Async/await Ticks Update

Review

• Awaiting a promise requires 3 jobs (ticks), whereas we intuitively expect it (and would like for performance reasons) to require only 1 job.
• Change involves using PromiseResolve instead of creating a new Promise to wrap the await operand.
• Significant performance gains in V8 from implementing this change.
• ChakraCore (mostly) already implements these semantics.
• Babel transformation uses ChakraCore semantics.
• Semantic tradeoff: for built-in, non-subclass promises with a monkey-patched "then", the "then" method will not be called.

Where did we leave off last time?

• Concern was raised by Mark Miller regarding a monkey-patching use case: Override "then" so that it returns frozen promise objects. Does this still work?

Update

• Analysis of monkey-patching use cases:
  1. Modifying the returned promise: (e.g. frozen promises): The non-monkey-patched promise resulting from PerformPromiseThen is never seen by user code.
  2. Wrap the input callbacks: (e.g. async "zone" tracking): For await, "then" only receives "resolve" and "reject", which cannot invoke user code.
• "Frozen promise" use case still works, because the promise resulting from PerformPromiseThen is never visible to user code.
• We believe this change makes the right tradeoffs and does not significantly impact known monkey-patching use cases.

Conclusion

• We would like to achieve consensus on making this change.
• We would like to make this change soon, while Babel transformations are still widely used.