diff options
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/pred/bpred_unit.cc | |
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/pred/bpred_unit.cc')
-rw-r--r-- | src/cpu/pred/bpred_unit.cc | 25 |
1 files changed, 16 insertions, 9 deletions
diff --git a/src/cpu/pred/bpred_unit.cc b/src/cpu/pred/bpred_unit.cc index 660475017..05770cba9 100644 --- a/src/cpu/pred/bpred_unit.cc +++ b/src/cpu/pred/bpred_unit.cc @@ -330,13 +330,9 @@ BPredUnit::update(const InstSeqNum &done_sn, ThreadID tid) while (!predHist[tid].empty() && predHist[tid].back().seqNum <= done_sn) { // Update the branch predictor with the correct results. - if (!predHist[tid].back().wasSquashed) { - update(tid, predHist[tid].back().pc, - predHist[tid].back().predTaken, - predHist[tid].back().bpHistory, false); - } else { - retireSquashed(tid, predHist[tid].back().bpHistory); - } + update(tid, predHist[tid].back().pc, + predHist[tid].back().predTaken, + predHist[tid].back().bpHistory, false); predHist[tid].pop_back(); } @@ -430,12 +426,23 @@ BPredUnit::squash(const InstSeqNum &squashed_sn, tid, hist_it->seqNum); } - // Have to get GHR here because the update deletes bpHistory + // Get the underlying Global History Register unsigned ghr = getGHR(tid, hist_it->bpHistory); + // There are separate functions for in-order and out-of-order + // branch prediction, but not for update. Therefore, this + // call should take into account that the mispredicted branch may + // be on the wrong path (i.e., OoO execution), and that the counter + // counter table(s) should not be updated. Thus, this call should + // restore the state of the underlying predictor, for instance the + // local/global histories. The counter tables will be updated when + // the branch actually commits. + + // Remember the correct direction for the update at commit. + pred_hist.front().predTaken = actually_taken; + update(tid, (*hist_it).pc, actually_taken, pred_hist.front().bpHistory, true); - hist_it->wasSquashed = true; if (actually_taken) { if (hist_it->wasReturn && !hist_it->usedRAS) { |