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

[Daniel McCrevan via Phabricator via llvm-commits]

dmccrevan updated this revision to Diff 301403.

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D90328/new/

https://reviews.llvm.org/D90328

Files:
  llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
  llvm/test/Transforms/DeadStoreElimination/MSSA/pr16520.ll


Index: llvm/test/Transforms/DeadStoreElimination/MSSA/pr16520.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/DeadStoreElimination/MSSA/pr16520.ll
@@ -0,0 +1,25 @@
+; ModuleID = 'pr16520.cc'
+target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define void @_Z1fbRb(i1 zeroext %b, i8* nocapture %r) {
+entry:
+  store i8 1, i8* %r, align 1
+  br i1 %b, label %if.then, label %if.else
+
+if.then:                                          ; preds = %entry
+  store i8 1, i8* %r, align 1
+  tail call void @_Z1gv()
+  br label %if.end
+
+if.else:                                          ; preds = %entry
+  tail call void @_Z1hv()
+  br label %if.end
+
+if.end:                                           ; preds = %if.else, %if.then
+  ret void
+}
+
+declare void @_Z1gv()
+
+declare void @_Z1hv()
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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