20% faster deserialize (#3655)

Co-authored-by: Jarred Sumner <709451+Jarred-Sumner@users.noreply.github.com>

src/bun.js/bindings/webcore/SerializedScriptValue.cpp

@@ -2571,6 +2571,19 @@ private:
         {
         }
 
+        CachedString(const Identifier& identifier)
+            : m_identifier(identifier)
+            , m_string(identifier.string())
+        {
+        }
+
+        Identifier identifier(JSC::VM& vm)
+        {
+            if (m_identifier.isEmpty())
+                m_identifier = Identifier::fromString(vm, string());
+            return m_identifier;
+        }
+
         JSValue jsString(JSGlobalObject* lexicalGlobalObject)
         {
             if (!m_jsString)
@@ -2583,6 +2596,7 @@ private:
     private:
         String m_string;
         JSValue m_jsString;
+        Identifier m_identifier;
     };
 
     struct CachedStringRef {
@@ -2956,6 +2970,38 @@ private:
         return true;
     }
 
+    static bool readIdentifier(JSC::VM& vm, const uint8_t*& ptr, const uint8_t* end, Identifier& str, unsigned length, bool is8Bit)
+    {
+        if (length >= std::numeric_limits<int32_t>::max() / sizeof(UChar))
+            return false;
+
+        if (is8Bit) {
+            if ((end - ptr) < static_cast<int>(length))
+                return false;
+            str = Identifier::fromString(vm, reinterpret_cast<const LChar*>(ptr), length);
+            ptr += length;
+            return true;
+        }
+
+        unsigned size = length * sizeof(UChar);
+        if ((end - ptr) < static_cast<int>(size))
+            return false;
+
+#if ASSUME_LITTLE_ENDIAN
+        str = Identifier::fromString(vm, reinterpret_cast<const UChar*>(ptr), length);
+        ptr += length * sizeof(UChar);
+#else
+        UChar* characters;
+        str = String::createUninitialized(length, characters);
+        for (unsigned i = 0; i < length; ++i) {
+            uint16_t c;
+            readLittleEndian(ptr, end, c);
+            characters[i] = c;
+        }
+#endif
+        return true;
+    }
+
     bool readStringData(CachedStringRef& cachedString)
     {
         bool scratch;
@@ -2998,6 +3044,42 @@ private:
         return true;
     }
 
+    bool readIdentifierData(JSC::VM& vm, CachedStringRef& cachedString, bool& wasTerminator)
+    {
+        if (m_failed)
+            return false;
+        uint32_t length = 0;
+        if (!read(length))
+            return false;
+        if (length == TerminatorTag) {
+            wasTerminator = true;
+            return false;
+        }
+        if (length == StringPoolTag) {
+            unsigned index = 0;
+            if (!readStringIndex(index)) {
+                fail();
+                return false;
+            }
+            if (index >= m_constantPool.size()) {
+                fail();
+                return false;
+            }
+            cachedString = CachedStringRef(&m_constantPool, index);
+            return true;
+        }
+        bool is8Bit = length & StringDataIs8BitFlag;
+        length &= ~StringDataIs8BitFlag;
+        Identifier identifier;
+        if (!readIdentifier(vm, m_ptr, m_end, identifier, length, is8Bit)) {
+            fail();
+            return false;
+        }
+        m_constantPool.append(identifier);
+        cachedString = CachedStringRef(&m_constantPool, m_constantPool.size() - 1);
+        return true;
+    }
+
     SerializationTag readTag()
     {
         if (m_ptr >= m_end)
@@ -4658,7 +4740,7 @@ DeserializationResult CloneDeserializer::deserialize()
         case ObjectStartVisitMember: {
             CachedStringRef cachedString;
             bool wasTerminator = false;
-            if (!readStringData(cachedString, wasTerminator)) {
+            if (!readIdentifierData(vm, cachedString, wasTerminator)) {
                 if (!wasTerminator)
                     goto error;
 
@@ -4669,11 +4751,11 @@ DeserializationResult CloneDeserializer::deserialize()
             }
 
             if (JSValue terminal = readTerminal()) {
-                putProperty(outputObjectStack.last(), Identifier::fromString(vm, cachedString->string()), terminal);
+                putProperty(outputObjectStack.last(), cachedString->identifier(vm), terminal);
                 goto objectStartVisitMember;
             }
             stateStack.append(ObjectEndVisitMember);
-            propertyNameStack.append(Identifier::fromString(vm, cachedString->string()));
+            propertyNameStack.append(cachedString->identifier(vm));
             goto stateUnknown;
         }
         case ObjectEndVisitMember: {