[SandboxVec][DependencyGraph] Fix dependency node iterators

This patch fixes a bug in the dependency node iterators that would incorrectly
not skip nodes that are not in the current DAG. This resulted in iterators
returning nullptr when dereferenced.

The fix is to update the existing "skip" function to not only skip
non-instruction values but also to skip instructions not in the DAG.

Author: Vasileios Porpodas

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

@@ -45,15 +45,6 @@ class DGNode;
 class MemDGNode;
 class DependencyGraph;
 
-/// While OpIt points to a Value that is not an Instruction keep incrementing
-/// it. \Returns the first iterator that points to an Instruction, or end.
-[[nodiscard]] static User::op_iterator skipNonInstr(User::op_iterator OpIt,
-                                                    User::op_iterator OpItE) {
-  while (OpIt != OpItE && !isa<Instruction>((*OpIt).get()))
-    ++OpIt;
-  return OpIt;
-}
-
 /// Iterate over both def-use and mem dependencies.
 class PredIterator {
   User::op_iterator OpIt;
@@ -72,6 +63,12 @@ class PredIterator {
   friend class DGNode;    // For constructor
   friend class MemDGNode; // For constructor
 
+  /// Skip iterators that don't point instructions or are outside \p DAG,
+  /// starting from \p OpIt and ending before \p OpItE.n
+  static User::op_iterator skipBadIt(User::op_iterator OpIt,
+                                     User::op_iterator OpItE,
+                                     const DependencyGraph &DAG);
+
 public:
   using difference_type = std::ptrdiff_t;
   using value_type = DGNode *;
@@ -135,8 +132,9 @@ class DGNode {
   bool comesBefore(const DGNode *Other) { return I->comesBefore(Other->I); }
   using iterator = PredIterator;
   virtual iterator preds_begin(DependencyGraph &DAG) {
-    return PredIterator(skipNonInstr(I->op_begin(), I->op_end()), I->op_end(),
-                        this, DAG);
+    return PredIterator(
+        PredIterator::skipBadIt(I->op_begin(), I->op_end(), DAG), I->op_end(),
+        this, DAG);
   }
   virtual iterator preds_end(DependencyGraph &DAG) {
     return PredIterator(I->op_end(), I->op_end(), this, DAG);
@@ -249,8 +247,8 @@ class MemDGNode final : public DGNode {
   }
   iterator preds_begin(DependencyGraph &DAG) override {
     auto OpEndIt = I->op_end();
-    return PredIterator(skipNonInstr(I->op_begin(), OpEndIt), OpEndIt,
-                        MemPreds.begin(), this, DAG);
+    return PredIterator(PredIterator::skipBadIt(I->op_begin(), OpEndIt, DAG),
+                        OpEndIt, MemPreds.begin(), this, DAG);
   }
   iterator preds_end(DependencyGraph &DAG) override {
     return PredIterator(I->op_end(), I->op_end(), MemPreds.end(), this, DAG);

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

@@ -14,6 +14,18 @@
 
 namespace llvm::sandboxir {
 
+User::op_iterator PredIterator::skipBadIt(User::op_iterator OpIt,
+                                          User::op_iterator OpItE,
+                                          const DependencyGraph &DAG) {
+  auto Skip = [&DAG](auto OpIt) {
+    auto *I = dyn_cast<Instruction>((*OpIt).get());
+    return I == nullptr || DAG.getNode(I) == nullptr;
+  };
+  while (OpIt != OpItE && Skip(OpIt))
+    ++OpIt;
+  return OpIt;
+}
+
 PredIterator::value_type PredIterator::operator*() {
   // If it's a DGNode then we dereference the operand iterator.
   if (!isa<MemDGNode>(N)) {
@@ -35,16 +47,16 @@ PredIterator &PredIterator::operator++() {
   if (!isa<MemDGNode>(N)) {
     assert(OpIt != OpItE && "Already at end!");
     ++OpIt;
-    // Skip operands that are not instructions.
-    OpIt = skipNonInstr(OpIt, OpItE);
+    // Skip operands that are not instructions or are outside the DAG.
+    OpIt = PredIterator::skipBadIt(OpIt, OpItE, *DAG);
     return *this;
   }
   // It's a MemDGNode, so if we are not at the end of the use-def iterator we
   // need to first increment that.
   if (OpIt != OpItE) {
     ++OpIt;
-    // Skip operands that are not instructions.
-    OpIt = skipNonInstr(OpIt, OpItE);
+    // Skip operands that are not instructions or are outside the DAG.
+    OpIt = PredIterator::skipBadIt(OpIt, OpItE, *DAG);
     return *this;
   }
   // It's a MemDGNode with OpIt == end, so we need to increment MemIt.

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

@@ -79,9 +79,6 @@ void Scheduler::scheduleAndUpdateReadyList(SchedBundle &Bndl) {
   for (DGNode *N : Bndl) {
     N->setScheduled(true);
     for (auto *DepN : N->preds(DAG)) {
-      // TODO: preds() should not return nullptr.
-      if (DepN == nullptr)
-        continue;
       DepN->decrUnscheduledSuccs();
       if (DepN->ready())
         ReadyList.insert(DepN);

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

@@ -313,6 +313,39 @@ define i8 @foo(i8 %v0, i8 %v1) {
   EXPECT_EQ(RetN->getNumUnscheduledSuccs(), 0u);
 }
 
+// Make sure we don't get null predecessors even if they are outside the DAG.
+TEST_F(DependencyGraphTest, NonNullPreds) {
+  parseIR(C, R"IR(
+define void @foo(ptr %ptr, i8 %val) {
+  %gep = getelementptr i8, ptr %ptr, i32 0
+  store i8 %val, ptr %gep
+  ret void
+}
+)IR");
+  llvm::Function *LLVMF = &*M->getFunction("foo");
+  sandboxir::Context Ctx(C);
+  auto *F = Ctx.createFunction(LLVMF);
+  auto *BB = &*F->begin();
+  auto It = BB->begin();
+  [[maybe_unused]] auto *GEP = cast<sandboxir::GetElementPtrInst>(&*It++);
+  auto *S0 = cast<sandboxir::StoreInst>(&*It++);
+  auto *Ret = cast<sandboxir::ReturnInst>(&*It++);
+
+  sandboxir::DependencyGraph DAG(getAA(*LLVMF), Ctx);
+  // The DAG doesn't include GEP.
+  DAG.extend({S0, Ret});
+
+  auto *S0N = DAG.getNode(S0);
+  // S0 has one operand (the GEP) that is outside the DAG and no memory
+  // predecessors. So pred_begin() should be == pred_end().
+  auto PredIt = S0N->preds_begin(DAG);
+  auto PredItE = S0N->preds_end(DAG);
+  EXPECT_EQ(PredIt, PredItE);
+  // Check preds().
+  for (auto *PredN : S0N->preds(DAG))
+    EXPECT_NE(PredN, nullptr);
+}
+
 TEST_F(DependencyGraphTest, MemDGNode_getPrevNode_getNextNode) {
   parseIR(C, R"IR(
 define void @foo(ptr %ptr, i8 %v0, i8 %v1) {