I was in his FOSDEM GC talk about whippet and found out that he doesn't know anything about the really good GC strategies. He only knows and implemented about dirt-slow mark-sweep, which is only needed for pointer stability from C callbacks, and then inmix which is the slow variant of a copying GC, just without the double heap requirement. But a conventional Cheney copying collector with 1-2 generations for minor sweeps is far better than those.

Until he adds a proper GC to whippet I don't trust anything he says about GC's. He even has the luxury for a precise GC because scheme carries the types along with its values.

And he doesn't know about colored pointers, using 2-5 bits for the GC state, nor nan-tagging. Probably neither about forwarding pointers.