I'm not sure what you mean by 'double precision' for an integer processor, though. I'm not an expert on DSP.
Then refer to the many articles about multiple precision arithmetic on the integer units of CPUs.
All you need is a carry out and a carry in.
Great example of how to design high end audio stuff without the audiofools getting in the way. Switching PSUs are perfectly fine to use, as are seemingly ordinary JRC opamps.
Maybe for cheap consumer crap, but not for pro gear. Even if you are one of those lucky people who can't hear any difference between an Amstrad hifi and a Technics one, there are good reasons for using a high end linear PSU and high end op-amps. That PSU is going to get a lot of hours on it, and you need it to remain consistent over that time. Same with the op-amps, you want them to be well matched and tested for many years of operation. The cheap stuff tends to be hand matched or they just don't bother, and good lucky getting data on ageing.
This is one of the most old-fashioned opinions about pro audio gear I've heard in a long time. As mentioned before; switching PSUs are in practically everything pro audio. Linear stuff is limited to - mostly - audiphool territory. Why? Well, you can control switching PSUs much better (i.e. you have actual feedback, so you can correct for line voltage and temperature), it's much easier to filter (switching frequencies are all at or above audio frequencies, so you can get 100dB+ damping on those within a reasonable physical size multi-stage LC filter) and higher efficiency means higher reliability in rackspace (because lower temps).
Even very high-end DAC/ADC modules are generally powered by at least a switching pre-reg with a very small linear stage dropping only a couple volts at most.
I've torn down a bunch of modern (2010+) pro audio gear (like, two mixing and recording stations from a local radio station, €100k+) over the past years and without exception there were no linear power supplies to be found. A few linear regulators for minor rails, but that's it. They do filter SMPSes out the wazoo, I've seen 3-stage 6-decade-spanning LC filters.