TournamentBP: Fix some bugs with table sizes and counters

globalHistoryBits, globalPredictorSize, and choicePredictorSize are decoupled.
globalHistoryBits controls how much history is kept, global and choice
predictor sizes control how much of that history is used when accessing
predictor tables. This way, global and choice predictors can actually be
different sizes, and it is no longer possible to walk off the predictor arrays
and cause a seg fault.

There are now individual thresholds for choice, global, and local saturating
counters, so that taken/not taken decisions are correct even when the
predictors' counters' sizes are different.

The interface for localPredictorSize has been removed from TournamentBP because
the value can be calculated from localHistoryBits.

Committed by: Nilay Vaish <nilay@cs.wisc.edu>

2 files changed, 83 insertions, 55 deletions

diff --git a/src/cpu/pred/tournament.cc b/src/cpu/pred/tournament.cc
index ba2b1decb..f36d30ae5 100644
--- a/src/cpu/pred/tournament.cc
+++ b/src/cpu/pred/tournament.cc
@@ -40,11 +40,11 @@
  * Authors: Kevin Lim
  */
 
+#include "base/bitfield.hh"
 #include "base/intmath.hh"
 #include "cpu/pred/tournament.hh"
 
-TournamentBP::TournamentBP(unsigned _localPredictorSize,
-                           unsigned _localCtrBits,
+TournamentBP::TournamentBP(unsigned _localCtrBits,
                            unsigned _localHistoryTableSize,
                            unsigned _localHistoryBits,
                            unsigned _globalPredictorSize,
@@ -53,28 +53,25 @@ TournamentBP::TournamentBP(unsigned _localPredictorSize,
                            unsigned _choicePredictorSize,
                            unsigned _choiceCtrBits,
                            unsigned _instShiftAmt)
-    : localPredictorSize(_localPredictorSize),
-      localCtrBits(_localCtrBits),
+    : localCtrBits(_localCtrBits),
       localHistoryTableSize(_localHistoryTableSize),
       localHistoryBits(_localHistoryBits),
       globalPredictorSize(_globalPredictorSize),
       globalCtrBits(_globalCtrBits),
       globalHistoryBits(_globalHistoryBits),
-      choicePredictorSize(_globalPredictorSize),
+      choicePredictorSize(_choicePredictorSize),
       choiceCtrBits(_choiceCtrBits),
       instShiftAmt(_instShiftAmt)
 {
-    if (!isPowerOf2(localPredictorSize)) {
-        fatal("Invalid local predictor size!\n");
-    }
+    localPredictorSize = ULL(1) << localHistoryBits;
 
-    //Setup the array of counters for the local predictor
+    //Set up the array of counters for the local predictor
     localCtrs.resize(localPredictorSize);
 
     for (int i = 0; i < localPredictorSize; ++i)
         localCtrs[i].setBits(localCtrBits);
 
-    localPredictorMask = floorPow2(localPredictorSize) - 1;
+    localPredictorMask = mask(localHistoryBits);
 
     if (!isPowerOf2(localHistoryTableSize)) {
         fatal("Invalid local history table size!\n");
@@ -86,9 +83,6 @@ TournamentBP::TournamentBP(unsigned _localPredictorSize,
     for (int i = 0; i < localHistoryTableSize; ++i)
         localHistoryTable[i] = 0;
 
-    // Setup the local history mask
-    localHistoryMask = (1 << localHistoryBits) - 1;
-
     if (!isPowerOf2(globalPredictorSize)) {
         fatal("Invalid global predictor size!\n");
     }
@@ -101,21 +95,45 @@ TournamentBP::TournamentBP(unsigned _localPredictorSize,
 
     //Clear the global history
     globalHistory = 0;
-    // Setup the global history mask
-    globalHistoryMask = (1 << globalHistoryBits) - 1;
+    // Set up the global history mask
+    // this is equivalent to mask(log2(globalPredictorSize)
+    globalHistoryMask = globalPredictorSize - 1;
 
     if (!isPowerOf2(choicePredictorSize)) {
         fatal("Invalid choice predictor size!\n");
     }
 
+    // Set up choiceHistoryMask
+    // this is equivalent to mask(log2(choicePredictorSize)
+    choiceHistoryMask = choicePredictorSize - 1;
+
     //Setup the array of counters for the choice predictor
     choiceCtrs.resize(choicePredictorSize);
 
     for (int i = 0; i < choicePredictorSize; ++i)
         choiceCtrs[i].setBits(choiceCtrBits);
 
-    // @todo: Allow for different thresholds between the predictors.
-    threshold = (1 << (localCtrBits - 1)) - 1;
+    //Set up historyRegisterMask
+    historyRegisterMask = mask(globalHistoryBits);
+
+    //Check that predictors don't use more bits than they have available
+    if (globalHistoryMask > historyRegisterMask) {
+        fatal("Global predictor too large for global history bits!\n");
+    }
+    if (choiceHistoryMask > historyRegisterMask) {
+        fatal("Choice predictor too large for global history bits!\n");
+    }
+
+    if (globalHistoryMask < historyRegisterMask &&
+        choiceHistoryMask < historyRegisterMask) {
+        inform("More global history bits than required by predictors\n");
+    }
+
+    // Set thresholds for the three predictors' counters
+    // This is equivalent to (2^(Ctr))/2 - 1
+    localThreshold  = (ULL(1) << (localCtrBits  - 1)) - 1;
+    globalThreshold = (ULL(1) << (globalCtrBits - 1)) - 1;
+    choiceThreshold = (ULL(1) << (choiceCtrBits - 1)) - 1;
 }
 
 inline
@@ -131,7 +149,7 @@ void
 TournamentBP::updateGlobalHistTaken()
 {
     globalHistory = (globalHistory << 1) | 1;
-    globalHistory = globalHistory & globalHistoryMask;
+    globalHistory = globalHistory & historyRegisterMask;
 }
 
 inline
@@ -139,7 +157,7 @@ void
 TournamentBP::updateGlobalHistNotTaken()
 {
     globalHistory = (globalHistory << 1);
-    globalHistory = globalHistory & globalHistoryMask;
+    globalHistory = globalHistory & historyRegisterMask;
 }
 
 inline
@@ -163,8 +181,8 @@ 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 Global History to Not Taken (clear LSB)
+    globalHistory &= (historyRegisterMask & ~ULL(1));
     //Update Local History to Not Taken
     localHistoryTable[local_history_idx] =
        localHistoryTable[local_history_idx] & (localPredictorMask & ~ULL(1));
@@ -184,13 +202,15 @@ TournamentBP::lookup(Addr &branch_addr, void * &bp_history)
     local_history_idx = calcLocHistIdx(branch_addr);
     local_predictor_idx = localHistoryTable[local_history_idx]
         & localPredictorMask;
-    local_prediction = localCtrs[local_predictor_idx].read() > threshold;
+    local_prediction = localCtrs[local_predictor_idx].read() > localThreshold;
 
     //Lookup in the global predictor to get its branch prediction
-    global_prediction = globalCtrs[globalHistory].read() > threshold;
+    global_prediction =
+      globalCtrs[globalHistory & globalHistoryMask].read() > globalThreshold;
 
     //Lookup in the choice predictor to see which one to use
-    choice_prediction = choiceCtrs[globalHistory].read() > threshold;
+    choice_prediction =
+      choiceCtrs[globalHistory & choiceHistoryMask].read() > choiceThreshold;
 
     // Create BPHistory and pass it back to be recorded.
     BPHistory *history = new BPHistory;
@@ -201,9 +221,7 @@ TournamentBP::lookup(Addr &branch_addr, void * &bp_history)
     history->localHistory = local_predictor_idx;
     bp_history = (void *)history;
 
-    assert(globalHistory < globalPredictorSize &&
-           local_history_idx < localHistoryTableSize &&
-           local_predictor_idx < localPredictorSize);
+    assert(local_history_idx < localHistoryTableSize);
 
     // Commented code is for doing speculative update of counters and
     // all histories.
@@ -283,10 +301,12 @@ TournamentBP::update(Addr &branch_addr, bool taken, void *bp_history,
                  // If the local prediction matches the actual outcome,
                  // decerement the counter.  Otherwise increment the
                  // counter.
+                 unsigned choice_predictor_idx =
+                   history->globalHistory & choiceHistoryMask;
                  if (history->localPredTaken == taken) {
-                     choiceCtrs[history->globalHistory].decrement();
+                     choiceCtrs[choice_predictor_idx].decrement();
                  } else if (history->globalPredTaken == taken) {
-                     choiceCtrs[history->globalHistory].increment();
+                     choiceCtrs[choice_predictor_idx].increment();
                  }
 
              }
@@ -295,13 +315,15 @@ 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 global_predictor_idx =
+               history->globalHistory & globalHistoryMask;
              if (taken) {
-                  globalCtrs[history->globalHistory].increment();
+                  globalCtrs[global_predictor_idx].increment();
                   if (old_local_pred_valid) {
                           localCtrs[old_local_pred_index].increment();
                   }
              } else {
-                  globalCtrs[history->globalHistory].decrement();
+                  globalCtrs[global_predictor_idx].decrement();
                   if (old_local_pred_valid) {
                           localCtrs[old_local_pred_index].decrement();
                   }
@@ -310,14 +332,14 @@ TournamentBP::update(Addr &branch_addr, bool taken, void *bp_history,
         if (squashed) {
              if (taken) {
                 globalHistory = (history->globalHistory << 1) | 1;
-                globalHistory = globalHistory & globalHistoryMask;
+                globalHistory = globalHistory & historyRegisterMask;
                 if (old_local_pred_valid) {
                     localHistoryTable[local_history_idx] =
                      (history->localHistory << 1) | 1;
                 }
              } else {
                 globalHistory = (history->globalHistory << 1);
-                globalHistory = globalHistory & globalHistoryMask;
+                globalHistory = globalHistory & historyRegisterMask;
                 if (old_local_pred_valid) {
                      localHistoryTable[local_history_idx] =
                      history->localHistory << 1;
@@ -330,9 +352,7 @@ TournamentBP::update(Addr &branch_addr, bool taken, void *bp_history,
 
     }
 
-    assert(globalHistory < globalPredictorSize &&
-           local_history_idx < localHistoryTableSize &&
-           local_predictor_idx < localPredictorSize);
+    assert(local_history_idx < localHistoryTableSize);
 
 
 }
diff --git a/src/cpu/pred/tournament.hh b/src/cpu/pred/tournament.hh
index f9df2a61d..35cfd8455 100644
--- a/src/cpu/pred/tournament.hh
+++ b/src/cpu/pred/tournament.hh
@@ -63,8 +63,7 @@ class TournamentBP
     /**
      * Default branch predictor constructor.
      */
-    TournamentBP(unsigned localPredictorSize,
-                 unsigned localCtrBits,
+    TournamentBP(unsigned localCtrBits,
                  unsigned localHistoryTableSize,
                  unsigned localHistoryBits,
                  unsigned globalPredictorSize,
@@ -181,10 +180,10 @@ class TournamentBP
     /** Local counters. */
     std::vector<SatCounter> localCtrs;
 
-    /** Size of the local predictor. */
+    /** Number of counters in the local predictor. */
     unsigned localPredictorSize;
 
-    /** Mask to get the proper index bits into the predictor. */
+    /** Mask to truncate values stored in the local history table. */
     unsigned localPredictorMask;
 
     /** Number of bits of the local predictor's counters. */
@@ -193,42 +192,49 @@ class TournamentBP
     /** Array of local history table entries. */
     std::vector<unsigned> localHistoryTable;
 
-    /** Size of the local history table. */
+    /** Number of entries in the local history table. */
     unsigned localHistoryTableSize;
 
-    /** Number of bits for each entry of the local history table.
-     *  @todo Doesn't this come from the size of the local predictor?
-     */
+    /** Number of bits for each entry of the local history table. */
     unsigned localHistoryBits;
 
-    /** Mask to get the proper local history. */
-    unsigned localHistoryMask;
-
     /** Array of counters that make up the global predictor. */
     std::vector<SatCounter> globalCtrs;
 
-    /** Size of the global predictor. */
+    /** Number of entries in the global predictor. */
     unsigned globalPredictorSize;
 
     /** Number of bits of the global predictor's counters. */
     unsigned globalCtrBits;
 
-    /** Global history register. */
+    /** Global history register. Contains as much history as specified by
+     *  globalHistoryBits. Actual number of bits used is determined by
+     *  globalHistoryMask and choiceHistoryMask. */
     unsigned globalHistory;
 
-    /** Number of bits for the global history. */
+    /** Number of bits for the global history. Determines maximum number of
+        entries in global and choice predictor tables. */
     unsigned globalHistoryBits;
 
-    /** Mask to get the proper global history. */
+    /** Mask to apply to globalHistory to access global history table.
+     *  Based on globalPredictorSize.*/
     unsigned globalHistoryMask;
 
+    /** Mask to apply to globalHistory to access choice history table.
+     *  Based on choicePredictorSize.*/
+    unsigned choiceHistoryMask;
+
+    /** Mask to control how much history is stored. All of it might not be
+     *  used. */
+    unsigned historyRegisterMask;
+
     /** Array of counters that make up the choice predictor. */
     std::vector<SatCounter> choiceCtrs;
 
-    /** Size of the choice predictor (identical to the global predictor). */
+    /** Number of entries in the choice predictor. */
     unsigned choicePredictorSize;
 
-    /** Number of bits of the choice predictor's counters. */
+    /** Number of bits in the choice predictor's counters. */
     unsigned choiceCtrBits;
 
     /** Number of bits to shift the instruction over to get rid of the word
@@ -236,10 +242,12 @@ class TournamentBP
      */
     unsigned instShiftAmt;
 
-    /** Threshold for the counter value; above the threshold is taken,
+    /** Thresholds for the counter value; above the threshold is taken,
      *  equal to or below the threshold is not taken.
      */
-    unsigned threshold;
+    unsigned localThreshold;
+    unsigned globalThreshold;
+    unsigned choiceThreshold;
 };
 
 #endif // __CPU_O3_TOURNAMENT_PRED_HH__