cpu: Fixes on the loop predictor part of LTAGE

Fixed the following fields of the loop predictor entries as described on
the LTAGE paper:
 - Age counter (it was 3 bits and it should be 8 bits)
 - Tag (it was 16 bits and it should be 14 bits). Also some times it used
   int variables and some times uint16_t, leading to wrong behaviour
 - Confidence counter (it was 2 bits ins some parts of the code and 3 bits
   in some other parts. It should be 2 bits)
 - Iteration counters (they were 16 bits and they should be 14 bits)
All the new sizes are now configurable

Change-Id: I8884c7454c1e510b65160eb4d5749d3259d34096
Signed-off-by: Pau Cabre <pau.cabre@metempsy.com>
Reviewed-on: https://gem5-review.googlesource.com/c/14216
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>

diff --git a/src/cpu/pred/BranchPredictor.py b/src/cpu/pred/BranchPredictor.py
index 1eeecde9531dc31f7e470814798cafd312a55b38..a42819419c76562ec7e4179abc98035b6199f5ba 100644 (file)
--- a/src/cpu/pred/BranchPredictor.py
+++ b/src/cpu/pred/BranchPredictor.py
@@ -106,3 +106,9 @@ class LTAGE(BranchPredictor):
     maxHist = Param.Unsigned(640, "Maximum history size of LTAGE")
     minTagWidth = Param.Unsigned(7, "Minimum tag size in tag tables")
 
+    loopTableAgeBits = Param.Unsigned(8, "Number of age bits per loop entry")
+    loopTableConfidenceBits = Param.Unsigned(2,
+            "Number of confidence bits per loop entry")
+    loopTableTagBits = Param.Unsigned(14, "Number of tag bits per loop entry")
+    loopTableIterBits = Param.Unsigned(14, "Nuber of iteration bits per loop")
+

diff --git a/src/cpu/pred/ltage.cc b/src/cpu/pred/ltage.cc
index 8d20d50568ad1d8dbd37936e0e85c34e24da5927..85ae2b3042c0b38cd97e83417cb5c8c77729e296 100644 (file)
--- a/src/cpu/pred/ltage.cc
+++ b/src/cpu/pred/ltage.cc
@@ -59,8 +59,20 @@ LTAGE::LTAGE(const LTAGEParams *params)
     minHist(params->minHist),
     maxHist(params->maxHist),
     minTagWidth(params->minTagWidth),
+    loopTableAgeBits(params->loopTableAgeBits),
+    loopTableConfidenceBits(params->loopTableConfidenceBits),
+    loopTableTagBits(params->loopTableTagBits),
+    loopTableIterBits(params->loopTableIterBits),
+    confidenceThreshold((1 << loopTableConfidenceBits) - 1),
+    loopTagMask((1 << loopTableTagBits) - 1),
+    loopNumIterMask((1 << loopTableIterBits) - 1),
     threadHistory(params->numThreads)
 {
+    // we use uint16_t type for these vales, so they cannot be more than
+    // 16 bits
+    assert(loopTableTagBits <= 16);
+    assert(loopTableIterBits <= 16);
+
     assert(params->histBufferSize > params->maxHist * 2);
     useAltPredForNewlyAllocated = 0;
     logTick = 19;
@@ -212,6 +224,20 @@ LTAGE::ctrUpdate(int8_t & ctr, bool taken, int nbits)
     }
 }
 
+// Up-down unsigned saturating counter
+void
+LTAGE::unsignedCtrUpdate(uint8_t & ctr, bool up, unsigned nbits)
+{
+    assert(nbits <= sizeof(uint8_t) << 3);
+    if (up) {
+        if (ctr < ((1 << nbits) - 1))
+            ctr++;
+    } else {
+        if (ctr)
+            ctr--;
+    }
+}
+
 // Bimodal prediction
 bool
 LTAGE::getBimodePred(Addr pc, BranchInfo* bi) const
@@ -248,12 +274,13 @@ LTAGE::getLoop(Addr pc, BranchInfo* bi) const
     bi->loopHit = -1;
     bi->loopPredValid = false;
     bi->loopIndex = lindex(pc);
-    bi->loopTag = ((pc) >> (instShiftAmt + logSizeLoopPred - 2));
+    bi->loopTag = ((pc) >> (instShiftAmt + logSizeLoopPred - 2)) & loopTagMask;
 
     for (int i = 0; i < 4; i++) {
         if (ltable[bi->loopIndex + i].tag == bi->loopTag) {
             bi->loopHit = i;
-            bi->loopPredValid = (ltable[bi->loopIndex + i].confidence >= 3);
+            bi->loopPredValid =
+                ltable[bi->loopIndex + i].confidence == confidenceThreshold;
             bi->currentIter = ltable[bi->loopIndex + i].currentIterSpec;
             if (ltable[bi->loopIndex + i].currentIterSpec + 1 ==
                 ltable[bi->loopIndex + i].numIter) {
@@ -274,7 +301,8 @@ LTAGE::specLoopUpdate(Addr pc, bool taken, BranchInfo* bi)
         if (taken != ltable[index].dir) {
             ltable[index].currentIterSpec = 0;
         } else {
-            ltable[index].currentIterSpec++;
+            ltable[index].currentIterSpec =
+                (ltable[index].currentIterSpec + 1) & loopNumIterMask;
         }
     }
 }
@@ -295,12 +323,12 @@ LTAGE::loopUpdate(Addr pc, bool taken, BranchInfo* bi)
                 return;
             } else if (bi->loopPred != bi->tagePred) {
                 DPRINTF(LTage, "Loop Prediction success:%lx\n",pc);
-                if (ltable[idx].age < 7)
-                    ltable[idx].age++;
+                unsignedCtrUpdate(ltable[idx].age, true, loopTableAgeBits);
             }
         }
 
-        ltable[idx].currentIter++;
+        ltable[idx].currentIter =
+            (ltable[idx].currentIter + 1) & loopNumIterMask;
         if (ltable[idx].currentIter > ltable[idx].numIter) {
             ltable[idx].confidence = 0;
             if (ltable[idx].numIter != 0) {
@@ -315,9 +343,8 @@ LTAGE::loopUpdate(Addr pc, bool taken, BranchInfo* bi)
             if (ltable[idx].currentIter == ltable[idx].numIter) {
                 DPRINTF(LTage, "Loop End predicted successfully:%lx\n", pc);
 
-                if (ltable[idx].confidence < 7) {
-                    ltable[idx].confidence++;
-                }
+                unsignedCtrUpdate(ltable[idx].confidence, true,
+                                  loopTableConfidenceBits);
                 //just do not predict when the loop count is 1 or 2
                 if (ltable[idx].numIter < 3) {
                     // free the entry
@@ -355,7 +382,7 @@ LTAGE::loopUpdate(Addr pc, bool taken, BranchInfo* bi)
                 ltable[idx].dir = !taken;
                 ltable[idx].tag = bi->loopTag;
                 ltable[idx].numIter = 0;
-                ltable[idx].age = 7;
+                ltable[idx].age = (1 << loopTableAgeBits) - 1;
                 ltable[idx].confidence = 0;
                 ltable[idx].currentIter = 1;
                 break;

diff --git a/src/cpu/pred/ltage.hh b/src/cpu/pred/ltage.hh
index a810fb5fd9bae0eb4d88bcacefafe6375c2c976f..2119156b7c313fb0f9338aaa38c57a5a1e411dfc 100644 (file)
--- a/src/cpu/pred/ltage.hh
+++ b/src/cpu/pred/ltage.hh
@@ -135,7 +135,7 @@ class LTAGE: public BPredUnit
         int altBank;
         int altBankIndex;
         int bimodalIndex;
-        int loopTag;
+        uint16_t loopTag;
         uint16_t currentIter;
 
         bool tagePred;
@@ -237,6 +237,15 @@ class LTAGE: public BPredUnit
      */
     void ctrUpdate(int8_t & ctr, bool taken, int nbits);
 
+    /**
+     * Updates an unsigned counter based on up/down parameter
+     * @param ctr Reference to counter to update.
+     * @param up Boolean indicating if the counter is incremented/decremented
+     * If true it is incremented, if false it is decremented
+     * @param nbits Counter width.
+     */
+    void unsignedCtrUpdate(uint8_t & ctr, bool up, unsigned nbits);
+
     /**
      * Get a branch prediction from the bimodal
      * predictor.
@@ -355,6 +364,14 @@ class LTAGE: public BPredUnit
     const unsigned minHist;
     const unsigned maxHist;
     const unsigned minTagWidth;
+    const unsigned loopTableAgeBits;
+    const unsigned loopTableConfidenceBits;
+    const unsigned loopTableTagBits;
+    const unsigned loopTableIterBits;
+
+    const uint8_t confidenceThreshold;
+    const uint16_t loopTagMask;
+    const uint16_t loopNumIterMask;
 
     std::vector<bool> btablePrediction;
     std::vector<bool> btableHysteresis;