cpu-o3: add 2Fetch features

- Introduced support for 2Fetch features in the branch predictor and Fetch Stage.
- Enhanced the FetchTargetQueue to manage next FTQ entries for 2Fetch functionality.
- Now if 2 fetchBlock in the same 64 byte fetchBuffer, it could be 2 fetched at the same cycle.

Change-Id: I3b112cc844c485d81cea1f4ed0ff221cb37d2782

Author: jensen-yan

src/cpu/o3/fetch.cc

@@ -2070,7 +2070,7 @@ Fetch::performInstructionFetch(ThreadID tid)
     // Main instruction fetch loop - process until fetch width or other limits
     StallReason stall = StallReason::NoStall;
     while (numInst < fetchWidth && fetchQueue[tid].size() < fetchQueueSize &&
-           !predictedBranch && !ftqEmpty() && !waitForVsetvl) {
+           !shouldStopFetchThisCycle(predictedBranch)) {
 
         // Check memory needs and supply bytes to decoder if required
         stall = checkMemoryNeeds(tid, pc_state, curMacroop);

src/cpu/o3/fetch.hh

@@ -514,8 +514,21 @@ class Fetch
     /** Profile the reasons of fetch stall. */
     void profileStall(ThreadID tid);
 
-
-    bool ftqEmpty() { return isDecoupledFrontend() && usedUpFetchTargets; }
+    /**
+     * Decide whether fetch should stop for this cycle based on frontend mode.
+     * - Decoupled frontend stops when the current FTQ supply is exhausted.
+     * - Non-decoupled frontend stops when a branch is predicted taken.
+     */
+    bool shouldStopFetchThisCycle(bool predictedBranch)
+    {
+        if (waitForVsetvl) {
+            return true;
+        }
+        if (isDecoupledFrontend()) {
+            return usedUpFetchTargets;
+        }
+        return predictedBranch;
+    }
 
     /** Set the reasons of all fetch stalls. */
     void setAllFetchStalls(StallReason stall);

src/cpu/o3/fetch.md

@@ -341,7 +341,16 @@ bool isFTBPred() { return branchPred->isFTB(); }
 bool isBTBPred() { return branchPred->isBTB(); }  // 主要使用的预测器类型
 
 // Track if FTQ is empty
-bool ftqEmpty() { return isDecoupledFrontend() && usedUpFetchTargets; }
+bool shouldStopFetchThisCycle(bool predictedBranch)
+{
+    if (waitForVsetvl) {
+        return true;
+    }
+    if (isDecoupledFrontend()) {
+        return usedUpFetchTargets;
+    }
+    return predictedBranch;
+}
 ```
 
 ### DecoupledBPUWithBTB 工作流程：
@@ -573,7 +582,7 @@ void fetch(bool &status_change) {
 void performInstructionFetch(ThreadID tid, Addr fetch_addr, bool &status_change) {
     // 主循环: 处理直到fetch宽度或其他限制
     while (numInst < fetchWidth && fetchQueue[tid].size() < fetchQueueSize &&
-           !predictedBranch && !ftqEmpty() && !waitForVsetvl) {
+           !shouldStopFetchThisCycle(predictedBranch)) {
 
         // 1. 检查内存需求并供给decoder
         stall = checkMemoryNeeds(tid, this_pc, curMacroop);

src/cpu/pred/BranchPredictor.py

@@ -1181,3 +1181,7 @@ class DecoupledBPUWithBTB(BranchPredictor):
     enableLoopPredictor = Param.Bool(False, "Use loop predictor to predict loop exit")
     enableJumpAheadPredictor = Param.Bool(False, "Use jump ahead predictor to skip no-need-to-predict blocks")
     resolveBlockThreshold = Param.Unsigned(8, "Consecutive resolve dequeue failures before blocking prediction once")
+
+    enable2Taken = Param.Bool(False, "Enable 2taken feature")
+    enable2Fetch = Param.Bool(False, "Enable 2fetch feature")
+    maxFetchBytesPerCycle = Param.Unsigned(64, "Maximum fetch bytes per cycle for 2fetch")

src/cpu/pred/btb/decoupled_bpred.cc

@@ -1,5 +1,6 @@
 #include "cpu/pred/btb/decoupled_bpred.hh"
 
+#include <algorithm>
 #include <array>
 
 #include "base/debug_helper.hh"
@@ -27,6 +28,7 @@ DecoupledBPUWithBTB::DecoupledBPUWithBTB(const DecoupledBPUWithBTBParams &p)
       enableLoopBuffer(p.enableLoopBuffer),
       enableLoopPredictor(p.enableLoopPredictor),
       enableJumpAheadPredictor(p.enableJumpAheadPredictor),
+      enable2Taken(p.enable2Taken),
       fetchTargetQueue(p.ftq_size),
       fetchStreamQueueSize(p.fsq_size),
       predictWidth(p.predictWidth),
@@ -45,7 +47,9 @@ DecoupledBPUWithBTB::DecoupledBPUWithBTB(const DecoupledBPUWithBTBParams &p)
       numStages(p.numStages),
       historyManager(16), // TODO: fix this
       resolveBlockThreshold(p.resolveBlockThreshold),
-      dbpBtbStats(this, p.numStages, p.fsq_size, maxInstsNum)
+      dbpBtbStats(this, p.numStages, p.fsq_size, maxInstsNum),
+      enable2Fetch(p.enable2Fetch),
+      maxFetchBytesPerCycle(p.maxFetchBytesPerCycle)
 {
     if (bpDBSwitches.size() > 0) {
         initDB();
@@ -160,7 +164,7 @@ DecoupledBPUWithBTB::tick()
         // Clear each predictor's output
         for (int i = 0; i < numStages; i++) {
             predsOfEachStage[i].btbEntries.clear();
-        }
+}
     }
 
     if (bpuState == BpuState::PREDICTION_OUTSTANDING && numOverrideBubbles > 0) {
@@ -436,7 +440,20 @@ DecoupledBPUWithBTB::decoupledPredict(const StaticInstPtr &inst,
     // Increment instruction counter for current FTQ entry
     currentFtqEntryInstNum++;
     if (run_out_of_this_entry) {
+        // Check if 2fetch is enabled, not fetched first FTQ yet, and if we can extend to the next FTQ
+        // NEW: 2Fetch extension check - before processing completion
+        dbpBtbStats.fetch2Attempts++;
+        if (enable2Fetch && !has1Fetched && canExtendToNextFTQ(pc, target_to_fetch)) {
+            DPRINTF(DecoupleBP, "2Fetch: extending to next FTQ in same cycle\n");
+            has1Fetched = true;
+            processFetchTargetCompletion(target_to_fetch);
+            extendToNextFTQ(pc);
+            // first fetchBlock is always taken, do not run out of FTQ now
+            return std::make_pair(true, false);
+        }
+
         processFetchTargetCompletion(target_to_fetch);
+        has1Fetched = false;    // reset 2fetch flag
     }
 
     DPRINTF(DecoupleBP, "Predict it %staken to %#lx\n", taken ? "" : "not ",
@@ -1372,6 +1389,93 @@ DecoupledBPUWithBTB::recoverHistoryForSquash(
 }
 
 
+// NEW: 2Fetch support methods implementation
+
+/**
+ * @brief Check if we can extend to next FTQ entry for 2fetch
+ *
+ * @param current_pc Current program counter
+ * @param current_ftq Current FTQ entry that is being completed
+ * @return true if 2fetch extension is possible
+ */
+bool
+DecoupledBPUWithBTB::canExtendToNextFTQ(const PCStateBase &current_pc, const FtqEntry &current_ftq)
+{
+    // Early exit if 2fetch is disabled
+    if (!enable2Fetch) {
+        return false;
+    }
+
+    if (!current_ftq.taken) {
+        DPRINTF(DecoupleBP, "2Fetch rejected: current FTQ is not taken\n");
+        dbpBtbStats.fetch2FirstNotTaken++;
+        return false;
+    }
+
+    // Check if next FTQ entry is available
+    if (!fetchTargetQueue.hasNext()) {
+        DPRINTF(DecoupleBP, "2Fetch rejected: no next FTQ entry available\n");
+        dbpBtbStats.fetch2NoNextFTQ++;
+        return false;
+    }
+
+    // Get next FTQ entry (without consuming it)
+    const auto &next_ftq = fetchTargetQueue.peekNext();
+    // current_ftq is passed as parameter
+
+    // Check if current PC is the jump target of the next FTQ start
+    if (current_pc.instAddr() != next_ftq.startPC) {
+        DPRINTF(DecoupleBP, "2Fetch rejected: PC %#x not at next FTQ start %#x\n",
+                current_pc.instAddr(), next_ftq.startPC);
+        dbpBtbStats.fetch2FirstNotAtStart++;
+        return false;
+    }
+
+    // Check if both FTQs fit in maxFetchBytesPerCycle window
+    Addr span = next_ftq.endPC - current_ftq.startPC;
+    if (span > maxFetchBytesPerCycle) {
+        DPRINTF(DecoupleBP, "2Fetch rejected: span %d exceeds %d bytes\n",
+                span, maxFetchBytesPerCycle);
+        dbpBtbStats.fetch2SpanTooLarge++;
+        return false;
+    }
+
+    DPRINTF(DecoupleBP, "2Fetch enabled: extending to next FTQ [%#x, %#x), total span: %d bytes\n",
+            next_ftq.startPC, next_ftq.endPC, span);
+    return true;
+}
+
+/**
+ * @brief Extend to process next FTQ entry for 2fetch
+ *
+ * @param pc Program counter reference to update
+ * @param seqNum Sequence number
+ * @param tid Thread ID
+ * @param currentLoopIter Current loop iteration
+ */
+void
+DecoupledBPUWithBTB::extendToNextFTQ(PCStateBase &pc)
+{
+    // Move to next FTQ entry
+    fetchTargetQueue.advance();
+    currentFtqEntryInstNum = 0;  // Reset instruction counter for new FTQ
+
+    // Get the new FTQ entry
+    const auto &target_to_fetch = fetchTargetQueue.getTarget();
+
+    DPRINTF(DecoupleBP, "Processing extended FTQ entry: [%#x, %#x)\n",
+            target_to_fetch.startPC, target_to_fetch.endPC);
+
+    // Set PC to start of new FTQ
+    auto &rpc = pc.as<GenericISA::PCStateWithNext>();
+    rpc.pc(target_to_fetch.startPC);
+    rpc.npc(target_to_fetch.startPC + 4);
+    rpc.uReset();
+
+    // Record successful 2fetch
+    dbpBtbStats.fetch2Successes++;
+}
+
 }  // namespace btb_pred
 
 }  // namespace branch_prediction

src/cpu/pred/btb/decoupled_bpred.hh

@@ -82,6 +82,9 @@ class DecoupledBPUWithBTB : public BPredUnit
     JumpAheadPredictor jap;
     bool enableJumpAheadPredictor{false};
 
+    // 2taken feature support
+    bool enable2Taken{true};  // Default enabled
+
   private:
     std::string _name;
 
@@ -377,6 +380,14 @@ class DecoupledBPUWithBTB : public BPredUnit
         statistics::Scalar s3PredWrongIttage;
         statistics::Scalar s3PredWrongRas;
 
+        // NEW: 2Fetch statistics
+        statistics::Scalar fetch2Attempts;        ///< Number of 2fetch attempts
+        statistics::Scalar fetch2Successes;       ///< Number of successful 2fetch cycles
+        statistics::Scalar fetch2SpanTooLarge;    ///< Rejected due to span > maxFetchBytes
+        statistics::Scalar fetch2NoNextFTQ;       ///< Rejected due to no next FTQ entry
+        statistics::Scalar fetch2FirstNotTaken;   ///< Rejected due to current FTQ is not taken
+        statistics::Scalar fetch2FirstNotAtStart; ///< Rejected due to current PC is not at next FTQ start
+
         DBPBTBStats(statistics::Group* parent, unsigned numStages, unsigned fsqSize, unsigned maxInstsNum);
     } dbpBtbStats;
 
@@ -999,6 +1010,36 @@ class DecoupledBPUWithBTB : public BPredUnit
      */
     int currentFtqEntryInstNum{0};
 
+    // NEW: 2Fetch support variables
+    /**
+     * @brief Enable 2fetch capability
+     */
+    bool enable2Fetch{true};
+
+    /**
+     * @brief Whether fetched first FTQ
+     */
+    bool has1Fetched{false};
+
+    /**
+     * @brief Maximum fetch bytes per cycle for 2fetch
+     */
+    unsigned maxFetchBytesPerCycle{64};
+
+    // NEW: 2Fetch support methods
+    /**
+     * @brief Check if we can extend to next FTQ entry for 2fetch
+     *
+     * @param current_pc Current program counter
+     * @param current_ftq Current FTQ entry that is being completed
+     * @return true if extension to next FTQ is possible
+     */
+    bool canExtendToNextFTQ(const PCStateBase &current_pc, const FtqEntry &current_ftq);
+
+    // Extend processing to next FTQ entry for 2fetch
+
+    void extendToNextFTQ(PCStateBase &pc);
+
     /**
      * @brief Dump statistics on program exit
      *

src/cpu/pred/btb/decoupled_bpred_stats.cc

@@ -505,8 +505,13 @@ DecoupledBPUWithBTB::DBPBTBStats::DBPBTBStats(
     ADD_STAT(s3PredWrongMbtb, statistics::units::Count::get(), "S3pred wrong blame mbtb "),
     ADD_STAT(s3PredWrongTage, statistics::units::Count::get(), "S3pred wrong blame tage "),
     ADD_STAT(s3PredWrongIttage, statistics::units::Count::get(), "S3pred wrong blame ittage "),
-    ADD_STAT(s3PredWrongRas, statistics::units::Count::get(), "S3pred wrong blame ras ")
-
+    ADD_STAT(s3PredWrongRas, statistics::units::Count::get(), "S3pred wrong blame ras "),
+    ADD_STAT(fetch2Attempts, statistics::units::Count::get(), "Number of 2fetch attempts"),
+    ADD_STAT(fetch2Successes, statistics::units::Count::get(), "Number of successful 2fetch cycles"),
+    ADD_STAT(fetch2SpanTooLarge, statistics::units::Count::get(), "Rejected due to span > maxFetchBytes"),
+    ADD_STAT(fetch2NoNextFTQ, statistics::units::Count::get(), "Rejected due to no next FTQ entry"),
+    ADD_STAT(fetch2FirstNotTaken, statistics::units::Count::get(), "Rejected due to current FTQ is not taken"),
+    ADD_STAT(fetch2FirstNotAtStart, statistics::units::Count::get(), "Rejected due to PC is != next FTQ start")
 {
     predsOfEachStage.init(numStages);
     commitPredsFromEachStage.init(numStages+1);

src/cpu/pred/btb/fetch_target_queue.cc

@@ -264,6 +264,60 @@ FetchTargetQueue::resetPC(Addr new_pc)
     fetchTargetEnqState.pc = new_pc;
 }
 
+// NEW: 2Fetch support methods implementation
+
+/**
+ * @brief Check if there is a next available FTQ entry
+ *
+ * @return true if next FTQ entry is available
+ */
+bool
+FetchTargetQueue::hasNext() const
+{
+    // Check if there's an entry with ID = fetchDemandTargetId + 1
+    auto next_it = ftq.find(fetchDemandTargetId + 1);
+    return next_it != ftq.end();
+}
+
+/**
+ * @brief Peek at the next FTQ entry without consuming it
+ *
+ * @return Reference to the next FTQ entry
+ */
+const FtqEntry&
+FetchTargetQueue::peekNext() const
+{
+    assert(hasNext());
+    auto next_it = ftq.find(fetchDemandTargetId + 1);
+    return next_it->second;
+}
+
+/**
+ * @brief Advance to the next FTQ entry without dequeuing current one
+ *
+ * Used for 2fetch when we want to process the next entry
+ * while keeping the current one active
+ */
+void
+FetchTargetQueue::advance()
+{
+    // Already moved to next target ID in processFetchTargetCompletion
+    // Update supply state to point to new target
+    auto next_it = ftq.find(fetchDemandTargetId);
+    if (next_it != ftq.end()) {
+        supplyFetchTargetState.valid = true;
+        supplyFetchTargetState.targetId = fetchDemandTargetId;
+        supplyFetchTargetState.entry = &(next_it->second);
+
+        DPRINTF(DecoupleBP,
+                "Advanced to next FTQ entry: ID %lu, PC [%#lx, %#lx)\n",
+                fetchDemandTargetId, next_it->second.startPC, next_it->second.endPC);
+    } else {
+        supplyFetchTargetState.valid = false;
+        supplyFetchTargetState.entry = nullptr;
+    }
+}
+
 }  // namespace btb_pred
 
 }  // namespace branch_prediction

src/cpu/pred/btb/fetch_target_queue.hh

@@ -157,6 +157,29 @@ class FetchTargetQueue
      */
     void finishCurrentFetchTarget();
 
+    // NEW: 2Fetch support methods
+    /**
+     * @brief Check if there is a next available FTQ entry
+     *
+     * @return true if next FTQ entry is available
+     */
+    bool hasNext() const;
+
+    /**
+     * @brief Peek at the next FTQ entry without consuming it
+     *
+     * @return Reference to the next FTQ entry
+     */
+    const FtqEntry& peekNext() const;
+
+    /**
+     * @brief Advance to the next FTQ entry without dequeuing current one
+     *
+     * Used for 2fetch when we want to process the next entry
+     * while keeping the current one active
+     */
+    void advance();
+
     /**
      * @brief Try to supply fetch with a target matching the demand PC
      *