BP: Fix several Branch Predictor issues.

1. Updates the Branch Predictor correctly to the state
   just after a mispredicted branch, if a squash occurs.
2. If a BTB does not find an entry, the branch is predicted not taken.
   The global history is modified to correctly reflect this prediction.
3. Local history is now updated at the fetch stage instead of
   execute stage.
4. In the Update stage of the branch predictor the local predictors are
   now correctly updated according to the state of local history during
   fetch stage.

This patch also improves performance by as much as 17% on some benchmarks

1 files changed, 53 insertions, 33 deletions

diff --git a/src/cpu/pred/tournament.cc b/src/cpu/pred/tournament.cc
index 9608dc011..3adafb608 100644
--- a/src/cpu/pred/tournament.cc
+++ b/src/cpu/pred/tournament.cc
@@ -1,4 +1,16 @@
 /*
+ * Copyright (c) 2011 ARM Limited
+ * All rights reserved
+ *
+ * The license below extends only to copyright in the software and shall
+ * not be construed as granting a license to any other intellectual
+ * property including but not limited to intellectual property relating
+ * to a hardware implementation of the functionality of the software
+ * licensed hereunder.  You may use the software subject to the license
+ * terms below provided that you ensure that this notice is replicated
+ * unmodified and in its entirety in all distributions of the software,
+ * modified or unmodified, in source code or in binary form.
+ *
  * Copyright (c) 2004-2006 The Regents of The University of Michigan
  * All rights reserved.
  *
@@ -146,6 +158,18 @@ TournamentBP::updateLocalHistNotTaken(unsigned local_history_idx)
         (localHistoryTable[local_history_idx] << 1);
 }
 
+
+void
+TournamentBP::BTBUpdate(Addr &branch_addr, void * &bp_history)
+{
+    unsigned local_history_idx = calcLocHistIdx(branch_addr);
+    //Update Global History to Not Taken
+    globalHistory = globalHistory & (globalHistoryMask - 1);
+    //Update Local History to Not Taken
+    localHistoryTable[local_history_idx] =
+       localHistoryTable[local_history_idx] & (localPredictorMask - 1);
+}
+
 bool
 TournamentBP::lookup(Addr &branch_addr, void * &bp_history)
 {
@@ -174,6 +198,7 @@ TournamentBP::lookup(Addr &branch_addr, void * &bp_history)
     history->localPredTaken = local_prediction;
     history->globalPredTaken = global_prediction;
     history->globalUsed = choice_prediction;
+    history->localHistory = local_predictor_idx;
     bp_history = (void *)history;
 
     assert(globalHistory < globalPredictorSize &&
@@ -184,30 +209,22 @@ TournamentBP::lookup(Addr &branch_addr, void * &bp_history)
     // all histories.
     if (choice_prediction) {
         if (global_prediction) {
-//            updateHistoriesTaken(local_history_idx);
-//            globalCtrs[globalHistory].increment();
-//            localCtrs[local_history_idx].increment();
             updateGlobalHistTaken();
+            updateLocalHistTaken(local_history_idx);
             return true;
         } else {
-//            updateHistoriesNotTaken(local_history_idx);
-//            globalCtrs[globalHistory].decrement();
-//            localCtrs[local_history_idx].decrement();
             updateGlobalHistNotTaken();
+            updateLocalHistNotTaken(local_history_idx);
             return false;
         }
     } else {
         if (local_prediction) {
-//            updateHistoriesTaken(local_history_idx);
-//            globalCtrs[globalHistory].increment();
-//            localCtrs[local_history_idx].increment();
             updateGlobalHistTaken();
+            updateLocalHistTaken(local_history_idx);
             return true;
         } else {
-//            updateHistoriesNotTaken(local_history_idx);
-//            globalCtrs[globalHistory].decrement();
-//            localCtrs[local_history_idx].decrement();
             updateGlobalHistNotTaken();
+            updateLocalHistNotTaken(local_history_idx);
             return false;
         }
     }
@@ -222,16 +239,18 @@ TournamentBP::uncondBr(void * &bp_history)
     history->localPredTaken = true;
     history->globalPredTaken = true;
     history->globalUsed = true;
+    history->localHistory = invalidPredictorIndex;
     bp_history = static_cast<void *>(history);
 
     updateGlobalHistTaken();
 }
 
 void
-TournamentBP::update(Addr &branch_addr, bool taken, void *bp_history)
+TournamentBP::update(Addr &branch_addr, bool taken, void *bp_history,
+                     bool squashed)
 {
     unsigned local_history_idx;
-    unsigned local_predictor_idx;
+    unsigned local_predictor_idx M5_VAR_USED;
     unsigned local_predictor_hist;
 
     // Get the local predictor's current prediction
@@ -259,33 +278,34 @@ TournamentBP::update(Addr &branch_addr, bool taken, void *bp_history)
         // resolution of the branch.  Global history is updated
         // speculatively and restored upon squash() calls, so it does not
         // need to be updated.
+        unsigned old_local_pred_index = history->localHistory
+                 & localPredictorMask;
         if (taken) {
-               localCtrs[local_predictor_idx].increment();
                globalCtrs[history->globalHistory].increment();
-
-               updateLocalHistTaken(local_history_idx);
+               if (old_local_pred_index != invalidPredictorIndex) {
+                      localCtrs[old_local_pred_index].increment();
+               }
         } else {
-               localCtrs[local_predictor_idx].decrement();
                globalCtrs[history->globalHistory].decrement();
-
-               updateLocalHistNotTaken(local_history_idx);
+               if (old_local_pred_index != invalidPredictorIndex) {
+                      localCtrs[old_local_pred_index].decrement();
+               }
        }
 
-       bool mispredict = false;
-
-       //global predictor used and mispredicted
-       if (history->globalUsed && history->globalPredTaken != taken)
-           mispredict = true;
-       //local predictor used and mispredicted
-       else if (!history->globalUsed && history->localPredTaken != taken)
-           mispredict = true;
-
-       if (mispredict) {
+       if (squashed) {
            if (taken) {
-              globalHistory = globalHistory | 1;
+              globalHistory = (history->globalHistory << 1) | 1;
+              globalHistory = globalHistory & globalHistoryMask;
+              if (history->localHistory != invalidPredictorIndex)
+                  localHistoryTable[local_history_idx] =
+                     (history->localHistory << 1) | 1;
            } else {
-              unsigned mask = globalHistoryMask - 1;
-              globalHistory = globalHistory & mask;
+              globalHistory = (history->globalHistory << 1);
+              globalHistory = globalHistory & globalHistoryMask;
+              if (history->localHistory != invalidPredictorIndex) {
+                  localHistoryTable[local_history_idx] =
+                     history->localHistory << 1;
+              }
            }
 
         }