[mlir][SliceAnalysis] Fix stack overflow in graph regions (llvm#139694)

This analysis currently just crashes when applied to a graph region that
has a use-def cycle. This PR fixes that by keeping track of the
operations the DFS has already visited when following use-def edges and
stopping once we visit an operation again.

Author: maerhart

mlir/include/mlir/Analysis/SliceAnalysis.h

@@ -65,8 +65,9 @@ using ForwardSliceOptions = SliceOptions;
 ///
 /// The implementation traverses the use chains in postorder traversal for
 /// efficiency reasons: if an operation is already in `forwardSlice`, no
-/// need to traverse its uses again. Since use-def chains form a DAG, this
-/// terminates.
+/// need to traverse its uses again. In the presence of use-def cycles in a
+/// graph region, the traversal stops at the first operation that was already
+/// visited (which is not added to the slice anymore).
 ///
 /// Upon return to the root call, `forwardSlice` is filled with a
 /// postorder list of uses (i.e. a reverse topological order). To get a proper
@@ -114,8 +115,9 @@ void getForwardSlice(Value root, SetVector<Operation *> *forwardSlice,
 ///
 /// The implementation traverses the def chains in postorder traversal for
 /// efficiency reasons: if an operation is already in `backwardSlice`, no
-/// need to traverse its definitions again. Since useuse-def chains form a DAG,
-/// this terminates.
+/// need to traverse its definitions again. In the presence of use-def cycles
+/// in a graph region, the traversal stops at the first operation that was
+/// already visited (which is not added to the slice anymore).
 ///
 /// Upon return to the root call, `backwardSlice` is filled with a
 /// postorder list of defs. This happens to be a topological order, from the

mlir/lib/Analysis/SliceAnalysis.cpp

@@ -26,7 +26,8 @@
 using namespace mlir;
 
 static void
-getForwardSliceImpl(Operation *op, SetVector<Operation *> *forwardSlice,
+getForwardSliceImpl(Operation *op, DenseSet<Operation *> &visited,
+                    SetVector<Operation *> *forwardSlice,
                     const SliceOptions::TransitiveFilter &filter = nullptr) {
   if (!op)
     return;
@@ -40,20 +41,41 @@ getForwardSliceImpl(Operation *op, SetVector<Operation *> *forwardSlice,
   for (Region &region : op->getRegions())
     for (Block &block : region)
       for (Operation &blockOp : block)
-        if (forwardSlice->count(&blockOp) == 0)
-          getForwardSliceImpl(&blockOp, forwardSlice, filter);
-  for (Value result : op->getResults()) {
-    for (Operation *userOp : result.getUsers())
-      if (forwardSlice->count(userOp) == 0)
-        getForwardSliceImpl(userOp, forwardSlice, filter);
-  }
+        if (forwardSlice->count(&blockOp) == 0) {
+          // We don't have to check if the 'blockOp' is already visited because
+          // there cannot be a traversal path from this nested op to the parent
+          // and thus a cycle cannot be closed here. We still have to mark it
+          // as visited to stop before visiting this operation again if it is
+          // part of a cycle.
+          visited.insert(&blockOp);
+          getForwardSliceImpl(&blockOp, visited, forwardSlice, filter);
+          visited.erase(&blockOp);
+        }
+
+  for (Value result : op->getResults())
+    for (Operation *userOp : result.getUsers()) {
+      // A cycle can only occur within a basic block (not across regions or
+      // basic blocks) because the parent region must be a graph region, graph
+      // regions are restricted to always have 0 or 1 blocks, and there cannot
+      // be a def-use edge from a nested operation to an operation in an
+      // ancestor region. Therefore, we don't have to but may use the same
+      // 'visited' set across regions/blocks as long as we remove operations
+      // from the set again when the DFS traverses back from the leaf to the
+      // root.
+      if (forwardSlice->count(userOp) == 0 && visited.insert(userOp).second)
+        getForwardSliceImpl(userOp, visited, forwardSlice, filter);
+
+      visited.erase(userOp);
+    }
 
   forwardSlice->insert(op);
 }
 
 void mlir::getForwardSlice(Operation *op, SetVector<Operation *> *forwardSlice,
                            const ForwardSliceOptions &options) {
-  getForwardSliceImpl(op, forwardSlice, options.filter);
+  DenseSet<Operation *> visited;
+  visited.insert(op);
+  getForwardSliceImpl(op, visited, forwardSlice, options.filter);
   if (!options.inclusive) {
     // Don't insert the top level operation, we just queried on it and don't
     // want it in the results.
@@ -69,8 +91,12 @@ void mlir::getForwardSlice(Operation *op, SetVector<Operation *> *forwardSlice,
 
 void mlir::getForwardSlice(Value root, SetVector<Operation *> *forwardSlice,
                            const SliceOptions &options) {
-  for (Operation *user : root.getUsers())
-    getForwardSliceImpl(user, forwardSlice, options.filter);
+  DenseSet<Operation *> visited;
+  for (Operation *user : root.getUsers()) {
+    visited.insert(user);
+    getForwardSliceImpl(user, visited, forwardSlice, options.filter);
+    visited.erase(user);
+  }
 
   // Reverse to get back the actual topological order.
   // std::reverse does not work out of the box on SetVector and I want an
@@ -80,6 +106,7 @@ void mlir::getForwardSlice(Value root, SetVector<Operation *> *forwardSlice,
 }
 
 static LogicalResult getBackwardSliceImpl(Operation *op,
+                                          DenseSet<Operation *> &visited,
                                           SetVector<Operation *> *backwardSlice,
                                           const BackwardSliceOptions &options) {
   if (!op || op->hasTrait<OpTrait::IsIsolatedFromAbove>())
@@ -93,8 +120,12 @@ static LogicalResult getBackwardSliceImpl(Operation *op,
 
   auto processValue = [&](Value value) {
     if (auto *definingOp = value.getDefiningOp()) {
-      if (backwardSlice->count(definingOp) == 0)
-        return getBackwardSliceImpl(definingOp, backwardSlice, options);
+      if (backwardSlice->count(definingOp) == 0 &&
+          visited.insert(definingOp).second)
+        return getBackwardSliceImpl(definingOp, visited, backwardSlice,
+                                    options);
+
+      visited.erase(definingOp);
     } else if (auto blockArg = dyn_cast<BlockArgument>(value)) {
       if (options.omitBlockArguments)
         return success();
@@ -107,7 +138,8 @@ static LogicalResult getBackwardSliceImpl(Operation *op,
       if (parentOp && backwardSlice->count(parentOp) == 0) {
         if (parentOp->getNumRegions() == 1 &&
             llvm::hasSingleElement(parentOp->getRegion(0).getBlocks())) {
-          return getBackwardSliceImpl(parentOp, backwardSlice, options);
+          return getBackwardSliceImpl(parentOp, visited, backwardSlice,
+                                      options);
         }
       }
     } else {
@@ -145,7 +177,10 @@ static LogicalResult getBackwardSliceImpl(Operation *op,
 LogicalResult mlir::getBackwardSlice(Operation *op,
                                      SetVector<Operation *> *backwardSlice,
                                      const BackwardSliceOptions &options) {
-  LogicalResult result = getBackwardSliceImpl(op, backwardSlice, options);
+  DenseSet<Operation *> visited;
+  visited.insert(op);
+  LogicalResult result =
+      getBackwardSliceImpl(op, visited, backwardSlice, options);
 
   if (!options.inclusive) {
     // Don't insert the top level operation, we just queried on it and don't

mlir/test/Dialect/Affine/slicing-utils.mlir

@@ -292,3 +292,26 @@ func.func @slicing_test_multiple_return(%arg0: index) -> (index, index) {
   %0:2 = "slicing-test-op"(%arg0, %arg0): (index, index) -> (index, index)
   return %0#0, %0#1 : index, index
 }
+
+// -----
+
+// FWD-LABEL: graph_region_with_cycle
+// BWD-LABEL: graph_region_with_cycle
+// FWDBWD-LABEL: graph_region_with_cycle
+func.func @graph_region_with_cycle() {
+  test.isolated_graph_region {
+    // FWD: matched: [[V0:%.+]] = "slicing-test-op"([[V1:%.+]]) : (i1) -> i1 forward static slice:
+    // FWD: [[V1]] = "slicing-test-op"([[V0]]) : (i1) -> i1 
+    // FWD: matched: [[V1]] = "slicing-test-op"([[V0]]) : (i1) -> i1 forward static slice:
+    // FWD: [[V0]] = "slicing-test-op"([[V1]]) : (i1) -> i1 
+    
+    // BWD: matched: [[V0:%.+]] = "slicing-test-op"([[V1:%.+]]) : (i1) -> i1 backward static slice:
+    // BWD: [[V1]] = "slicing-test-op"([[V0]]) : (i1) -> i1 
+    // BWD: matched: [[V1]] = "slicing-test-op"([[V0]]) : (i1) -> i1 backward static slice:
+    // BWD: [[V0]] = "slicing-test-op"([[V1]]) : (i1) -> i1 
+    %0 = "slicing-test-op"(%1) : (i1) -> i1
+    %1 = "slicing-test-op"(%0) : (i1) -> i1
+  }
+
+  return
+}