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globalHistoryBits, globalPredictorSize, and choicePredictorSize are decoupled.
globalHistoryBits controls how much history is kept, global and choice
predictor sizes control how much of that history is used when accessing
predictor tables. This way, global and choice predictors can actually be
different sizes, and it is no longer possible to walk off the predictor arrays
and cause a seg fault.
There are now individual thresholds for choice, global, and local saturating
counters, so that taken/not taken decisions are correct even when the
predictors' counters' sizes are different.
The interface for localPredictorSize has been removed from TournamentBP because
the value can be calculated from localHistoryBits.
Committed by: Nilay Vaish <nilay@cs.wisc.edu>
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Fix some issues with the local predictor and the way it's indexed.
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Change RAS to fix issues with predicated call/return instructions.
Handled all cases in the life of a predicated call and return instruction.
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1. Updates the Branch Predictor correctly to the state
just after a mispredicted branch, if a squash occurs.
2. If a BTB does not find an entry, the branch is predicted not taken.
The global history is modified to correctly reflect this prediction.
3. Local history is now updated at the fetch stage instead of
execute stage.
4. In the Update stage of the branch predictor the local predictors are
now correctly updated according to the state of local history during
fetch stage.
This patch also improves performance by as much as 17% on some benchmarks
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Branch predictor could not predict a branch in a nested loop because:
1. The global history was not updated after a mispredict squash.
2. The global history was updated in the fetch stage. The choice predictors
that were updated used the changed global history. This is incorrect, as
it incorporates the state of global history after the branch in
encountered. Fixed update to choice predictor using the global history
state before the branch happened.
3. The global predictor table was also updated using the global history state
before the branch happened as above.
Additionally, parameters to initialize ctr and history size were reversed.
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Suppose the saturating counters of a branch predictor contain n bits. When the
counter is between 0 and (2^(n-1) - 1), boundaries included, the branch is
predicted as not taken. When the counter is between 2^(n-1) and (2^n - 1),
boundaries included, the branch is predicted as taken.
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--HG--
rename : src/cpu/2bit_local_pred.cc => src/cpu/pred/2bit_local.cc
rename : src/cpu/o3/2bit_local_pred.hh => src/cpu/pred/2bit_local.hh
rename : src/cpu/btb.cc => src/cpu/pred/btb.cc
rename : src/cpu/o3/btb.hh => src/cpu/pred/btb.hh
rename : src/cpu/ras.cc => src/cpu/pred/ras.cc
rename : src/cpu/o3/ras.hh => src/cpu/pred/ras.hh
rename : src/cpu/tournament_pred.cc => src/cpu/pred/tournament.cc
rename : src/cpu/o3/tournament_pred.hh => src/cpu/pred/tournament.hh
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