Review Board 2.0.15

cpu: disallow speculative update of the conditional branch predictor tables (o3)

Review Request #3727 - Created Nov. 18, 2016 and submitted

Arthur Perais

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)

Review request changed
Updated (Feb. 18, 2017, 5:38 p.m.)

Status: Closed (submitted)