tensorflow/compiler/mlir/hlo/lib/Dialect/mhlo/transforms/sink_constants_to_control_flow.cc

/* Copyright 2020 The TensorFlow Authors. All Rights Reserved.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
==============================================================================*/

#include "llvm/ADT/DenseMap.h"
#include "llvm/Support/Casting.h"
#include "mlir-hlo/Dialect/mhlo/IR/hlo_ops.h"
#include "mlir-hlo/Dialect/mhlo/transforms/PassDetail.h"
#include "mlir/Dialect/Func/IR/FuncOps.h"
#include "mlir/IR/Operation.h"
#include "mlir/Pass/Pass.h"
#include "mlir/Pass/PassManager.h"
#include "mlir/Support/LLVM.h"
#include "mlir/Transforms/RegionUtils.h"

namespace mlir {
namespace mhlo {

namespace {

// A pass that sinks constants implicitly captured in control flow regions. This
// is necessary to export to XLA.
//
// TODO(b/203775547): Any value used within the region that is defined outside
// of op's region should be sank to the regions and not just the constants. Ops
// such as If and While whose computations doesn't require fixed signature like
// Sort or Reduce have an option to pass outside values as operands of the op to
// avoid recomputing those within internally. Note that doing so is the only
// option in case of values defined outside that are BlockArguments of any of
// the parent region.
class SinkConstantsToControlFlowPass
    : public SinkConstantsToControlFlowPassBase<
          SinkConstantsToControlFlowPass> {
  void runOnOperation() override {
    getOperation().walk([](Operation* op) {
      for (Region& region : op->getRegions()) SinkToRegion(&region);
    });
  }

 private:
  // Performs constant sinking into a region.
  static void SinkToRegion(Region* region) {
    llvm::DenseMap<Value, Operation*> sunk_constant;
    visitUsedValuesDefinedAbove({*region}, [&](OpOperand* use) {
      Value constant = use->get();
      auto* op = constant.getDefiningOp();
      if (!op || !op->hasTrait<mlir::OpTrait::ConstantLike>()) return;
      auto map_entry = sunk_constant.try_emplace(constant, nullptr);
      if (!map_entry.second) {
        // This constant has already been cloned into the region, reuse it.
        use->set(map_entry.first->getSecond()->getResult(0));
        if (op->use_empty()) op->erase();
        return;
      }
      if (constant.hasOneUse()) {
        op->moveBefore(&region->front().front());
        return;
      }
      map_entry.first->getSecond() = op->clone();
      region->front().getOperations().insert(region->front().begin(),
                                             map_entry.first->getSecond());
      use->set(map_entry.first->getSecond()->getResult(0));
    });
  }
};

}  // anonymous namespace

// TODO(hinsu): Rename this pass and move to a different file along with the
// generalization to make all ops isolated from above.
std::unique_ptr<OperationPass<func::FuncOp>>
createSinkConstantsToControlFlowPass() {
  return std::make_unique<SinkConstantsToControlFlowPass>();
}

}  // namespace mhlo
}  // namespace mlir