[mlir] Use-after-free checker for the Transform dialect

[ftynse]

Summary

The Transform dialect uses the side effect modeling mechanism to record the
effects of the transform ops on the mapping between Transform IR values and
Payload IR ops. Introduce a checker pass that warns if a Transform IR value is
used after it has been freed (consumed). This pass is mostly intended as a
debugging aid in addition to the verification/assertion mechanisms in the
transform interpreter. It reports all potential use-after-free situations.
The implementation makes a series of simplifying assumptions to be simple and
conservative. A more advanced implementation would rely on the data flow-like
analysis associated with a side-effect resource rather than a value, which is
currently not supported by the analysis infrastructure.

[springerm]

mlir/include/mlir/Dialect/Transform/IR/TransformOps.td

52–54

I've seen code that simply uses DeclareOpInterfaceMethods<RegionBranchOpInterface>, so maybe it even works without explicitly listing all interface methods.

mlir/include/mlir/Dialect/Transform/Transforms/Passes.td

18

typo

29

typo

mlir/lib/Dialect/Transform/IR/TransformOps.cpp

297

should this be operand_begin?

318

is this needed?

[ftynse]

mlir/include/mlir/Dialect/Transform/IR/TransformOps.td

52–54

This makes the generator emit declarations for methods that have the default implementation, also signaling the reader that these methods will be reimplemented. Without listing them, no declaration will be emitted and the code will fail to compile.

mlir/lib/Dialect/Transform/IR/TransformOps.cpp

297

No, we want to return an empty range.

318

Yes, otherwise we may get an unused argument warning.

[ftynse]

Review.

mlir/include/mlir/Dialect/Transform/CMakeLists.txt

 add_subdirectory(IR)
+add_subdirectory(Transforms)

mlir/include/mlir/Dialect/Transform/IR/TransformOps.h

 //===- TransformDialect.h - Transform dialect operations --------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef MLIR_DIALECT_TRANSFORM_IR_TRANSFORMOPS_H
 #define MLIR_DIALECT_TRANSFORM_IR_TRANSFORMOPS_H
 
 #include "mlir/Dialect/PDL/IR/PDLTypes.h"
 #include "mlir/Dialect/Transform/IR/TransformInterfaces.h"
 #include "mlir/IR/OpDefinition.h"
 #include "mlir/IR/OpImplementation.h"
 #include "mlir/IR/SymbolTable.h"
+#include "mlir/Interfaces/ControlFlowInterfaces.h"
 
 #define GET_OP_CLASSES
 #include "mlir/Dialect/Transform/IR/TransformOps.h.inc"
 
 #endif // MLIR_DIALECT_TRANSFORM_IR_TRANSFORMOPS_H

mlir/include/mlir/Dialect/Transform/IR/TransformOps.td

 //===- TransformOps.td - Transform dialect operations ------*- tablegen -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef MLIR_DIALECT_TRANSFORM_IR_TRANSFORMOPS
 #define MLIR_DIALECT_TRANSFORM_IR_TRANSFORMOPS
 
+include "mlir/Interfaces/ControlFlowInterfaces.td"
 include "mlir/IR/OpAsmInterface.td"
 include "mlir/IR/SymbolInterfaces.td"
 include "mlir/Dialect/PDL/IR/PDLTypes.td"
 include "mlir/Dialect/Transform/IR/TransformDialect.td"
 include "mlir/Dialect/Transform/IR/TransformEffects.td"
 include "mlir/Dialect/Transform/IR/TransformInterfaces.td"
 
 def PDLMatchOp : TransformDialectOp<"pdl_match",
def PDLMatchOp : TransformDialectOp<"pdl_match",
   let results = (outs
     Res<PDL_Operation, "Handle to the matched Payload IR ops",
         [TransformMappingAlloc, TransformMappingWrite]>:$matched);
 
   let assemblyFormat = "$pattern_name `in` $root attr-dict";
 }
 
 def SequenceOp : TransformDialectOp<"sequence",
-    [DeclareOpInterfaceMethods<TransformOpInterface>,
+    [DeclareOpInterfaceMethods<RegionBranchOpInterface,
+        ["getSuccessorEntryOperands", "getSuccessorRegions",
+         "getRegionInvocationBounds"]>,
+     DeclareOpInterfaceMethods<TransformOpInterface>,
      DeclareOpInterfaceMethods<MemoryEffectsOpInterface>,
      OpAsmOpInterface, PossibleTopLevelTransformOpTrait,
      SingleBlockImplicitTerminator<"::mlir::transform::YieldOp">]> {
   let summary = "Contains a sequence of other transform ops to apply";
   let description = [{
     The transformations indicated by the sequence are applied in order of their
     appearance. Each value produced by a transformation within the sequence
     corresponds to an operation or a group of operations in the payload IR.

mlir/include/mlir/Dialect/Transform/Transforms/CMakeLists.txt

+set(LLVM_TARGET_DEFINITIONS Passes.td)
+mlir_tablegen(Passes.h.inc -gen-pass-decls -name Transform)
+add_public_tablegen_target(MLIRTransformDialectTransformsIncGen)
+
+add_mlir_doc(Passes TransformPasses ./ -gen-pass-doc)

mlir/include/mlir/Dialect/Transform/Transforms/Passes.h

+//===- CheckUses.h - Expensive transform value validity checks --*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef MLIR_DIALECT_TRANSFORM_TRANSFORMS_PASSES_H
+#define MLIR_DIALECT_TRANSFORM_TRANSFORMS_PASSES_H
+
+#include "mlir/Pass/Pass.h"
+#include <memory>
+
+namespace mlir {
+class Pass;
+
+namespace transform {
+std::unique_ptr<Pass> createCheckUsesPass();
+
+#define GEN_PASS_REGISTRATION
+#include "mlir/Dialect/Transform/Transforms/Passes.h.inc"
+} // namespace transform
+} // namespace mlir
+
+#endif // MLIR_DIALECT_TRANSFORM_TRANSFORMS_PASSES_H

mlir/include/mlir/Dialect/Transform/Transforms/Passes.td

+//===-- Passes.td - Transform dialect pass definitions -----*- tablegen -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef MLIR_DIALECT_TRANSFORM_TRANSFORMS_PASSES
+#define MLIR_DIALECT_TRANSFORM_TRANSFORMS_PASSES
+
+include "mlir/Pass/PassBase.td"
+
+def CheckUses : Pass<"transform-dialect-check-uses"> {
+  let summary = "warn about potential use-after-free in the transform dialect";
+  let description = [{
+    This pass analyzes operations from the transform dialect and its extensions
+    and warns if a tranfsorm IR value may be used by an operation after it was
+    "freed" by some other operation, as described by side effects on the
+    `TransformMappingResource`. This statically detects situations that lead to
+    errors when interpreting the Transform IR.
+
+    The pass is capable of handling branching control flow and reports all
+    _potential_ use-after-free siutations, e.g., a may-use-after-free is
+    reported if at least one of the control flow paths between the definition of
+    a value and its use contains an operation with a "free" effect on the
+    `TransformMappingResource`. It does not currently perform an SCCP-style data
+    flow analysis to prove that some branches are not taken, however, SCCP and
+    other contorl flow simplifications can be performed on the transform IR
+    prior to this pass provided that transform ops implement the relevant
+    control flow interfaces.
+  }];
+  let constructor = "::mlir::transform::createCheckUsesPass()";
+}
+
+#endif // MLIR_DIALECT_TRANSFORM_TRANSFORMS_PASSES

mlir/include/mlir/InitAllPasses.h

 #include "mlir/Dialect/Quant/Passes.h"
 #include "mlir/Dialect/SCF/Passes.h"
 #include "mlir/Dialect/SPIRV/Transforms/Passes.h"
 #include "mlir/Dialect/Shape/Transforms/Passes.h"
 #include "mlir/Dialect/SparseTensor/Pipelines/Passes.h"
 #include "mlir/Dialect/SparseTensor/Transforms/Passes.h"
 #include "mlir/Dialect/Tensor/Transforms/Passes.h"
 #include "mlir/Dialect/Tosa/Transforms/Passes.h"
+#include "mlir/Dialect/Transform/Transforms/Passes.h"
 #include "mlir/Dialect/Vector/Transforms/Passes.h"
 #include "mlir/Transforms/Passes.h"
 
 #include <cstdlib>
 
 namespace mlir {
 
 // This function may be called to register the MLIR passes with the
inline void registerAllPasses() {
   LLVM::registerLLVMPasses();
   memref::registerMemRefPasses();
   quant::registerQuantPasses();
   registerSCFPasses();
   registerShapePasses();
   spirv::registerSPIRVPasses();
   tensor::registerTensorPasses();
   tosa::registerTosaOptPasses();
+  transform::registerTransformPasses();
   vector::registerVectorPasses();
 
   // Dialect pipelines
   sparse_tensor::registerSparseTensorPipelines();
 }
 
 } // namespace mlir
 
 #endif // MLIR_INITALLPASSES_H_

mlir/lib/Dialect/Transform/CMakeLists.txt

 add_subdirectory(IR)
+add_subdirectory(Transforms)

mlir/lib/Dialect/Transform/IR/TransformOps.cpp

 //===- TransformDialect.cpp - Transform dialect operations ----------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include "mlir/Dialect/Transform/IR/TransformOps.h"
 #include "mlir/Dialect/PDL/IR/PDLOps.h"
 #include "mlir/Dialect/Transform/IR/TransformDialect.h"
 #include "mlir/Dialect/Transform/IR/TransformInterfaces.h"
 #include "mlir/IR/Builders.h"
 #include "mlir/IR/OpImplementation.h"
 #include "mlir/IR/PatternMatch.h"
+#include "mlir/Interfaces/ControlFlowInterfaces.h"
 #include "mlir/Rewrite/FrozenRewritePatternSet.h"
 #include "mlir/Rewrite/PatternApplicator.h"
 #include "llvm/ADT/ScopeExit.h"
 
 using namespace mlir;
 
 #define GET_OP_CLASSES
 #include "mlir/Dialect/Transform/IR/TransformOps.cpp.inc"
for (Operation &op : *getBodyBlock()) {
 
     SmallVector<MemoryEffects::EffectInstance, 2> nestedEffects;
     iface.getEffectsOnValue(getBodyBlock()->getArgument(0), nestedEffects);
     for (const auto &effect : nestedEffects)
       effects.emplace_back(effect.getEffect(), getRoot(), effect.getResource());
   }
 }
 
+OperandRange transform::SequenceOp::getSuccessorEntryOperands(unsigned index) {
+  assert(index == 0 && "unexpected region index");
+  if (getOperation()->getNumOperands() == 1)
+    return getOperation()->getOperands();
+  return OperandRange(getOperation()->operand_end(),
+                      getOperation()->operand_end());
+}
+
+void transform::SequenceOp::getSuccessorRegions(
+    Optional<unsigned> index, ArrayRef<Attribute> operands,
+    SmallVectorImpl<RegionSuccessor> &regions) {
+  if (!index.hasValue()) {
+    Region *bodyRegion = &getBody();
+    regions.emplace_back(bodyRegion, !operands.empty()
+                                         ? bodyRegion->getArguments()
+                                         : Block::BlockArgListType());
+    return;
+  }
+
+  assert(*index == 0 && "unexpected region index");
+  regions.emplace_back(getOperation()->getResults());
+}
+
+void transform::SequenceOp::getRegionInvocationBounds(
+    ArrayRef<Attribute> operands, SmallVectorImpl<InvocationBounds> &bounds) {
+  (void)operands;
+  bounds.emplace_back(1, 1);
+}
+
 //===----------------------------------------------------------------------===//
 // WithPDLPatternsOp
 //===----------------------------------------------------------------------===//
 
 LogicalResult
 transform::WithPDLPatternsOp::apply(transform::TransformResults &results,
                                     transform::TransformState &state) {
   OwningOpRef<ModuleOp> pdlModuleOp =

mlir/lib/Dialect/Transform/Transforms/CMakeLists.txt

+add_mlir_dialect_library(MLIRTransformDialectTransforms
+  CheckUses.cpp
+
+  DEPENDS
+  MLIRTransformDialectTransformsIncGen
+
+  LINK_LIBS PUBLIC
+  MLIRTransformDialect
+  MLIRIR
+  MLIRPass
+)

mlir/lib/Dialect/Transform/Transforms/CheckUses.cpp

+//===- CheckUses.cpp - Expensive transform value validity checks ----------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This file defines a pass that performs expensive opt-in checks for Transform
+// dialect values being potentially used after they have been consumed.
+//
+//===----------------------------------------------------------------------===//
+
+#include "mlir/Analysis/DataFlowAnalysis.h"
+#include "mlir/Dialect/Transform/IR/TransformInterfaces.h"
+#include "mlir/Dialect/Transform/Transforms/Passes.h"
+#include "mlir/Interfaces/SideEffectInterfaces.h"
+#include "mlir/Pass/Pass.h"
+#include "llvm/ADT/SetOperations.h"
+
+using namespace mlir;
+
+namespace {
+
+/// Returns a reference to a cached set of blocks that are reachable from the
+/// given block via edges computed by the `getNextNodes` function. For example,
+/// if `getNextNodes` returns successors of a block, this will return the set of
+/// reachable blocks; if it returns predecessors of a block, this will return
+/// the set of blocks from which the given block can be reached. The block is
+/// considered reachable form itself only if there is a cycle.
+template <typename FnTy>
+const llvm::SmallPtrSet<Block *, 4> &
+getReachableImpl(Block *block, FnTy getNextNodes,
+                 DenseMap<Block *, llvm::SmallPtrSet<Block *, 4>> &cache) {
+  auto it = cache.find(block);
+  if (it != cache.end())
+    return it->getSecond();
+
+  llvm::SmallPtrSet<Block *, 4> &reachable = cache[block];
+  SmallVector<Block *> worklist;
+  worklist.push_back(block);
+  while (!worklist.empty()) {
+    Block *current = worklist.pop_back_val();
+    for (Block *predecessor : getNextNodes(current)) {
+      // The block is reachable from its transitive predecessors. Only add
+      // them to the worklist if they weren't already visited.
+      if (reachable.insert(predecessor).second)
+        worklist.push_back(predecessor);
+    }
+  }
+  return reachable;
+}
+
+/// An analysis that identifies whether a value allocated by a Transform op may
+/// be used by another such op after it may have been freed by a third op on
+/// some control flow path. This is conceptually similar to a data flow
+/// analysis, but relies on side effects related to particular values that
+/// currently cannot be modeled by the MLIR data flow analysis framework (also,
+/// the lattice element would be rather expensive as it would need to include
+/// live and/or freed values for each operation).
+///
+/// This analysis is conservatively pessimisic: it will consider that a value
+/// may be freed if it is freed on any possible control flow path between its
+/// allocation and a relevant use, even if the control never actually flows
+/// through the operation that frees the value. It also does not differentiate
+/// between may- (freed on at least one control flow path) and must-free (freed
+/// on all possible control flow paths) because it would require expensive graph
+/// algorithms.
+///
+/// It is intended as an additional non-blocking verification or debugging aid
+/// for ops in the Transform dialect. It leverages the requirement for Transform
+/// dialect ops to implement the MemoryEffectsOpInterface, and expects the
+/// values in the Transform IR to have an allocation effect on the
+/// TransformMappingResource when defined.
+class TransformOpMemFreeAnalysis {
+public:
+  MLIR_DEFINE_EXPLICIT_INTERNAL_INLINE_TYPE_ID(TransformOpMemFreeAnalysis)
+
+  /// Computes the analysis for Transform ops nested in the given operation.
+  explicit TransformOpMemFreeAnalysis(Operation *root) {
+    root->walk([&](Operation *op) {
+      if (isa<transform::TransformOpInterface>(op)) {
+        collectFreedValues(op);
+        return WalkResult::skip();
+      }
+      return WalkResult::advance();
+    });
+  }
+
+  /// A list of operations that may be deleting a value. Non-empty list
+  /// contextually converts to boolean "true" value.
+  class PotentialDeleters {
+  public:
+    /// Creates an empty list that corresponds to the value being live.
+    static PotentialDeleters live() { return PotentialDeleters({}); }
+
+    /// Creates a list from the operations that may be deleting the value.
+    static PotentialDeleters maybeFreed(ArrayRef<Operation *> deleters) {
+      return PotentialDeleters(deleters);
+    }
+
+    /// Converts to "true" if there are operations that may be deleting the
+    /// value.
+    explicit operator bool() const { return !deleters.empty(); }
+
+    /// Concatenates the lists of operations that may be deleting the value. The
+    /// value is known to be live if the reuslting list is still empty.
+    PotentialDeleters &operator|=(const PotentialDeleters &other) {
+      llvm::append_range(deleters, other.deleters);
+      return *this;
+    }
+
+    /// Returns the list of ops that may be deleting the value.
+    ArrayRef<Operation *> getOps() const { return deleters; }
+
+  private:
+    /// Constructs the list from the given operations.
+    explicit PotentialDeleters(ArrayRef<Operation *> ops) {
+      llvm::append_range(deleters, ops);
+    }
+
+    /// The list of operations that may be deleting the value.
+    SmallVector<Operation *> deleters;
+  };
+
+  /// Returns the list of operations that may be deleting the operand value on
+  /// any control flow path between the definition of the value and its use as
+  /// the given operand. For the purposes of this analysis, the value is
+  /// considered to be allocated at its definition point and never re-allocated.
+  PotentialDeleters isUseLive(OpOperand &operand) {
+    const llvm::SmallPtrSet<Operation *, 2> &deleters = freedBy[operand.get()];
+    if (deleters.empty())
+      return live();
+
+#ifndef NDEBUG
+    // Check that the definition point actually allcoates the value.
+    Operation *valueSource =
+        operand.get().isa<OpResult>()
+            ? operand.get().getDefiningOp()
+            : operand.get().getParentBlock()->getParentOp();
+    auto iface = cast<MemoryEffectOpInterface>(valueSource);
+    SmallVector<MemoryEffects::EffectInstance> instances;
+    iface.getEffectsOnResource(transform::TransformMappingResource::get(),
+                               instances);
+    assert(hasEffect<MemoryEffects::Allocate>(instances, operand.get()) &&
+           "expected the op defining the value to have an allocation effect "
+           "on it");
+#endif
+
+    // Collect ancestors of the use operation.
+    Block *defBlock = operand.get().getParentBlock();
+    SmallVector<Operation *> ancestors;
+    Operation *ancestor = operand.getOwner();
+    do {
+      ancestors.push_back(ancestor);
+      if (ancestor->getParentRegion() == defBlock->getParent())
+        break;
+      ancestor = ancestor->getParentOp();
+    } while (true);
+    std::reverse(ancestors.begin(), ancestors.end());
+
+    // Consider the control flow from the definition point of the value to its
+    // use point. If the use is located in some nested region, consider the path
+    // from the entry block of the region to the use.
+    for (Operation *ancestor : ancestors) {
+      // The block should be considered partially if it is the block that
+      // contains the definition (allocation) of the value being used, and the
+      // value is defined in the middle of the block, i.e., is not a block
+      // argument.
+      bool isOutermost = ancestor == ancestors.front();
+      bool isFromBlockPartial = isOutermost && operand.get().isa<OpResult>();
+
+      // Check if the value may be freed by operations between its definition
+      // (allocation) point in its block and the terminator of the block or the
+      // ancestor of the use if it is located in the same block. This is only
+      // done for partial blocks here, full blocks will be considered below
+      // similarly to other blocks.
+      if (isFromBlockPartial) {
+        bool defUseSameBlock = ancestor->getBlock() == defBlock;
+        // Consider all ops from the def to its block terminator, except the
+        // when the use is in the same block, in which case only consider the
+        // ops until the user.
+        if (PotentialDeleters potentialDeleters = isFreedInBlockAfter(
+                operand.get().getDefiningOp(), operand.get(),
+                defUseSameBlock ? ancestor : nullptr))
+          return potentialDeleters;
+      }
+
+      // Check if the value may be freed by opeations preceding the ancestor in
+      // its block. Skip the check for partial blocks that contain both the
+      // definition and the use point, as this has been already checked above.
+      if (!isFromBlockPartial || ancestor->getBlock() != defBlock) {
+        if (PotentialDeleters potentialDeleters =
+                isFreedInBlockBefore(ancestor, operand.get()))
+          return potentialDeleters;
+      }
+
+      // Check if the value may be freed by operations in any of the blocks
+      // between the definition point (in the outermost region) or the entry
+      // block of the region (in other regions) and the operand or its ancestor
+      // in the region. This includes the entire "form" block if (1) the block
+      // has not been considered as partial above and (2) the block can be
+      // reached again through some control-flow loop. This includes the entire
+      // "to" block if it can be reached form itself through some control-flow
+      // cycle, regardless of whether it has been visited before.
+      Block *ancestorBlock = ancestor->getBlock();
+      Block *from =
+          isOutermost ? defBlock : &ancestorBlock->getParent()->front();
+      if (PotentialDeleters potentialDeleters =
+              isMaybeFreedOnPaths(from, ancestorBlock, operand.get(),
+                                  /*alwaysIncludeFrom=*/!isFromBlockPartial))
+        return potentialDeleters;
+    }
+    return live();
+  }
+
+private:
+  /// Make PotentialDeleters constructors available with shorter names.
+  static PotentialDeleters maybeFreed(ArrayRef<Operation *> deleters) {
+    return PotentialDeleters::maybeFreed(deleters);
+  }
+  static PotentialDeleters live() { return PotentialDeleters::live(); }
+
+  /// Returns the list of operations that may be deleting the given value betwen
+  /// the first and last operations, non-inclusive. `getNext` indicates the
+  /// direction of the traversal.
+  PotentialDeleters
+  isFreedBetween(Value value, Operation *first, Operation *last,
+                 llvm::function_ref<Operation *(Operation *)> getNext) const {
+    auto it = freedBy.find(value);
+    if (it == freedBy.end())
+      return live();
+    const llvm::SmallPtrSet<Operation *, 2> &deleters = it->getSecond();
+    for (Operation *op = getNext(first); op != last; op = getNext(op)) {
+      if (deleters.contains(op))
+        return maybeFreed(op);
+    }
+    return live();
+  }
+
+  /// Returns the list of operations that may be deleting the given value
+  /// between `root` and `before` values. `root` is expected to be in the same
+  /// block as `before` and precede it. If `before` is null, consider all
+  /// operations until the end of the block including the terminator.
+  PotentialDeleters isFreedInBlockAfter(Operation *root, Value value,
+                                        Operation *before = nullptr) const {
+    return isFreedBetween(value, root, before,
+                          [](Operation *op) { return op->getNextNode(); });
+  }
+
+  /// Returns the list of operations that may be deleting the given value
+  /// between the entry of the block and the `root` operation.
+  PotentialDeleters isFreedInBlockBefore(Operation *root, Value value) const {
+    return isFreedBetween(value, root, nullptr,
+                          [](Operation *op) { return op->getPrevNode(); });
+  }
+
+  /// Returns the list of operations that may be deleting the given value on
+  /// any of the control flow paths between the "form" and the "to" block. The
+  /// operations from any block visited on any control flow path are
+  /// consdiered. The "from" block is considered if there is a control flow
+  /// cycle going through it, i.e., if there is a possibility that all
+  /// operations in this block are visited or if the `alwaysIncludeFrom` flag is
+  /// set. The "to" block is considered only if there is a control flow cycle
+  /// going through it.
+  PotentialDeleters isMaybeFreedOnPaths(Block *from, Block *to, Value value,
+                                        bool alwaysIncludeFrom) {
+    // Find all blocks that lie on any path between "from" and "to", i.e., the
+    // intersection of blocks reachable from "from" and blocks from which "to"
+    // is rechable.
+    const llvm::SmallPtrSet<Block *, 4> &sources = getReachableFrom(to);
+    if (!sources.contains(from))
+      return live();
+
+    llvm::SmallPtrSet<Block *, 4> reachable(getReachable(from));
+    llvm::set_intersect(reachable, sources);
+
+    // If requested, include the "from" block that may not be present in the set
+    // of visited blocks when there is no cycle going through it.
+    if (alwaysIncludeFrom)
+      reachable.insert(from);
+
+    // Join potential deleters from all blocks as we don't know here which of
+    // the paths through the control flow is taken.
+    PotentialDeleters potentialDeleters = live();
+    for (Block *block : reachable) {
+      for (Operation &op : *block) {
+        if (freedBy[value].count(&op))
+          potentialDeleters |= maybeFreed(&op);
+      }
+    }
+    return potentialDeleters;
+  }
+
+  /// Popualtes `reachable` with the set of blocks that are rechable from the
+  /// given block. A block is considered reachable from itself if there is a
+  /// cycle in the control-flow graph that invovles the block.
+  const llvm::SmallPtrSet<Block *, 4> &getReachable(Block *block) {
+    return getReachableImpl(
+        block, [](Block *b) { return b->getSuccessors(); }, reachableCache);
+  }
+
+  /// Populates `sources` with the set of blocks from which the given block is
+  /// reachable.
+  const llvm::SmallPtrSet<Block *, 4> &getReachableFrom(Block *block) {
+    return getReachableImpl(
+        block, [](Block *b) { return b->getPredecessors(); },
+        reachableFromCache);
+  }
+
+  /// Returns true of `instances` contains an effect of `EffectTy` on `value`.
+  template <typename EffectTy>
+  static bool hasEffect(ArrayRef<MemoryEffects::EffectInstance> instances,
+                        Value value) {
+    return llvm::any_of(instances,
+                        [&](const MemoryEffects::EffectInstance &instance) {
+                          return instance.getValue() == value &&
+                                 isa<EffectTy>(instance.getEffect());
+                        });
+  }
+
+  /// Records the values that are being freed by an operation or any of its
+  /// children in `freedBy`.
+  void collectFreedValues(Operation *root) {
+    SmallVector<MemoryEffects::EffectInstance> instances;
+    root->walk([&](Operation *child) {
+      // TODO: extend this to conservatively handle operations with undeclared
+      // side effects as maybe freeing the operands.
+      auto iface = cast<MemoryEffectOpInterface>(child);
+      instances.clear();
+      iface.getEffectsOnResource(transform::TransformMappingResource::get(),
+                                 instances);
+      for (Value operand : child->getOperands()) {
+        if (hasEffect<MemoryEffects::Free>(instances, operand)) {
+          // All parents of the operation that frees a value should be
+          // considered as potentially freeing the value as well.
+          //
+          // TODO: differentiate between must-free/may-free as well as between
+          // this op having the effect and children having the effect. This may
+          // require some analysis of all control flow paths through the nested
+          // regions as well as a mechanism to separate proper side effects from
+          // those obtained by nesting.
+          Operation *parent = child;
+          do {
+            freedBy[operand].insert(parent);
+            if (parent == root)
+              break;
+            parent = parent->getParentOp();
+          } while (true);
+        }
+      }
+    });
+  }
+
+  /// The mapping from a value to operations that have a Free memory effect on
+  /// the TransformMappingResource and associated with this value, or to
+  /// Transform operations transitively containing such operations.
+  DenseMap<Value, llvm::SmallPtrSet<Operation *, 2>> freedBy;
+
+  /// Caches for sets of reachable blocks.
+  DenseMap<Block *, llvm::SmallPtrSet<Block *, 4>> reachableCache;
+  DenseMap<Block *, llvm::SmallPtrSet<Block *, 4>> reachableFromCache;
+};
+
+#define GEN_PASS_CLASSES
+#include "mlir/Dialect/Transform/Transforms/Passes.h.inc"
+
+//// A simple pass that warns about any use of a value by a transform operation
+// that may be using the value after it has been freed.
+class CheckUsesPass : public CheckUsesBase<CheckUsesPass> {
+public:
+  void runOnOperation() override {
+    auto &analysis = getAnalysis<TransformOpMemFreeAnalysis>();
+
+    getOperation()->walk([&](Operation *child) {
+      for (OpOperand &operand : child->getOpOperands()) {
+        TransformOpMemFreeAnalysis::PotentialDeleters deleters =
+            analysis.isUseLive(operand);
+        if (!deleters)
+          continue;
+
+        InFlightDiagnostic diag = child->emitWarning()
+                                  << "operand #" << operand.getOperandNumber()
+                                  << " may be used after free";
+        diag.attachNote(operand.get().getLoc()) << "allocated here";
+        for (Operation *d : deleters.getOps()) {
+          diag.attachNote(d->getLoc()) << "freed here";
+        }
+      }
+    });
+  }
+};
+
+} // namespace
+
+namespace mlir {
+namespace transform {
+std::unique_ptr<Pass> createCheckUsesPass() {
+  return std::make_unique<CheckUsesPass>();
+}
+} // namespace transform
+} // namespace mlir

mlir/test/Dialect/Transform/check-use-after-free.mlir

+// RUN: mlir-opt %s --transform-dialect-check-uses --split-input-file --verify-diagnostics
+
+func.func @use_after_free_branching_control_flow() {
+  // expected-note @below {{allocated here}}
+  %0 = transform.test_produce_param_or_forward_operand 42
+  transform.test_transform_op_with_regions {
+    "transform.test_branching_transform_op_terminator"() : () -> ()
+  },
+  {
+  ^bb0:
+    "transform.test_branching_transform_op_terminator"()[^bb1, ^bb2] : () -> ()
+  ^bb1:
+    // expected-note @below {{freed here}}
+    transform.test_consume_operand_if_matches_param_or_fail %0[42]
+    "transform.test_branching_transform_op_terminator"()[^bb3] : () -> ()
+  ^bb2:
+    "transform.test_branching_transform_op_terminator"()[^bb3] : () -> ()
+  ^bb3:
+    // expected-warning @below {{operand #0 may be used after free}}
+    transform.sequence %0 {
+    ^bb0(%arg0: !pdl.operation):
+    }
+    "transform.test_branching_transform_op_terminator"() : () -> ()
+  }
+  return
+}
+
+// -----
+
+func.func @use_after_free_in_nested_op() {
+  // expected-note @below {{allocated here}}
+  %0 = transform.test_produce_param_or_forward_operand 42
+  // expected-note @below {{freed here}}
+  transform.test_transform_op_with_regions {
+    "transform.test_branching_transform_op_terminator"() : () -> ()
+  },
+  {
+  ^bb0:
+    "transform.test_branching_transform_op_terminator"()[^bb1, ^bb2] : () -> ()
+  ^bb1:
+    transform.test_consume_operand_if_matches_param_or_fail %0[42]
+    "transform.test_branching_transform_op_terminator"()[^bb3] : () -> ()
+  ^bb2:
+    "transform.test_branching_transform_op_terminator"()[^bb3] : () -> ()
+  ^bb3:
+    "transform.test_branching_transform_op_terminator"() : () -> ()
+  }
+  // expected-warning @below {{operand #0 may be used after free}}
+  transform.sequence %0 {
+    ^bb0(%arg0: !pdl.operation):
+  }
+  return
+}
+
+// -----
+
+func.func @use_after_free_recursive_side_effects() {
+  transform.sequence {
+  ^bb0(%arg0: !pdl.operation):
+    // expected-note @below {{allocated here}}
+    %0 = transform.sequence %arg0 attributes { ord = 1 } {
+    ^bb1(%arg1: !pdl.operation):
+      yield %arg1 : !pdl.operation
+    } : !pdl.operation
+    transform.sequence %0 attributes { ord = 2 } {
+    ^bb2(%arg2: !pdl.operation):
+    }
+    transform.sequence %0 attributes { ord = 3 } {
+    ^bb3(%arg3: !pdl.operation):
+    }
+    
+    // `transform.sequence` has recursive side effects so it has the same "free"
+    // as the child op it contains.
+    // expected-note @below {{freed here}}
+    transform.sequence %0 attributes { ord = 4 } {
+    ^bb4(%arg4: !pdl.operation):
+      test_consume_operand_if_matches_param_or_fail %0[42]
+    }
+    // expected-warning @below {{operand #0 may be used after free}}
+    transform.sequence %0 attributes { ord = 5 } {
+    ^bb3(%arg3: !pdl.operation):
+    }
+  }
+  return
+}
+
+// -----
+
+func.func @use_after_free() {
+  transform.sequence {
+  ^bb0(%arg0: !pdl.operation):
+    // expected-note @below {{allocated here}}
+    %0 = transform.sequence %arg0 attributes { ord = 1 } {
+    ^bb1(%arg1: !pdl.operation):
+      yield %arg1 : !pdl.operation
+    } : !pdl.operation
+    transform.sequence %0 attributes { ord = 2 } {
+    ^bb2(%arg2: !pdl.operation):
+    }
+    transform.sequence %0 attributes { ord = 3 } {
+    ^bb3(%arg3: !pdl.operation):
+    }
+    
+    // expected-note @below {{freed here}}
+    test_consume_operand_if_matches_param_or_fail %0[42]
+    // expected-warning @below {{operand #0 may be used after free}}
+    transform.sequence %0 attributes { ord = 5 } {
+    ^bb3(%arg3: !pdl.operation):
+    }
+  }
+  return
+}
+
+// -----
+
+// In the case of a control flow cycle, the operation that uses the value may
+// precede the one that frees it in the same block. Both operations should
+// be reported as use-after-free.
+func.func @use_after_free_self_cycle() {
+  // expected-note @below {{allocated here}}
+  %0 = transform.test_produce_param_or_forward_operand 42
+  transform.test_transform_op_with_regions {
+    "transform.test_branching_transform_op_terminator"() : () -> ()
+  },
+  {
+  ^bb0:
+    "transform.test_branching_transform_op_terminator"()[^bb1] : () -> ()
+  ^bb1:
+    // expected-warning @below {{operand #0 may be used after free}}
+    transform.sequence %0 {
+    ^bb0(%arg0: !pdl.operation):
+    }
+    // expected-warning @below {{operand #0 may be used after free}}
+    // expected-note @below {{freed here}}
+    transform.test_consume_operand_if_matches_param_or_fail %0[42]
+    "transform.test_branching_transform_op_terminator"()[^bb1, ^bb2] : () -> ()
+  ^bb2:
+    "transform.test_branching_transform_op_terminator"() : () -> ()
+  }
+  return
+}
+
+
+// -----
+
+// Check that the "free" that happens in a cycle is also reported as potential
+// use-after-free.
+func.func @use_after_free_cycle() {
+  // expected-note @below {{allocated here}}
+  %0 = transform.test_produce_param_or_forward_operand 42
+  transform.test_transform_op_with_regions {
+    "transform.test_branching_transform_op_terminator"() : () -> ()
+  },
+  {
+  ^bb0:
+    "transform.test_branching_transform_op_terminator"()[^bb1, ^bb2] : () -> ()
+  ^bb1:
+    // expected-warning @below {{operand #0 may be used after free}}
+    // expected-note @below {{freed here}}
+    transform.test_consume_operand_if_matches_param_or_fail %0[42]
+    "transform.test_branching_transform_op_terminator"()[^bb2, ^bb3] : () -> ()
+  ^bb2:
+    "transform.test_branching_transform_op_terminator"()[^bb1] : () -> ()
+  ^bb3:
+    "transform.test_branching_transform_op_terminator"() : () -> ()
+  }
+  return
+}
+

mlir/test/lib/Dialect/Transform/TestTransformDialectExtension.cpp

return extension->updateMapping(state.getPayloadOps(getOperand()).front(),
                                   getOperation());
 }
 
 LogicalResult mlir::test::TestRemoveTestExtensionOp::apply(
     transform::TransformResults &results, transform::TransformState &state) {
   state.removeExtension<TestTransformStateExtension>();
   return success();
 }
+LogicalResult mlir::test::TestTransformOpWithRegions::apply(
+    transform::TransformResults &results, transform::TransformState &state) {
+  return success();
+}
+
+void mlir::test::TestTransformOpWithRegions::getEffects(
+    SmallVectorImpl<MemoryEffects::EffectInstance> &effects) {}
+
+LogicalResult mlir::test::TestBranchingTransformOpTerminator::apply(
+    transform::TransformResults &results, transform::TransformState &state) {
+  return success();
+}
+
+void mlir::test::TestBranchingTransformOpTerminator::getEffects(
+    SmallVectorImpl<MemoryEffects::EffectInstance> &effects) {}
 
 namespace {
 /// Test extension of the Transform dialect. Registers additional ops and
 /// declares PDL as dependent dialect since the additional ops are using PDL
 /// types for operands and results.
 class TestTransformDialectExtension
     : public transform::TransformDialectExtension<
           TestTransformDialectExtension> {

mlir/test/lib/Dialect/Transform/TestTransformDialectExtension.td

 def TestRemoveTestExtensionOp
   : Op<Transform_Dialect, "test_remove_test_extension",
        [DeclareOpInterfaceMethods<TransformOpInterface>,
         NoSideEffect]> {
   let assemblyFormat = "attr-dict";
   let cppNamespace = "::mlir::test";
 }
 
+def TestTransformOpWithRegions
+  : Op<Transform_Dialect, "test_transform_op_with_regions",
+       [DeclareOpInterfaceMethods<TransformOpInterface>,
+        DeclareOpInterfaceMethods<MemoryEffectsOpInterface>]> {
+  let regions = (region AnyRegion:$first, AnyRegion:$second);
+  let assemblyFormat = "attr-dict-with-keyword regions";
+  let cppNamespace = "::mlir::test";
+}
+
+def TestBranchingTransformOpTerminator
+  : Op<Transform_Dialect, "test_branching_transform_op_terminator",
+       [Terminator, DeclareOpInterfaceMethods<TransformOpInterface>,
+        DeclareOpInterfaceMethods<MemoryEffectsOpInterface>]> {
+  let successors = (successor VariadicSuccessor<AnySuccessor>:$succ);
+  let cppNamespace = "::mlir::test";
+}
 
 #endif // MLIR_TESTTRANSFORMDIALECTEXTENSION_TD

utils/bazel/llvm-project-overlay/mlir/BUILD.bazel

deps = [
         ":SparseTensorTransforms",
         ":TensorDialect",
         ":TensorInferTypeOpInterfaceImpl",
         ":TensorTilingInterfaceImpl",
         ":TensorTransforms",
         ":TosaDialect",
         ":TosaToLinalg",
         ":TransformDialect",
+        ":TransformDialectTransforms",
         ":Transforms",
         ":TransformsPassIncGen",
         ":VectorOps",
         ":VectorToLLVM",
         ":VectorToROCDL",
         ":VectorToSCF",
         ":VectorToSPIRV",
         ":VectorTransforms",
deps = [
         ":Transforms",
     ],
 )
 
 td_library(
     name = "TransformDialectTdFiles",
     srcs = glob(["include/mlir/Dialect/Transform/IR/*.td"]),
     deps = [
+        ":ControlFlowInterfacesTdFiles",
         ":OpBaseTdFiles",
         ":PDLDialectTdFiles",
         ":SideEffectInterfacesTdFiles",
     ],
 )
 
 gentbl_cc_library(
     name = "TransformDialectInterfacesIncGen",
gentbl_cc_library(
     deps = [":TransformDialectTdFiles"],
 )
 
 cc_library(
     name = "TransformDialect",
     srcs = glob(["lib/Dialect/Transform/IR/*.cpp"]),
     hdrs = glob(["include/mlir/Dialect/Transform/IR/*.h"]),
     deps = [
+        ":ControlFlowInterfaces",
         ":IR",
         ":PDLDialect",
         ":PDLInterpDialect",
         ":Rewrite",
         ":SideEffectInterfaces",
         ":Support",
         ":TransformDialectIncGen",
         ":TransformDialectInterfacesIncGen",
         ":TransformOpsIncGen",
         "//llvm:Support",
     ],
 )
 
 td_library(
+    name = "TransformDialectTransformsTdFiles",
+    srcs = glob(["include/mlir/Dialect/Transform/Transforms/*.td"]),
+    deps = [
+        ":PassBaseTdFiles",
+        ":TransformDialectTdFiles",
+    ],
+)
+
+gentbl_cc_library(
+    name = "TransformDialectTransformsIncGen",
+    strip_include_prefix = "include",
+    tbl_outs = [
+        (
+            [
+                "-gen-pass-decls",
+                "-name=Transform",
+            ],
+            "include/mlir/Dialect/Transform/Transforms/Passes.h.inc",
+        ),
+    ],
+    tblgen = ":mlir-tblgen",
+    td_file = "include/mlir/Dialect/Transform/Transforms/Passes.td",
+    deps = [":TransformDialectTransformsTdFiles"],
+)
+
+cc_library(
+    name = "TransformDialectTransforms",
+    srcs = glob(["lib/Dialect/Transform/Transforms/*.cpp"]),
+    hdrs = glob(["include/mlir/Dialect/Transform/Transforms/*.h"]),
+    deps = [
+        ":Analysis",
+        ":IR",
+        ":Pass",
+        ":SideEffectInterfaces",
+        ":TransformDialect",
+        ":TransformDialectTransformsIncGen",
+        "//llvm:Support",
+    ],
+)
+
+td_library(
     name = "ComplexOpsTdFiles",
     srcs = [
         "include/mlir/Dialect/Complex/IR/ComplexBase.td",
         "include/mlir/Dialect/Complex/IR/ComplexOps.td",
     ],
     includes = ["include"],
     deps = [
         ":InferTypeOpInterfaceTdFiles",