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authorArthur Perais <arthur.perais@inria.fr>2016-12-21 15:07:16 -0600
committerArthur Perais <arthur.perais@inria.fr>2016-12-21 15:07:16 -0600
commit497cc2d373d1559aaae0263635b88f670fd239cd (patch)
tree7ad8f55f589fe1e04bc3bc3764794c83e13f1788 /ext/dsent/model/ModelGen.cc
parent34065f8d5f51e165b56d12a6d88092332809f0b9 (diff)
downloadgem5-497cc2d373d1559aaae0263635b88f670fd239cd.tar.xz
cpu: disallow speculative update of branch predictor tables (o3)
The Minor and o3 cpu models share the branch prediction code. Minor relies on the BPredUnit::squash() function to update the branch predictor tables on a branch mispre- diction. This is fine because Minor executes in-order, so the update is on the correct path. However, this causes the branch predictor to be updated on out-of-order branch mispredictions when using the o3 model, which should not be the case. This patch guards against speculative update of the branch prediction tables. On a branch misprediction, BPredUnit::squash() calls BpredUnit::update(..., squashed = true). The underlying branch predictor tests against the value of squashed. If it is true, it restores any speculatively updated internal state it might have (e.g., global/local branch history), then returns. If false, it updates its prediction tables. Previously, exist- ing predictors did not test against the "squashed" parameter. To accomodate for this change, the Minor model must now call BPredUnit::squash() then BPredUnit::update(..., squashed = false) on branch mispredictions. Before, calling BpredUnit::squash() performed the prediction tables update. The effect is a slight MPKI improvement when using the o3 model. A further patch should perform the same modifications for the indirect target predictor and BTB (less critical). Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
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