/* Copyright 2019 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 <cassert>
#include <memory>
#include <queue>
#include <utility>

#include "llvm/ADT/STLExtras.h"
#include "mlir/IR/SymbolTable.h"  // from @llvm-project
#include "mlir/Pass/Pass.h"  // from @llvm-project
#include "mlir/Support/LogicalResult.h"  // from @llvm-project
#include "mlir/Transforms/GreedyPatternRewriteDriver.h"  // from @llvm-project
#include "tensorflow/compiler/mlir/tensorflow/ir/tf_device.h"
#include "tensorflow/compiler/mlir/tensorflow/transforms/decompose_resource_ops.h"
#include "tensorflow/compiler/mlir/tensorflow/transforms/passes.h"

namespace mlir {
namespace TFDevice {
namespace {

constexpr char kBadDecompositionMessage[] =
    "Resource ops decomposition did not converge";

// Decomposing resource ops should not take more than a few iterations (2-3) to
// converge as only a few patterns create new resource ops that can be further
// decomposed. The rest of the iterations are enough to clean up any dead ops
// created by decomposition.
constexpr int kMaxIterations = 20;

// Populates `reachable_functions` with all functions that can be reached from
// device cluster ops.
void PopulateClusterReachableFunctions(
    ModuleOp module, SmallPtrSetImpl<Operation*>& reachable_functions) {
  SymbolTableCollection table;
  SymbolUserMap symbol_map(table, module);

  // Create map from caller to set of all callee(s).
  llvm::DenseMap<func::FuncOp, llvm::DenseSet<func::FuncOp>> caller_callee_map;

  // Use worklist to populate the set of reachable functions.
  std::queue<func::FuncOp> function_worklist;

  // Iterates over all functions within the module to (1) create caller-callee
  // map, and (2) initialize function worklist with functions referenced from
  // device cluster ops.
  for (auto func : module.getOps<func::FuncOp>()) {
    for (auto user : symbol_map.getUsers(func)) {
      // Populate caller-callee map.
      if (func::FuncOp caller = user->getParentOfType<func::FuncOp>())
        caller_callee_map[caller].insert(func);
      // Initialize function worklist with functions refrerenced in device
      // cluster.
      if (auto cluster = user->getParentOfType<tf_device::ClusterOp>()) {
        if (reachable_functions.insert(func).second)
          function_worklist.push(func);
      }
    }
  }

  // Uses worklist algorithm to insert all functions reachable from device
  // cluster ops.
  while (!function_worklist.empty()) {
    func::FuncOp caller = function_worklist.front();
    function_worklist.pop();
    for (auto callee : caller_callee_map[caller]) {
      if (reachable_functions.insert(callee).second)
        function_worklist.push(callee);
    }
  }
}

// Applies patterns locally on ops within `cluster` until convergence or
// `max_iterations` are reached. Returns failure if resource ops decomposition
// does not converge after `max_iterations`.
// TODO(prakalps): This can be useful to a lot of other passes in bridge.
// Extract out as a separate utility.
LogicalResult ApplyPatternsLocallyUntilConverged(
    Operation* op_with_regions, FrozenRewritePatternSet& patterns,
    int max_iterations) {
  bool changed = true;
  int iteration = 0;
  while (changed && (iteration++ < max_iterations)) {
    changed = false;
    auto walk_result =
        op_with_regions->walk([&patterns, &changed](Operation* operation) {
          GreedyRewriteConfig config;
          config.setStrictness(mlir::GreedyRewriteStrictness::ExistingOps);
          bool op_erased;
          if (failed(applyOpPatternsGreedily(operation, patterns, config,
                                             &op_erased)))
            return WalkResult::interrupt();
          changed |= op_erased;
          return WalkResult::advance();
        });
    if (walk_result.wasInterrupted()) return failure();
  }
  // Return failure is `op_with_region` was modified changed in last iteration.
  return success(!changed);
}

// Applies patterns in only device clusters and functions reachable from such
// clusters. Returns failure if it fails to converge in `max_iterations`.
// TODO(prakalps): This can be useful to a lot of other passes in bridge.
// Extract out as a separate utility.
LogicalResult ApplyPatternsInClusterAndReachableFunctions(
    ModuleOp module, FrozenRewritePatternSet& patterns, int max_iterations) {
  SmallPtrSet<Operation*, 16> reachable_functions;
  PopulateClusterReachableFunctions(module, reachable_functions);

  // Apply patterns to reachable functions.
  for (Operation* op : reachable_functions) {
    assert(isa<func::FuncOp>(op));
    if (failed(applyPatternsGreedily(op, patterns))) {
      return op->emitError() << kBadDecompositionMessage;
    }
  }

  // Apply patterns to device cluster ops.
  // Note: This module search for cluster ops is a bit wasteful as we could have
  // collected many cluster ops when we were populating reachable functions. But
  // we would still need to do a walk to find all clusters that do not
  // reference any function.
  for (func::FuncOp func : module.getOps<func::FuncOp>()) {
    // If we have already applied patterns to a function then we can skip
    // applying patterns to any device clusters it contains.
    if (reachable_functions.contains(func)) continue;

    auto walk_result = func.walk([&](tf_device::ClusterOp cluster) {
      // Cluster ops are not isolated from above so we cannot use
      // `applyPatternsGreedily` utility. Instead we apply patterns
      // locally on each op within the cluster until convergence.
      if (failed(ApplyPatternsLocallyUntilConverged(cluster, patterns,
                                                    max_iterations))) {
        cluster.emitError() << kBadDecompositionMessage;
        return WalkResult::interrupt();
      }
      return WalkResult::advance();
    });
    if (walk_result.wasInterrupted()) return failure();
  }

  return success();
}

#define GEN_PASS_DEF_DECOMPOSERESOURCEOPSPASS
#include "tensorflow/compiler/mlir/tensorflow/transforms/tf_device_passes.h.inc"

struct DecomposeResourceOpsPass
    : public impl::DecomposeResourceOpsPassBase<DecomposeResourceOpsPass> {
  void runOnOperation() override {
    // Add lowering patterns to the list.
    RewritePatternSet patterns(&getContext());
    TF::PopulateDecomposeResourceOpsPatterns(&getContext(), &patterns);

    if (failed(applyPatternsGreedily(getOperation(), std::move(patterns)))) {
      getOperation().emitError() << kBadDecompositionMessage;
      signalPassFailure();
    }
  }
};

#define GEN_PASS_DEF_DECOMPOSERESOURCEOPSINCLUSTERPASS
#include "tensorflow/compiler/mlir/tensorflow/transforms/tf_device_passes.h.inc"

struct DecomposeResourceOpsInClusterPass
    : public impl::DecomposeResourceOpsInClusterPassBase<
          DecomposeResourceOpsInClusterPass> {
  void runOnOperation() override {
    // Add lowering patterns to the list.
    RewritePatternSet patterns(&getContext());
    TF::PopulateDecomposeResourceOpsPatterns(&getContext(), &patterns);
    FrozenRewritePatternSet frozen_patterns(std::move(patterns));

    if (failed(ApplyPatternsInClusterAndReachableFunctions(
            getOperation(), frozen_patterns, kMaxIterations)))
      signalPassFailure();
  }
};

}  // namespace

std::unique_ptr<OperationPass<func::FuncOp>> CreateDecomposeResourceOpsPass() {
  return std::make_unique<DecomposeResourceOpsPass>();
}

std::unique_ptr<OperationPass<ModuleOp>>
CreateDecomposeResourceOpsInClusterPass() {
  return std::make_unique<DecomposeResourceOpsInClusterPass>();
}

}  // namespace TFDevice
}  // namespace mlir
