cpu: Fix incorrect speculative branch predictor behavior

When a branch mispredicted gem5 would squash all history after and including
the mispredicted branch.  However, the mispredicted branch is still speculative
and its history is required to rollback state if another, older, branch
mispredicts.  This leads to things like RAS corruption.

src/cpu/pred/2bit_local.hh

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2011 ARM Limited
+ * Copyright (c) 2011, 2014 ARM Limited
  * All rights reserved
  *
  * The license below extends only to copyright in the software and shall
@@ -92,6 +92,9 @@ class LocalBP : public BPredUnit
      */
     void update(Addr branch_addr, bool taken, void *bp_history, bool squashed);
 
+    void retireSquashed(void *bp_history)
+    { assert(bp_history == NULL); }
+
     void squash(void *bp_history)
     { assert(bp_history == NULL); }
 

src/cpu/pred/bpred_unit.hh

@@ -178,6 +178,13 @@ class BPredUnit : public SimObject
      */
     virtual void update(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(void *bp_history) = 0;
 
     /**
      * Updates the BTB with the target of a branch.
@@ -200,7 +207,7 @@ 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)
+              wasCall(0), wasReturn(0), wasSquashed(0)
         {}
 
         bool operator==(const PredictorHistory &entry) const {
@@ -234,7 +241,7 @@ class BPredUnit : public SimObject
         /** Whether or not the RAS was used. */
         bool usedRAS;
 
-        /* Wether or not the RAS was pushed */
+        /* Whether or not the RAS was pushed */
         bool pushedRAS;
 
         /** Whether or not the instruction was a call. */
@@ -242,6 +249,9 @@ class BPredUnit : public SimObject
 
         /** Whether or not the instruction was a return. */
         bool wasReturn;
+
+        /** Whether this instruction has already mispredicted/updated bp */
+        bool wasSquashed;
     };
 
     typedef std::deque<PredictorHistory> History;

src/cpu/pred/bpred_unit_impl.hh

@@ -372,8 +372,12 @@ 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.
-        update(predHist[tid].back().pc, predHist[tid].back().predTaken,
+        if (!predHist[tid].back().wasSquashed) {
-               predHist[tid].back().bpHistory, false);
+            update(predHist[tid].back().pc, predHist[tid].back().predTaken,
+                predHist[tid].back().bpHistory, false);
+        } else {
+            retireSquashed(predHist[tid].back().bpHistory);
+        }
 
         predHist[tid].pop_back();
     }
@@ -465,12 +469,15 @@ BPredUnit::squash(const InstSeqNum &squashed_sn,
 
         update((*hist_it).pc, actually_taken,
                pred_hist.front().bpHistory, true);
+        hist_it->wasSquashed = true;
+
         if (actually_taken) {
             if (hist_it->wasReturn && !hist_it->usedRAS) {
                  DPRINTF(Branch, "[tid: %i] Incorrectly predicted"
                          "  return [sn:%i] PC: %s\n", tid, hist_it->seqNum,
                          hist_it->pc);
                  RAS[tid].pop();
+                 hist_it->usedRAS = true;
             }
 
             DPRINTF(Branch,"[tid: %i] BTB Update called for [sn:%i]"
@@ -488,23 +495,16 @@ BPredUnit::squash(const InstSeqNum &squashed_sn,
                         " to: %i, target: %s.\n", tid,
                         hist_it->RASIndex, hist_it->RASTarget);
                 RAS[tid].restore(hist_it->RASIndex, hist_it->RASTarget);
-
+                hist_it->usedRAS = false;
            } else if (hist_it->wasCall && hist_it->pushedRAS) {
                  //Was a Call but predicated false. Pop RAS here
                  DPRINTF(Branch, "[tid: %i] Incorrectly predicted"
                          "  Call [sn:%i] PC: %s Popping RAS\n", tid,
                          hist_it->seqNum, hist_it->pc);
                  RAS[tid].pop();
+                 hist_it->pushedRAS = false;
            }
         }
-        DPRINTF(Branch, "[tid:%i]: Removing history for [sn:%i]"
-                " PC %s  Actually Taken: %i\n", tid, hist_it->seqNum,
-                hist_it->pc, actually_taken);
-
-        pred_hist.erase(hist_it);
-
-        DPRINTF(Branch, "[tid:%i]: predHist.size(): %i\n", tid,
-                                         predHist[tid].size());
     } else {
         DPRINTF(Branch, "[tid:%i]: [sn:%i] pred_hist empty, can't "
                 "update.\n", tid, squashed_sn);

src/cpu/pred/tournament.cc

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2011 ARM Limited
+ * Copyright (c) 2011, 2014 ARM Limited
  * All rights reserved
  *
  * The license below extends only to copyright in the software and shall
@@ -346,10 +346,10 @@ TournamentBP::update(Addr branch_addr, bool taken, void *bp_history,
                 }
              }
 
+        } else {
+            // We're done with this history, now delete it.
+            delete history;
         }
-        // We're done with this history, now delete it.
-        delete history;
-
     }
 
     assert(local_history_idx < localHistoryTableSize);
@@ -357,6 +357,13 @@ TournamentBP::update(Addr branch_addr, bool taken, void *bp_history,
 
 }
 
+void
+TournamentBP::retireSquashed(void *bp_history)
+{
+    BPHistory *history = static_cast<BPHistory *>(bp_history);
+    delete history;
+}
+
 void
 TournamentBP::squash(void *bp_history)
 {

src/cpu/pred/tournament.hh

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2011 ARM Limited
+ * Copyright (c) 2011, 2014 ARM Limited
  * All rights reserved
  *
  * The license below extends only to copyright in the software and shall
@@ -104,6 +104,8 @@ class TournamentBP : public BPredUnit
      */
     void update(Addr branch_addr, bool taken, void *bp_history, bool squashed);
 
+    void retireSquashed(void *bp_history);
+
     /**
      * Restores the global branch history on a squash.
      * @param bp_history Pointer to the BPHistory object that has the