[tsapeta]

Summary

React Native reads every props object with Object::getPropertyNames, then getValueAtIndex, getString, getProperty(String) and utf8 for each name. In Hermes, for-in keeps the names of ordinary objects as symbols, so this converts each symbol to a string, wraps it in a JS array, reads it back, and converts the string to a symbol again for the property lookup.

This adds getPropertyNameIDs to JSI:

// IRuntime
virtual std::vector<PropNameID> getPropertyNameIDs(const Object& object) = 0;
// Object
std::vector<PropNameID> getPropertyNameIDs(IRuntime& runtime) const;

It returns the same names as getPropertyNames, in the same order, as PropNameIDs and without a JS array.

• It follows getStringData and setPrototypeOf: an IRuntime method with a default implementation in jsi::Runtime (jsi.cpp), built on getPropertyNames, so every runtime has it.
• RuntimeDecorator forwards it, and WithRuntimeDecorator forwards it inside Around, so the thread-safe runtime takes its lock.
• TracingRuntime uses the default implementation, so a trace records the getPropertyNames and other calls that it's equivalent to, and replay doesn't need a new event.
• Hermes returns the for-in symbols directly, and converts only string and index keys with stringToSymbolID (index keys with numberToStringPrimitive first). The values can then be read with getProperty(PropNameID), which looks the symbol up directly.

#2319 (forEachProperty) adds a walk that also reads the values and gives UTF-8 names, for callers that read each name once. This one is for callers that keep the names, for example to read the same properties again with getProperty.

Test Plan

• New JSITest.GetPropertyNameIDsTest in jsi/test/testlib.cpp, for the runtime's implementation and, through a RuntimeDecorator, for the default one: for objects with string, index, non-ASCII, symbol, inherited, getter and non-enumerable properties, an array and an empty object, the names and getProperty values are the same as with getPropertyNames.
• APITests: all 439 tests pass. hermesABIRuntimeWrapper, hermesSandboxRuntime, synthTrace and traceInterpreter build.
• A timing loop on top of APITests (macOS host, Release, median of 9 samples of 200,000 iterations, not part of this PR) for the walk React Native does: names, values and UTF-8 names, on objects with number-valued props.

Walk	makeHermesRuntime	makeThreadSafeHermesRuntime
getPropertyNames + strings, 1 prop	97 ns	154 ns
getPropertyNameIDs, 1 prop	46 ns	82 ns
getPropertyNames + strings, 6 props	477 ns	702 ns
getPropertyNameIDs, 6 props	173 ns	325 ns
getPropertyNames + strings, 20 props	1,534 ns	2,356 ns
getPropertyNameIDs, 20 props	551 ns	1,143 ns

#2317 makes the getPropertyNames walk about 45% faster on its own; with both, the PropNameID walk is still about a third faster than the string walk.

[tmikov]

This breaks the ABI like #2319. Worth discussing whether two separate calls for essentially the same thing are needed.