lsbFU is a load/store for binary floating point, which converts to and from internal representation. There is also a lsdFU, but Gold doesn’t support decimal in native.
Most machines have a distinct internal representation for FP and convert on load and store. We had one that kept the number denormalized, but it turned out not to work. The current simplified form does (apparently) work, but it’s not clear that the gain is worth the complication, and lsbFU may go away later.