[SandboxVec][DAG] Update MemDGNode chain upon instr deletion

llvm/include/llvm/Transforms/Vectorize/SandboxVectorizer/DependencyGraph.h

@@ -342,11 +342,7 @@ class DependencyGraph {
   void notifyCreateInstr(Instruction *I);
   /// Called by the callbacks when instruction \p I is about to get
   /// deleted.
-  void notifyEraseInstr(Instruction *I) {
-    InstrToNodeMap.erase(I);
-    // TODO: Update the dependencies.
-    // TODO: Update the MemDGNode chain to remove the node if needed.
-  }
+  void notifyEraseInstr(Instruction *I);
 
 public:
   /// This constructor also registers callbacks.

llvm/lib/Transforms/Vectorize/SandboxVectorizer/DependencyGraph.cpp

@@ -370,6 +370,22 @@ void DependencyGraph::notifyCreateInstr(Instruction *I) {
   }
 }
 
+void DependencyGraph::notifyEraseInstr(Instruction *I) {
+  // Update the MemDGNode chain if this is a memory node.
+  if (auto *MemN = dyn_cast_or_null<MemDGNode>(getNodeOrNull(I))) {
+    auto *PrevMemN = getMemDGNodeBefore(MemN, /*IncludingN=*/false);
+    auto *NextMemN = getMemDGNodeAfter(MemN, /*IncludingN=*/false);
+    if (PrevMemN != nullptr)
+      PrevMemN->NextMemN = NextMemN;
+    if (NextMemN != nullptr)
+      NextMemN->PrevMemN = PrevMemN;
+  }
+
+  InstrToNodeMap.erase(I);
+
+  // TODO: Update the dependencies.
+}
+
 Interval<Instruction> DependencyGraph::extend(ArrayRef<Instruction *> Instrs) {
   if (Instrs.empty())
     return {};

llvm/unittests/Transforms/Vectorize/SandboxVectorizer/DependencyGraphTest.cpp

@@ -880,6 +880,16 @@ define void @foo(ptr %ptr, i8 %v1, i8 %v2, i8 %v3, i8 %arg) {
   S2->eraseFromParent();
   auto *DeletedN = DAG.getNodeOrNull(S2);
   EXPECT_TRUE(DeletedN == nullptr);
+
+  // Check the MemDGNode chain.
+  auto *S1MemN = cast<sandboxir::MemDGNode>(DAG.getNode(S1));
+  auto *S3MemN = cast<sandboxir::MemDGNode>(DAG.getNode(S3));
+  EXPECT_EQ(S1MemN->getNextNode(), S3MemN);
+  EXPECT_EQ(S3MemN->getPrevNode(), S1MemN);
+
+  // Check the chain when we erase the top node.
+  S1->eraseFromParent();
+  EXPECT_EQ(S3MemN->getPrevNode(), nullptr);
+
   // TODO: Check the dependencies to/from NewSN after they land.
-  // TODO: Check the MemDGNode chain.
 }