[mlir][Transforms] Fix crash in reconcile-unrealized-casts

Author: matthias-springer

mlir/lib/Transforms/Utils/DialectConversion.cpp

@@ -3306,9 +3306,13 @@ LogicalResult OperationConverter::convertOperations(ArrayRef<Operation *> ops) {
 void mlir::reconcileUnrealizedCasts(
     ArrayRef<UnrealizedConversionCastOp> castOps,
     SmallVectorImpl<UnrealizedConversionCastOp> *remainingCastOps) {
+  // Set of all cast ops for faster lookups.
+  DenseSet<Operation *> castOpSet;
+  for (UnrealizedConversionCastOp op : castOps)
+    castOpSet.insert(op);
+
+  // A worklist of cast ops to process.
   SetVector<UnrealizedConversionCastOp> worklist(llvm::from_range, castOps);
-  // This set is maintained only if `remainingCastOps` is provided.
-  DenseSet<Operation *> erasedOps;
 
   // Helper function that adds all operands to the worklist that are an
   // unrealized_conversion_cast op result.
@@ -3337,39 +3341,73 @@ void mlir::reconcileUnrealizedCasts(
   // Process ops in the worklist bottom-to-top.
   while (!worklist.empty()) {
     UnrealizedConversionCastOp castOp = worklist.pop_back_val();
-    if (castOp->use_empty()) {
-      // DCE: If the op has no users, erase it. Add the operands to the
-      // worklist to find additional DCE opportunities.
-      enqueueOperands(castOp);
-      if (remainingCastOps)
-        erasedOps.insert(castOp.getOperation());
-      castOp->erase();
-      continue;
-    }
 
     // Traverse the chain of input cast ops to see if an op with the same
     // input types can be found.
     UnrealizedConversionCastOp nextCast = castOp;
     while (nextCast) {
       if (nextCast.getInputs().getTypes() == castOp.getResultTypes()) {
+        if (llvm::any_of(nextCast.getInputs(), [&](Value v) {
+              return v.getDefiningOp() == castOp;
+            })) {
+          // Ran into a cycle.
+          break;
+        }
+
         // Found a cast where the input types match the output types of the
-        // matched op. We can directly use those inputs and the matched op can
-        // be removed.
+        // matched op. We can directly use those inputs.
         enqueueOperands(castOp);
         castOp.replaceAllUsesWith(nextCast.getInputs());
-        if (remainingCastOps)
-          erasedOps.insert(castOp.getOperation());
-        castOp->erase();
         break;
       }
       nextCast = getInputCast(nextCast);
     }
   }
 
-  if (remainingCastOps)
-    for (UnrealizedConversionCastOp op : castOps)
-      if (!erasedOps.contains(op.getOperation()))
+  // A set of all alive cast ops. I.e., ops whose results are (transitively)
+  // used by an op that is not a cast op.
+  DenseSet<Operation *> liveOps;
+
+  // Helper function that marks the given op and all ops transitively reachable
+  // input cast ops as alive.
+  auto markOpLive = [&](Operation *op) {
+    SmallVector<Operation *> worklist;
+    worklist.push_back(op);
+    while (!worklist.empty()) {
+      Operation *op = worklist.pop_back_val();
+      if (liveOps.insert(op).second) {
+        // Successfully inserted: the op is live. Add its operands to the
+        // worklist to mark them live.
+        for (Value v : op->getOperands())
+          if (castOpSet.contains(v.getDefiningOp()))
+            worklist.push_back(v.getDefiningOp());
+      }
+    }
+  };
+
+  // Find all alive cast ops.
+  for (UnrealizedConversionCastOp op : castOps) {
+    // If any of the users is not a cast op, mark the current op (and its
+    // input ops) as live.
+    if (llvm::any_of(op->getUsers(), [&](Operation *user) {
+          return !castOpSet.contains(user);
+        }))
+      markOpLive(op);
+  }
+
+  // Erase all dead cast ops.
+  for (UnrealizedConversionCastOp op : castOps) {
+    if (liveOps.contains(op)) {
+      // Op is alive and was not erased. Add it to the remaining cast ops.
+      if (remainingCastOps)
         remainingCastOps->push_back(op);
+      continue;
+    }
+
+    // Op is dead. Erase it.
+    op->dropAllUses();
+    op->erase();
+  }
 }
 
 //===----------------------------------------------------------------------===//

mlir/test/Conversion/ReconcileUnrealizedCasts/reconcile-unrealized-casts.mlir

@@ -194,3 +194,53 @@ func.func @emptyCast() -> index {
     %0 = builtin.unrealized_conversion_cast to index
     return %0 : index
 }
+
+// -----
+
+// CHECK-LABEL: test.graph_region
+//  CHECK-NEXT:   "test.return"() : () -> ()
+test.graph_region {
+  %0 = builtin.unrealized_conversion_cast %2 : i32 to i64
+  %1 = builtin.unrealized_conversion_cast %0 : i64 to i16
+  %2 = builtin.unrealized_conversion_cast %1 : i16 to i32
+  "test.return"() : () -> ()
+}
+
+// -----
+
+// CHECK-LABEL: test.graph_region
+//  CHECK-NEXT:   %[[cast0:.*]] = builtin.unrealized_conversion_cast %[[cast2:.*]] : i32 to i64
+//  CHECK-NEXT:   %[[cast1:.*]] = builtin.unrealized_conversion_cast %[[cast0]] : i64 to i16
+//  CHECK-NEXT:   %[[cast2]] = builtin.unrealized_conversion_cast %[[cast1]] : i16 to i32
+//  CHECK-NEXT:   "test.user"(%[[cast2]]) : (i32) -> ()
+//  CHECK-NEXT:   "test.return"() : () -> ()
+test.graph_region {
+  %0 = builtin.unrealized_conversion_cast %2 : i32 to i64
+  %1 = builtin.unrealized_conversion_cast %0 : i64 to i16
+  %2 = builtin.unrealized_conversion_cast %1 : i16 to i32
+  "test.user"(%2) : (i32) -> ()
+  "test.return"() : () -> ()
+}
+
+// -----
+
+// CHECK-LABEL: test.graph_region
+//  CHECK-NEXT:   "test.return"() : () -> ()
+test.graph_region {
+  %0 = builtin.unrealized_conversion_cast %0 : i32 to i32
+  "test.return"() : () -> ()
+}
+
+// -----
+
+// CHECK-LABEL: test.graph_region
+//  CHECK-NEXT:   %[[c0:.*]] = arith.constant
+//  CHECK-NEXT:   %[[cast:.*]]:2 = builtin.unrealized_conversion_cast %[[c0]], %[[cast]]#1 : i32, i32 to i32, i32
+//  CHECK-NEXT:   "test.user"(%[[cast]]#0) : (i32) -> ()
+//  CHECK-NEXT:   "test.return"() : () -> ()
+test.graph_region {
+  %cst = arith.constant 0 : i32
+  %0, %1 = builtin.unrealized_conversion_cast %cst, %1 : i32, i32 to i32, i32
+  "test.user"(%0) : (i32) -> ()
+  "test.return"() : () -> ()
+}

mlir/test/Integration/Dialect/MemRef/assume-alignment-runtime-verification.mlir

@@ -1,7 +1,8 @@
 // RUN: mlir-opt %s -generate-runtime-verification \
 // RUN:     -expand-strided-metadata \
 // RUN:     -test-cf-assert \
-// RUN:     -convert-to-llvm | \
+// RUN:     -convert-to-llvm \
+// RUN:     -reconcile-unrealized-casts | \
 // RUN: mlir-runner -e main -entry-point-result=void \
 // RUN:     -shared-libs=%mlir_runner_utils 2>&1 | \
 // RUN: FileCheck %s

mlir/test/Integration/Dialect/MemRef/atomic-rmw-runtime-verification.mlir

@@ -1,6 +1,7 @@
 // RUN: mlir-opt %s -generate-runtime-verification \
 // RUN:     -test-cf-assert \
-// RUN:     -convert-to-llvm | \
+// RUN:     -convert-to-llvm \
+// RUN:     -reconcile-unrealized-casts | \
 // RUN: mlir-runner -e main -entry-point-result=void \
 // RUN:     -shared-libs=%mlir_runner_utils 2>&1 | \
 // RUN: FileCheck %s

mlir/test/Integration/Dialect/MemRef/store-runtime-verification.mlir

@@ -1,6 +1,7 @@
 // RUN: mlir-opt %s -generate-runtime-verification \
 // RUN:     -test-cf-assert \
-// RUN:     -convert-to-llvm | \
+// RUN:     -convert-to-llvm \
+// RUN:     -reconcile-unrealized-casts | \
 // RUN: mlir-runner -e main -entry-point-result=void \
 // RUN:     -shared-libs=%mlir_runner_utils 2>&1 | \
 // RUN: FileCheck %s