cpu: Change writeback modeling for outstanding instructions

As highlighed on the mailing list gem5's writeback modeling can impact
performance.  This patch removes the limitation on maximum outstanding issued
instructions, however the number that can writeback in a single cycle is still
respected in instToCommit().

configs/common/O3_ARM_v7a.py

@@ -126,7 +126,6 @@ class O3_ARM_v7a_3(DerivO3CPU):
     dispatchWidth = 6
     issueWidth = 8
     wbWidth = 8
-    wbDepth = 1
     fuPool = O3_ARM_v7a_FUP()
     iewToCommitDelay = 1
     renameToROBDelay = 1

src/cpu/o3/O3CPU.py

@@ -84,7 +84,6 @@ class DerivO3CPU(BaseCPU):
     dispatchWidth = Param.Unsigned(8, "Dispatch width")
     issueWidth = Param.Unsigned(8, "Issue width")
     wbWidth = Param.Unsigned(8, "Writeback width")
-    wbDepth = Param.Unsigned(1, "Writeback depth")
     fuPool = Param.FUPool(DefaultFUPool(), "Functional Unit pool")
 
     iewToCommitDelay = Param.Cycles(1, "Issue/Execute/Writeback to commit "

src/cpu/o3/iew.hh

@@ -219,49 +219,6 @@ class DefaultIEW
     /** Returns if the LSQ has any stores to writeback. */
     bool hasStoresToWB(ThreadID tid) { return ldstQueue.hasStoresToWB(tid); }
 
-    void incrWb(InstSeqNum &sn)
-    {
-        ++wbOutstanding;
-        if (wbOutstanding == wbMax)
-            ableToIssue = false;
-        DPRINTF(IEW, "wbOutstanding: %i [sn:%lli]\n", wbOutstanding, sn);
-        assert(wbOutstanding <= wbMax);
-#ifdef DEBUG
-        wbList.insert(sn);
-#endif
-    }
-
-    void decrWb(InstSeqNum &sn)
-    {
-        if (wbOutstanding == wbMax)
-            ableToIssue = true;
-        wbOutstanding--;
-        DPRINTF(IEW, "wbOutstanding: %i [sn:%lli]\n", wbOutstanding, sn);
-        assert(wbOutstanding >= 0);
-#ifdef DEBUG
-        assert(wbList.find(sn) != wbList.end());
-        wbList.erase(sn);
-#endif
-    }
-
-#ifdef DEBUG
-    std::set<InstSeqNum> wbList;
-
-    void dumpWb()
-    {
-        std::set<InstSeqNum>::iterator wb_it = wbList.begin();
-        while (wb_it != wbList.end()) {
-            cprintf("[sn:%lli]\n",
-                    (*wb_it));
-            wb_it++;
-        }
-    }
-#endif
-
-    bool canIssue() { return ableToIssue; }
-
-    bool ableToIssue;
-
     /** Check misprediction  */
     void checkMisprediction(DynInstPtr &inst);
 
@@ -452,19 +409,9 @@ class DefaultIEW
      */
     unsigned wbCycle;
 
-    /** Number of instructions in flight that will writeback. */
-
-    /** Number of instructions in flight that will writeback. */
-    int wbOutstanding;
-
     /** Writeback width. */
     unsigned wbWidth;
 
-    /** Writeback width * writeback depth, where writeback depth is
-     * the number of cycles of writing back instructions that can be
-     * buffered. */
-    unsigned wbMax;
-
     /** Number of active threads. */
     ThreadID numThreads;
 

src/cpu/o3/iew_impl.hh

@@ -76,7 +76,6 @@ DefaultIEW<Impl>::DefaultIEW(O3CPU *_cpu, DerivO3CPUParams *params)
       issueToExecuteDelay(params->issueToExecuteDelay),
       dispatchWidth(params->dispatchWidth),
       issueWidth(params->issueWidth),
-      wbOutstanding(0),
       wbWidth(params->wbWidth),
       numThreads(params->numThreads)
 {
@@ -109,12 +108,8 @@ DefaultIEW<Impl>::DefaultIEW(O3CPU *_cpu, DerivO3CPUParams *params)
         fetchRedirect[tid] = false;
     }
 
-    wbMax = wbWidth * params->wbDepth;
-
     updateLSQNextCycle = false;
 
-    ableToIssue = true;
-
     skidBufferMax = (3 * (renameToIEWDelay * params->renameWidth)) + issueWidth;
 }
 
@@ -635,8 +630,6 @@ DefaultIEW<Impl>::instToCommit(DynInstPtr &inst)
             ++wbCycle;
             wbNumInst = 0;
         }
-
-        assert((wbCycle * wbWidth + wbNumInst) <= wbMax);
     }
 
     DPRINTF(IEW, "Current wb cycle: %i, width: %i, numInst: %i\nwbActual:%i\n",
@@ -1263,7 +1256,6 @@ DefaultIEW<Impl>::executeInsts()
 
             ++iewExecSquashedInsts;
 
-            decrWb(inst->seqNum);
             continue;
         }
 
@@ -1502,8 +1494,6 @@ DefaultIEW<Impl>::writebackInsts()
             }
             writebackCount[tid]++;
         }
-
-        decrWb(inst->seqNum);
     }
 }
 

src/cpu/o3/inst_queue_impl.hh

@@ -756,7 +756,6 @@ InstructionQueue<Impl>::scheduleReadyInsts()
     int total_issued = 0;
 
     while (total_issued < (totalWidth - total_deferred_mem_issued) &&
-           iewStage->canIssue() &&
            order_it != order_end_it) {
         OpClass op_class = (*order_it).queueType;
 
@@ -861,7 +860,6 @@ InstructionQueue<Impl>::scheduleReadyInsts()
 
             listOrder.erase(order_it++);
             statIssuedInstType[tid][op_class]++;
-            iewStage->incrWb(issuing_inst->seqNum);
         } else {
             statFuBusy[op_class]++;
             fuBusy[tid]++;

src/cpu/o3/lsq_unit.hh

@@ -762,7 +762,6 @@ LSQUnit<Impl>::read(Request *req, Request *sreqLow, Request *sreqHigh,
             // Tell IQ/mem dep unit that this instruction will need to be
             // rescheduled eventually
             iewStage->rescheduleMemInst(load_inst);
-            iewStage->decrWb(load_inst->seqNum);
             load_inst->clearIssued();
             ++lsqRescheduledLoads;
 
@@ -889,12 +888,6 @@ LSQUnit<Impl>::read(Request *req, Request *sreqLow, Request *sreqHigh,
 
         ++lsqCacheBlocked;
 
-        // If the first part of a split access succeeds, then let the LSQ
-        // handle the decrWb when completeDataAccess is called upon return
-        // of the requested first part of data
-        if (!completedFirst)
-            iewStage->decrWb(load_inst->seqNum);
-
         // There's an older load that's already going to squash.
         if (isLoadBlocked && blockedLoadSeqNum < load_inst->seqNum)
             return NoFault;

src/cpu/o3/lsq_unit_impl.hh

@@ -109,9 +109,7 @@ LSQUnit<Impl>::completeDataAccess(PacketPtr pkt)
     }
 
     assert(!cpu->switchedOut());
-    if (inst->isSquashed()) {
-        iewStage->decrWb(inst->seqNum);
-    } else {
+    if (!inst->isSquashed()) {
         if (!state->noWB) {
             if (!TheISA::HasUnalignedMemAcc || !state->isSplit ||
                 !state->isLoad) {
@@ -1130,7 +1128,6 @@ LSQUnit<Impl>::writeback(DynInstPtr &inst, PacketPtr pkt)
 
     // Squashed instructions do not need to complete their access.
     if (inst->isSquashed()) {
-        iewStage->decrWb(inst->seqNum);
         assert(!inst->isStore());
         ++lsqIgnoredResponses;
         return;