No joke intended?
Yes, clock speeds for PowerPCs and x86s are generally not comparable, just like between AMDs and Intels they are not either (that was why AMD did things like release Athlons with a separate number ostensibly representing the "equivalent Intel clock").
In the case of the G4 and the P4, which had a huge gulf between them w/r/t clock speed, the difference is primarily pipelining. The G4 has a very shallow pipeline which does a lot of work per clock cycle, while the P4 is deeply pipelined and does minimal steps (but a large number of them). For this reason, the P4 requires a much higher clock speed to do the same work that the G4 does, although the payoff for the P4 is that the design can be clocked faster and faster as long as heat is not an issue because the small pipeline steps lend themselves to being done more quickly. On the other hand, the G4 doesn't get as hot and can do things more efficiently per tick, so choose your poison.
Note that pipelining is not the whole story, but it *is* a lot of it.
The gulf narrowed with the G5, which has a more pipelined architecture and uses some mechanisms for u-ops that are analogous to the P4, although an equivalent MIPS-benchmark G5 is still in general clocked a significant percentage under a P4 of whatever generation (I don't know all Intel's stupid code names
).
For those who are interested, just to complete the course, the G1 was the original PowerPC 601 in the Power Macintosh 6100, 7100, 7200, 7500, 8100 and 8200; the G2 sort of refers to the PPC 603 and 604 series en masse. Neither of these are official terms.
EDIT: I should add that massive pipelining is no panacea, especially if you have sucky branch prediction. On the P4, if its pipeline gets invalidated -- such as a branch predicted wrong and you have to start executing from an unexpected location -- you have a huge number of clock cycles wasted doing nothing while we wait to clear the pipeline. This is such a feared consequence on the P4 that there is a lot of silicon dedicated to accurately keeping the pipeline full. The G4 doesn't have (much of) this problem because its pipeline is much shallower and any burp will filter out in a few ticks.
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