Make JSCRuntime::createValue 35% faster (#27016)

Summary:
JSC does some sort of thread safety locking on every single JSC API call, which makes them ridiculously expensive for a large number of calls (such as when passing a large array over the RN bridge). It would be great if we could lock and unlock once for an entire sequence of JSC calls… but in the meantime, the less we call JSC the better.

![unknown](https://user-images.githubusercontent.com/183747/67624956-08bd6e00-f838-11e9-8f65-e077693f113d.png)

In my benchmark environment (Nozbe/WatermelonDB#541), the time spent in JSCRuntime::createValue went down from 1.07s to 0.69s by changing JSValueIsXXXX calls to a single JSValueGetType call.

The underlying implementation does the same thing: https://github.com/WebKit/webkit/blob/master/Source/JavaScriptCore/API/JSValueRef.cpp#L58

## Changelog

[General] [Fixed] - Make JSCRuntime::createValue faster
Pull Request resolved: #27016

Reviewed By: RSNara

Differential Revision: D18769047

Pulled By: mhorowitz

fbshipit-source-id: 9d1ee28840303f7721e065c1b3c347e354cd7fef

ReactCommon/jsi/JSCRuntime.cpp

@@ -444,10 +444,18 @@ bool JSCRuntime::isInspectable() {
 namespace {
 
 bool smellsLikeES6Symbol(JSGlobalContextRef ctx, JSValueRef ref) {
-  // Empirically, an es6 Symbol is not an object, but its type is
+  // Since iOS 13, JSValueGetType will return kJSTypeSymbol
+  // Before: Empirically, an es6 Symbol is not an object, but its type is
   // object.  This makes no sense, but we'll run with it.
-  return (
-      !JSValueIsObject(ctx, ref) && JSValueGetType(ctx, ref) == kJSTypeObject);
+  // https://github.com/WebKit/webkit/blob/master/Source/JavaScriptCore/API/JSValueRef.cpp#L79-L82
+
+  JSType type = JSValueGetType(ctx, ref);
+
+  if (type == /* kJSTypeSymbol */ 6) {
+    return true;
+  }
+
+  return (!JSValueIsObject(ctx, ref) && type == kJSTypeObject);
 }
 
 } // namespace
@@ -1339,27 +1347,37 @@ jsi::Object JSCRuntime::createObject(JSObjectRef obj) const {
 }
 
 jsi::Value JSCRuntime::createValue(JSValueRef value) const {
-  if (JSValueIsNumber(ctx_, value)) {
-    return jsi::Value(JSValueToNumber(ctx_, value, nullptr));
-  } else if (JSValueIsBoolean(ctx_, value)) {
-    return jsi::Value(JSValueToBoolean(ctx_, value));
-  } else if (JSValueIsNull(ctx_, value)) {
-    return jsi::Value(nullptr);
-  } else if (JSValueIsUndefined(ctx_, value)) {
-    return jsi::Value();
-  } else if (JSValueIsString(ctx_, value)) {
-    JSStringRef str = JSValueToStringCopy(ctx_, value, nullptr);
-    auto result = jsi::Value(createString(str));
-    JSStringRelease(str);
-    return result;
-  } else if (JSValueIsObject(ctx_, value)) {
-    JSObjectRef objRef = JSValueToObject(ctx_, value, nullptr);
-    return jsi::Value(createObject(objRef));
-  } else if (smellsLikeES6Symbol(ctx_, value)) {
-    return jsi::Value(createSymbol(value));
-  } else {
-    // WHAT ARE YOU
-    abort();
+  JSType type = JSValueGetType(ctx_, value);
+
+  switch (type) {
+    case kJSTypeNumber:
+      return jsi::Value(JSValueToNumber(ctx_, value, nullptr));
+    case kJSTypeBoolean:
+      return jsi::Value(JSValueToBoolean(ctx_, value));
+    case kJSTypeNull:
+      return jsi::Value(nullptr);
+    case kJSTypeUndefined:
+      return jsi::Value();
+    case kJSTypeString: {
+      JSStringRef str = JSValueToStringCopy(ctx_, value, nullptr);
+      auto result = jsi::Value(createString(str));
+      JSStringRelease(str);
+      return result;
+    }
+    case kJSTypeObject: {
+      JSObjectRef objRef = JSValueToObject(ctx_, value, nullptr);
+      return jsi::Value(createObject(objRef));
+    }
+// TODO: Uncomment this when all supported JSC versions have this symbol
+//    case kJSTypeSymbol:
+    default: {
+      if (smellsLikeES6Symbol(ctx_, value)) {
+        return jsi::Value(createSymbol(value));
+      } else {
+        // WHAT ARE YOU
+        abort();
+      }
+    }
   }
 }