On the effect of branch mispredictions in the Squeak VM

John M McIntosh johnmci at mac.com
Mon Jul 7 19:10:24 UTC 2003


> John, one question about your results: How do they measure up with the
> results coming from #tinyBenchmarks? I don't think there are very many
> bytecodes involved either so (in theory) the results obtained should be
> exactly in line.
>
> Cheers,
> - Andreas

Hi, I'm not sure what you are asking for here?


I see someone talked about the branch prediction logic in powerpc  
versus intel. I'll note that apple
once had a tool to instrument a program, then run and collect branch  
statistics. This data was then
reapplied to the binary to set tag bits on branches because you can  
indicate which way a branch
should usually go as part of the instruction. (a hint)

For the 604e I found this made a measurable improvement for the squeak  
VM.  However for the
G3 the gain was far less because the branch predictor logic was so much  
better. I've not gone
back to visit that, besides it's another time consuming step in the  
build process. However I note some
discussion of this issue for the G5, so we'll see.


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John M. McIntosh <johnmci at smalltalkconsulting.com> 1-800-477-2659
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