On Thu, 6 Aug 2009, Linus Torvalds wrote: > > Anyway, on P4 I think you have two double-speed integer issue ports (ie > max four ops per cycle), but only one of them takes a rotate, and only in > the first half of the cycle (ie just one shift per cycle). > > And afaik, that is actually the _improved_ state in Prescott. The older > P4's didn't have a full shifter unit at all, iirc: shifts were "complex > instructions" in Northwood and weren't even single-clock. Yeah, verified. Google for northwood "barrel shifter" and you'll find a lot of it. Basically, older P4's will I think shift one bit at a time. So while even Prescott is relatively weak in the shifter department, pre-prescott (Willamette and Northwood) are _really_ weak. If your P4 is one of those, you really shouldn't use it to decide on optimizations. Linus -- To unsubscribe from this list: send the line "unsubscribe git" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html