Hi Ali,
I've moved this thread to the dev list because it looks like I'll be
making this patch.
One thing I'm unsure of is that currently the global and choice
predictor tables end up being the same size (for all practical
purposes). I don't see any reason why this should necessarily be the
case, but I'm not an expert on branch predictors. Thoughts?
-Erik
On 25/10/12 17:24, Ali Saidi wrote:
Hi Erik,
I posted a patch last night that fixed some issues with the local
predictor, but it all certainly could be cleaned up. We'd be very
happy to commit patches that clean up and parameters and code to
address the issues you've identified.
Thanks,
Ali
On 25.10.2012 11:31, Erik Tomusk wrote:
Hi All,
It seems to me that some of the tournament branch predictor parameters
are redundant, at least given how the predictor is currently
implemented. Has anyone else run into this?
Specifically, the tournament predictor takes nine parameters that can be
configured independently:
localPredictorSize,
localCtrBits,
localHistoryTableSize,
localHistoryBits,
globalPredictorSize,
globalHistoryBits,
globalCtrBits,
choicePredictorSize,
choiceCtrBits
However, the code assumes that 2^(globalHistoryBits) =
globalPredictorSize = choicePredictorSize, and 2^(localHistoryBits) =
localPredictorSize. These assumptions aren't tested, so it's possible to
crash gem5 if, for example, 2^(globalHistoryBits)> globalPredictorSize.
There's also a redundancy in the three *CtrBits. The prediction
threshold is based on localCtrBits, but used for the other two as well.
The *CtrBits can be defined independently, but it looks like the
saturating counters' thresholds will be skewed unless all the *CtrBits
are the same. (There is a comment that the thresholds should be separated)
So the question is, how much of this is bug, and how much is feature?
Has this caused problems for anyone?
Thanks,
Erik
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