Changeset 11707:1d085f66c4ca
cpu: disallow speculative update of the conditional 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).

Fast regressions (SE)

booting Linux (FS)