(Answer) (Category) distributed.net Faq-O-Matic : (Category) Project: RC5-72 : (Answer) Why are PowerPC-based and (most) Intel-based computers so much faster than other platforms on RC5-72?
Integral to the mathematics of the RC5 algorithm are 32-bit rotate operations.

For whatever reason, the designers of the IA32 (32bit Intel x86) and the PowerPC architectures decided to implement the rotate function as a hardware instruction.

Many other CPUs do not have built-in hardware rotate instructions and must emulate the operation by (at the very least) two shifts and a logical OR. This handicap is why many non-32bit-Intel [1] and non-PowerPC computers run RC5 slower than one might expect based on real-world benchmarks. It is also the main reason why the RC5 client is a poor benchmark to use in determining the speed or performance of a particular CPU.

[1] The IA32 architecture is that used by the Intel 80386, 80486, Pentium, Pentium Pro, Pentium II, Pentium III and Pentium 4 processors. The Pentium 4 has a slow, multi-cycle hardware rotate instruction up until the Prescott revision.

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