[PATCH] D90328: Eliminates dead store of an exisiting value

[Daniel McCrevan via Phabricator via llvm-commits]

dmccrevan created this revision.
Herald added subscribers: llvm-commits, hiraditya.
Herald added a project: LLVM.
dmccrevan requested review of this revision.

Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D90328

Files:
  llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp


Index: llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
+++ llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
@@ -2367,6 +2367,46 @@
     return false;
   }
 
+  bool eliminateDeadStoresOfExisitingValues() {
+    bool MadeChange = false;
+    LLVM_DEBUG(dbgs() << "Trying to eliminate MemoryDefs that write the "
+                         "already exisiting value\n");
+    for (int I = MemDefs.size() - 1; I >= 0; I--) {
+      MemoryDef *Def = MemDefs[I];
+      SmallVector<MemoryAccess *, 4> WorkList;
+      SmallPtrSet<MemoryAccess *, 8> Visited;
+      auto PushMemUses = [&WorkList, &Visited](MemoryAccess *Acc) {
+        if (!Visited.insert(Acc).second)
+          return;
+        for (Use &U : Acc->uses())
+          WorkList.push_back(cast<MemoryAccess>(U.getUser()));
+      };
+      PushMemUses(Def);
+      for (unsigned I = 0; I < WorkList.size(); I++) {
+        if (WorkList.size() >= MemorySSAScanLimit) {
+          LLVM_DEBUG(dbgs() << "  ... hit exploration limit.\n");
+          return false;
+        }
+        MemoryAccess *UseAccess = WorkList[I];
+        if (isa<MemoryPhi>(UseAccess))
+          continue;
+
+        Instruction *UseInst = cast<MemoryUseOrDef>(UseAccess)->getMemoryInst();
+        if (MemoryDef *UseDef = dyn_cast<MemoryDef>(UseAccess)) {
+          MemoryAccess *tmp = UseDef->getDefiningAccess();
+          Instruction *tmpi = cast<MemoryUseOrDef>(tmp)->getMemoryInst();
+          if (tmpi->isIdenticalTo(UseInst)) {
+            deleteDeadInstruction(UseInst);
+            MadeChange = true;
+          } else {
+            PushMemUses(UseDef);
+          }
+        }
+      }
+    }
+    return MadeChange;
+  }
+
   /// Eliminate writes to objects that are not visible in the caller and are not
   /// accessed before returning from the function.
   bool eliminateDeadWritesAtEndOfFunction() {
@@ -2632,7 +2672,7 @@
       }
     }
   }
-
+  MadeChange |= State.eliminateDeadStoresOfExisitingValues();
   if (EnablePartialOverwriteTracking)
     for (auto &KV : State.IOLs)
       MadeChange |= removePartiallyOverlappedStores(State.DL, KV.second, TLI);


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