What did you intend to use the high bits for?
Nothing right now. Just speculation. I was actually pondering how our headers are rather svelte compared to many languages. Java, for instance, saves a full word for a hash. There are endless bits of metadata to speculate about... - whether or not the heap object is frozen to disallow further modification (e.g. for MutVars) - whether or not the heap object is part of a contiguous CNF - a flag to invoke some alternate GC behavior - metadata to avoid dereferencing the info table pointer in some circumstances (common type tags?) - you could even index 2^16 per-type values that live off of the info table But most likely it would have to be something that would give across the board perf improvements that would pay for the extra cost in masking. Good point re: HW prefetching.