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, 1 insertions, 12 deletions

diff --git a/src/cpu/pred/bpred_unit.hh b/src/cpu/pred/bpred_unit.hh
index 3f9cbc057..b890dc332 100644
--- a/src/cpu/pred/bpred_unit.hh
+++ b/src/cpu/pred/bpred_unit.hh
@@ -179,14 +179,6 @@ class BPredUnit : public SimObject
      */
     virtual void update(ThreadID tid, Addr instPC, bool taken,
                         void *bp_history, bool squashed) = 0;
-     /**
-     * Deletes the associated history with a branch, performs no predictor
-     * updates.  Used for branches that mispredict and update tables but
-     * are still speculative and later retire.
-     * @param bp_history History to delete associated with this predictor
-     */
-    virtual void retireSquashed(ThreadID tid, void *bp_history) = 0;
-
     /**
      * Updates the BTB with the target of a branch.
      * @param inst_PC The branch's PC that will be updated.
@@ -211,7 +203,7 @@ class BPredUnit : public SimObject
                          ThreadID _tid)
             : seqNum(seq_num), pc(instPC), bpHistory(bp_history), RASTarget(0),
               RASIndex(0), tid(_tid), predTaken(pred_taken), usedRAS(0), pushedRAS(0),
-              wasCall(0), wasReturn(0), wasSquashed(0), wasIndirect(0)
+              wasCall(0), wasReturn(0), wasIndirect(0)
         {}
 
         bool operator==(const PredictorHistory &entry) const {
@@ -254,9 +246,6 @@ class BPredUnit : public SimObject
         /** Whether or not the instruction was a return. */
         bool wasReturn;
 
-        /** Whether this instruction has already mispredicted/updated bp */
-        bool wasSquashed;
-
         /** Wether this instruction was an indirect branch */
         bool wasIndirect;
     };