[kannanvijayan]

There's a bit of time yes, but for an engine that relies on this format (e.g. spidermonkey), the assumptions associated with the value boxing format would have leaked into the codebase all over the place. It's the kind of thing that's far less painful to take care of when you don't need to do it than when you need to do it.

But fair point on the aligned pointers - that would give you some free bits to keep using, but it gets ugly.

You're right about the 51 bits - I always get mixed up about whether it's 12 bits of exponent, or the 12 includes the sign. Point is it puts some hard constraints on a pretty large number of high bits of a pointer being free, as opposed to an alignment requirement for low-bit tagging which will never run out of bits.