template <typename Char>
 Handle<Object> JsonParser<Char>::BuildJsonObject(
     const JsonContinuation& cont,
-    const std::vector<JsonProperty>& property_stack, Handle<Map> feedback) {
+    const SmallVector<JsonProperty>& property_stack, Handle<Map> feedback) {
   size_t start = cont.index;
   int length = static_cast<int>(property_stack.size() - start);
   int named_length = length - cont.elements;
@@ -645,7 +645,7 @@ Handle<Object> JsonParser<Char>::BuildJsonObject(
 template <typename Char>
 Handle<Object> JsonParser<Char>::BuildJsonArray(
     const JsonContinuation& cont,
-    const std::vector<Handle<Object>>& element_stack) {
+    const SmallVector<Handle<Object>>& element_stack) {
   size_t start = cont.index;
   int length = static_cast<int>(element_stack.size() - start);
 
@@ -686,12 +686,10 @@ Handle<Object> JsonParser<Char>::BuildJsonArray(
 template <typename Char>
 MaybeHandle<Object> JsonParser<Char>::ParseJsonValue() {
   std::vector<JsonContinuation> cont_stack;
-  std::vector<JsonProperty> property_stack;
-  std::vector<Handle<Object>> element_stack;
+  SmallVector<JsonProperty> property_stack;
+  SmallVector<Handle<Object>> element_stack;
 
   cont_stack.reserve(16);
-  property_stack.reserve(16);
-  element_stack.reserve(16);
 
   JsonContinuation cont(isolate_, JsonContinuation::kReturn, 0);
 
@@ -833,7 +831,7 @@ MaybeHandle<Object> JsonParser<Char>::ParseJsonValue() {
             }
           }
           value = BuildJsonObject(cont, property_stack, feedback);
-          property_stack.resize(cont.index);
+          property_stack.resize_no_init(cont.index);
           Expect(JsonToken::RBRACE);
 
           // Return the object.
@@ -852,7 +850,7 @@ MaybeHandle<Object> JsonParser<Char>::ParseJsonValue() {
           if (V8_LIKELY(Check(JsonToken::COMMA))) break;
 
           value = BuildJsonArray(cont, element_stack);
-          element_stack.resize(cont.index);
+          element_stack.resize_no_init(cont.index);
           Expect(JsonToken::RBRACK);
 
           // Return the array.

src/json/json-parser.h

@@ -5,6 +5,7 @@
 #ifndef V8_JSON_JSON_PARSER_H_
 #define V8_JSON_JSON_PARSER_H_
 
+#include "src/base/small-vector.h"
 #include "src/execution/isolate.h"
 #include "src/heap/factory.h"
 #include "src/objects/objects.h"
@@ -155,6 +156,8 @@ class JsonParser final {
   static constexpr uc32 kInvalidUnicodeCharacter = static_cast<uc32>(-1);
 
  private:
+  template <typename T>
+  using SmallVector = base::SmallVector<T, 16>;
   struct JsonContinuation {
     enum Type : uint8_t { kReturn, kObjectProperty, kArrayElement };
     JsonContinuation(Isolate* isolate, Type type, size_t index)
@@ -287,10 +290,10 @@ class JsonParser final {
 
   Handle<Object> BuildJsonObject(
       const JsonContinuation& cont,
-      const std::vector<JsonProperty>& property_stack, Handle<Map> feedback);
+      const SmallVector<JsonProperty>& property_stack, Handle<Map> feedback);
   Handle<Object> BuildJsonArray(
       const JsonContinuation& cont,
-      const std::vector<Handle<Object>>& element_stack);
+      const SmallVector<Handle<Object>>& element_stack);
 
   // Mark that a parsing error has happened at the current character.
   void ReportUnexpectedCharacter(uc32 c);