Author: Robert Hyatt
Date: 19:16:39 03/03/00
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On March 03, 2000 at 20:05:57, Tom Kerrigan wrote: >On March 03, 2000 at 17:08:58, Robert Hyatt wrote: > >>Problem is the compilers don't know what is going on. IE how many "hidden" >>registers does the architecture have for renaming? Intel (nor anyone else) >>will make this a 'constant'. > >But my point is, why have register renaming at all. I can list a dozen good >processors that don't do it. I would like to know the exact percentage speedup >it gives you. That is probably lost in the benchmark data. But in the case of intel, with 8 (barely) usable registers, it would be impossible to keep multiple pipes busy due to register conflicts. Renaming solves this nicely and frees up parallel streams of instructions to keep the pipes busy... Some don't need to do this, like the sparc/ultrasparcs, because they have 32 accessible registers for programming. But 8? What a decision... :) > >>And how does the P5 do more per cycle than a P6 when the p6 can do three >>ops/cycle, while the P5 drags along at a max of 2, and it requires a very >>good compiler to do two at a time??? > >TSCP (1.42) on an original Pentium/200 searches 136 NPS/MHz. > >On a Pentium II/300, it searches 119 NPS/MHz. > >So the Pentium appears to be 15% faster, despite its lack of out-of-order >execution, branch predition, speculative execution, register renaming, >reservation stations, blah blah blah. That looks like a compiler issue. I ran crafty on a P5/233mmx, and a P6/200, and the P5 was about 70% as fast (I have a P5/233 notebook laying around, and while it was not bad, it was definitely slower than a p6/200 machine for all the things I tested (ie linux kernel builds, crafty testing, etc.) > >I assume this is because the P5 has shorter pipes and doesn't have to flush them >all the time due to speculative execution gone wrong. > >(BTW, the K5 has almost everything beat. It searches 173 NPS/MHz, and it doesn't >do anything particularly fancy either.) same point as above. For crafty, the K* processors are slower. I have not delved into why... > >-Tom
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