Author: Tom Kerrigan
Date: 11:20:19 02/13/03
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On February 12, 2003 at 23:20:52, Matt Taylor wrote: >80% accuracy when you do it at runtime. The compiler can know the -exact- >probabilities of each branch and take advantage of this. The compiler can know >with near-100% certainty where most branches will go. The only variable is >input, and every combination of input is assumed to have equal probability. That's absurd. Dynamic branch prediction is over 90% accurate (over 95% for the P4) and static branch prediction is at best 80% accurate, and that's profile directed. You tell me which is closer to 100%. The reasons should be obvious if you think about it. >>What, exactly, do you think the point of predication is, then? It's to allow >>instructions to execute before the condition is determined, in other words, out >>of order. (Or at least in order without being dependent.) If you think >>predicated instructions are only executed after the condition is determined, >>then what is the difference between a "predicated branch" and a normal branch, >>besides some extra instructions? >Predication avoids small conditional branches such as the infamous abs, max, and >min functions. Sure, you can avoid having an actual branch instruction. I'm asking you to think deeper. How does that make the processor go any faster? >>And every other SPEC program shows that "in practice" McKinley is clearly slower >>than a P4. >So there are two results, and you prefer to throw away one rather than >attempting an explanation. No, more like 12 results and in only one case does the Itanium 2 outperform the P4. And I think I've done a very good job explaining why Crafty runs faster on the I2 than the P4. -Tom
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