This would allow us to fold more things.
Created attachment 434939 [details]
(In reply to Alexey Shvayka from comment #1)
> Created attachment 434939 [details]
is-undefined-fold-tricky-spec-object-other 30.6171+-0.7028 ^ 21.8494+-0.6913 ^ definitely 1.4013x faster
Comment on attachment 434939 [details]
View in context: https://bugs.webkit.org/attachment.cgi?id=434939&action=review
> + TypeOfIsObject / TypeOfIsFunction by-type folding can't be improved in the same way:
> + even if SpecObjectOther is a proven non-masquerader, we don't know if it's callable.
Can you replace the FIXME's with comments to this effect? This is an easy detail to forget. I would say something like - we could use the masquerade watchpoint to prove it's not undefined, but we don't have a trick for callability.