Tagged Unboxed Floating Point Numbers
The fundamental approaches taken by every implementation I have found in the literature are to either 1) box all floating point numbers, or 2) to use a combination of boxed floating point numbers and untagged floating point numbers. As one might imagine, (1) can cause serious performance degradation for numerically intensive programs, due to the need to create new boxed objects to store the result of each floating point computation. As for (2), there are numerous papers that discuss various compilation strategies for finding opportunities to use untagged unboxed floating point numbers, but these techniques appear to to be limited to particular problem domains, since they mainly try to convert vectors of floating point numbers to be untagged and unboxed. Nowhere have I found any mention whatsoever of using tagged unboxed floating point numbers.
