V8 could probably implement the backing object "trick" with some trouble. I'm half-hoping that Nova will show it to be worth their while and that they will eventually do it. It will be a major refactoring of the engine, however.

The heap vector "trick" is basically impossible, I believe. It wouldn't be a refactoring so much as it would be a complete rewrite of the engine. The entirety of V8 assumes it deals in pointers, and all of that would need to change to using indexes instead. I will eat my hat if they do it. Without heap vectors they can still split object data apart using pointer-keyed hash maps, so maybe they could take advantage of some of the ideas still.

V8 does offer ways to run code without optimisations, which we can use for a more apples-to-apples comparison. The most important optimisation that Nova really needs before any big performance comparisons become meaningful is property access inline caching, which requires implementing object shapes.

I'd say that once object shapes are done, then limited performance comparisons can probably be made, especially if V8's JIT is disabled.

To be fair, pointer compression is morally and memory-wise similar to indexing a vector.

So is the whole point of this project to convince V8 to adopt a particular optimization?