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| author | Arthur Perais <arthur.perais@inria.fr> | 2016-12-21 15:07:16 -0600 |
|---|---|---|
| committer | Arthur Perais <arthur.perais@inria.fr> | 2016-12-21 15:07:16 -0600 |
| commit | 497cc2d373d1559aaae0263635b88f670fd239cd (patch) | |
| tree | 7ad8f55f589fe1e04bc3bc3764794c83e13f1788 /src/cpu/minor | |
| parent | 34065f8d5f51e165b56d12a6d88092332809f0b9 (diff) | |
| download | gem5-497cc2d373d1559aaae0263635b88f670fd239cd.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>
Diffstat (limited to 'src/cpu/minor')
| -rw-r--r-- | src/cpu/minor/fetch2.cc | 8 |
1 files changed, 8 insertions, 0 deletions
diff --git a/src/cpu/minor/fetch2.cc b/src/cpu/minor/fetch2.cc index 394fe8549..15d362da7 100644 --- a/src/cpu/minor/fetch2.cc +++ b/src/cpu/minor/fetch2.cc @@ -152,6 +152,10 @@ Fetch2::updateBranchPrediction(const BranchData &branch) DPRINTF(Branch, "Unpredicted branch seen inst: %s\n", *inst); branchPredictor.squash(inst->id.fetchSeqNum, branch.target, true, inst->id.threadId); + // Update after squashing to accomodate O3CPU + // using the branch prediction code. + branchPredictor.update(inst->id.fetchSeqNum, + inst->id.threadId); break; case BranchData::CorrectlyPredictedBranch: /* Predicted taken, was taken */ @@ -164,6 +168,10 @@ Fetch2::updateBranchPrediction(const BranchData &branch) DPRINTF(Branch, "Branch mis-predicted inst: %s\n", *inst); branchPredictor.squash(inst->id.fetchSeqNum, branch.target /* Not used */, false, inst->id.threadId); + // Update after squashing to accomodate O3CPU + // using the branch prediction code. + branchPredictor.update(inst->id.fetchSeqNum, + inst->id.threadId); break; case BranchData::BadlyPredictedBranchTarget: /* Predicted taken, was taken but to a different target */ |