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>

1 files changed, 6 insertions, 0 deletions

diff --git a/src/cpu/pred/2bit_local.cc b/src/cpu/pred/2bit_local.cc
index 9e1c781c5..6f821e94c 100644
--- a/src/cpu/pred/2bit_local.cc
+++ b/src/cpu/pred/2bit_local.cc
@@ -123,6 +123,12 @@ LocalBP::update(ThreadID tid, Addr branch_addr, bool taken, void *bp_history,
     assert(bp_history == NULL);
     unsigned local_predictor_idx;
 
+    // No state to restore, and we do not update on the wrong
+    // path.
+    if (squashed) {
+        return;
+    }
+
     // Update the local predictor.
     local_predictor_idx = getLocalIndex(branch_addr);