[NFC] Refactor memory ops cluster method

[qiucf]

Summary

Current way of implementation in BaseMemOpsClusterMutation::apply is a little bit obscure. We can directly use a map from store chain ID to set of memory instrs to make it simpler, so that future improvements (there's indeed some) are easier to read, update and review.

[merge_guards_bot]

Unit tests: pass. 61168 tests passed, 0 failed and 728 were skipped.

clang-tidy: pass.

clang-format: pass.

Build artifacts: diff.json, clang-tidy.txt, clang-format.patch, CMakeCache.txt, console-log.txt, test-results.xml

[steven.zhang]

I like this patch, as it really takes me a while to understand what it is before your patch.

[qiucf]

Ping..

llvm/lib/CodeGen/MachineScheduler.cpp

if (TII->shouldClusterMemOps(*MemOpRecords[Idx].BaseOp,
       ++ClusterLength;
     } else
       ClusterLength = 1;
   }
 }
 
 /// Callback from DAG postProcessing to create cluster edges for loads.
 void BaseMemOpClusterMutation::apply(ScheduleDAGInstrs *DAG) {
-  // Map DAG NodeNum to store chain ID.
-  DenseMap<unsigned, unsigned> StoreChainIDs;
-  // Map each store chain to a set of dependent MemOps.
-  SmallVector<SmallVector<SUnit*,4>, 32> StoreChainDependents;
+  // Map DAG NodeNum to a set of dependent MemOps in store chain.
+  DenseMap<unsigned, SmallVector<SUnit *, 4>> StoreChains;
   for (SUnit &SU : DAG->SUnits) {
     if ((IsLoad && !SU.getInstr()->mayLoad()) ||
         (!IsLoad && !SU.getInstr()->mayStore()))
       continue;
 
     unsigned ChainPredID = DAG->SUnits.size();
     for (const SDep &Pred : SU.Preds) {
       if (Pred.isCtrl()) {
         ChainPredID = Pred.getSUnit()->NodeNum;
         break;
       }
     }
-    // Check if this chain-like pred has been seen
-    // before. ChainPredID==MaxNodeID at the top of the schedule.
-    unsigned NumChains = StoreChainDependents.size();
-    std::pair<DenseMap<unsigned, unsigned>::iterator, bool> Result =
-      StoreChainIDs.insert(std::make_pair(ChainPredID, NumChains));
-    if (Result.second)
-      StoreChainDependents.resize(NumChains + 1);
-    StoreChainDependents[Result.first->second].push_back(&SU);
+    // Insert the SU to corresponding store chain.
+    auto &Chain = StoreChains.FindAndConstruct(ChainPredID).second;
+    Chain.push_back(&SU);
   }
 
   // Iterate over the store chains.
-  for (auto &SCD : StoreChainDependents)
-    clusterNeighboringMemOps(SCD, DAG);
+  for (auto &SCD : StoreChains)
+    clusterNeighboringMemOps(SCD.second, DAG);
 }
 
 //===----------------------------------------------------------------------===//
 // CopyConstrain - DAG post-processing to encourage copy elimination.
 //===----------------------------------------------------------------------===//
 
 namespace {