RC5 involves a large number of integer additions, rotates and XORs. It
doesn't require floating point calculations and won't, in general,
benefit from them. There has been quite a lot of recent discussion on
whether or not it might be possible to boost keyrates (on x86
architectures at least) by taking advantage of the fact that there are
separate pipelines for integer and floating point instructions. (We
leave it to the reader to figure out how to do floating-point XORs and
Currently none of the clients attempt to make use of FPUs and we believe any use of the FPU will result in slower clients rather than faster ones. At least one bright person has suggested this is not necessarily the case and indeed he may be right. If anyone can develop an RC5-72 core that takes advantage of the x86 FPU for an overall client speed boost we would be very interested in hearing from them!
If you are interested in looking into it the x86 core code is
available and can be downloaded from http://www.distributed.net/source/.