Interestingly the issues sounds a lot like what a high performance Haskell implementation would run into. One additional note though on GC: highly tuned PL imp

Interestingly the issues sounds a lot like what a high performance Haskell implementation would run into. One additional note though on GC: highly tuned PL implementation requires completely control over object layout, pointer representation, and GC. (!) No off-the-shelf GC will ever be perfect for all languages, thus I think WASM would be far better off doing whatever it could to facilitate runtimes implementing their own. (!) Example from the lazy world: lazy closures when evaluated and updated often results in an indirection. We rely on GC to shortcut these. Sometimes we even have GC perform trivial constant-time evaluations when we know the result is smaller than the suspended evaluation. Example from the Lisp world: cdr-coded cons cells can cut size in half, but no generic GC would be able to do this.