llvm/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/Dialect/Transform/Transforms/Passes.h"

#include "mlir/Dialect/Transform/Interfaces/TransformInterfaces.h"
#include "mlir/Interfaces/SideEffectInterfaces.h"
#include "mlir/Pass/Pass.h"
#include "llvm/ADT/SetOperations.h"

namespace mlir {
namespace transform {
#define GEN_PASS_DEF_CHECKUSESPASS
#include "mlir/Dialect/Transform/Transforms/Passes.h.inc"
} // namespace transform
} // namespace mlir

usingnamespacemlir;

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) { … }

/// 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 { … };

//// 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 transform::impl::CheckUsesPassBase<CheckUsesPass> { … };

} // namespace