common_subexpression_elimination.cpp

#include <torch/csrc/jit/passes/common_subexpression_elimination.h>

#include <torch/csrc/jit/ir/alias_analysis.h>
#include <torch/csrc/jit/ir/ir.h>
#include <torch/csrc/jit/ir/node_hashing.h>
#include <torch/csrc/jit/jit_log.h>

#include <unordered_map>

namespace torch {
namespace jit {
namespace {

struct CommonSubexpressionEliminator {
  CommonSubexpressionEliminator(std::shared_ptr<Graph> graph)
      : graph_(std::move(graph)) {}

  bool run(std::function<Node*(Node*)> parent_lookup_fn) {
    return run(graph_->block(), std::move(parent_lookup_fn));
  }

  // The function implements common subexpression elimination.
  // Since the nodes are visited in topological order, one pass is enough.
  // returns true if CSE made changes to a graph
  bool run(Block* block, std::function<Node*(Node*)> parent_lookup_fn) {
    std::unordered_set<Node*, HashNode, EqualNode> subexprs;
    bool changed = false;
    for (auto it = block->nodes().begin(); it != block->nodes().end(); ++it) {
      auto node = *it;
      if (node->hasSideEffects()) {
        GRAPH_DEBUG("Node was skipped due to side effects:\n", *node);
        continue;
      }
      if (node->isNondeterministic()) {
        GRAPH_DEBUG("Node was skipped due to its non determinism:\n", *node);
        continue;
      }

      if (!node->blocks().empty()) {
        // Traverse sub-blocks.
        for (auto block : node->blocks()) {
          changed |= run(block, [&](Node* n) {
            auto existing = subexprs.find(n);
            if (existing != subexprs.end()) {
              return *existing;
            }

            return parent_lookup_fn(n);
          });
        }

        continue;
      }

      if (getOrCreateAliasDb().hasWriters(node)) {
        GRAPH_DEBUG("Node was skipped due to alias analysis result:\n", *node);
        // Do NOT have enough information to do CSE on these nodes.
        continue;
      }

      // Check for CSE opportunities in the parent block.
      auto parent_lookup = parent_lookup_fn(node);
      auto g_out = node->owningGraph()->outputs();
      if (parent_lookup != nullptr) {
        if (!getOrCreateAliasDb().safeToChangeAliasingRelationship(
                node->outputs(), parent_lookup->outputs())) {
          continue;
        }

        GRAPH_UPDATE("Replacing\n", *node, "with\n", *parent_lookup);
        changed = true;
        node->replaceAllUsesWith(parent_lookup);
        it.destroyCurrent();
        continue;
      }

      // Check whether the same subexpression already exists.
      auto subit = subexprs.insert(node);
      if (!subit.second) {
        // Subexpression exists, replace the uses of node, and destroy it.
        auto existing = *subit.first;

        // don't introduce new aliasing among graph outputs
        if (getOrCreateAliasDb().mayContainAlias(
                node->outputs(), node->owningGraph()->outputs()) &&
            getOrCreateAliasDb().mayContainAlias(existing->outputs(), g_out)) {
          continue;
        }

        GRAPH_UPDATE("Replacing\n", *node, "with\n", *existing);
        changed = true;
        node->replaceAllUsesWith(existing);
        // Destroy the node.
        it.destroyCurrent();
      }
    }

    return changed;
  }

  AliasDb& getOrCreateAliasDb() {
    if (!alias_db_) {
      alias_db_ = std::make_unique<AliasDb>(graph_);
    }

    return *alias_db_;
  }

 private:
  std::unique_ptr<AliasDb> alias_db_;
  std::shared_ptr<Graph> graph_;
};

} // namespace

bool EliminateCommonSubexpression(const std::shared_ptr<Graph>& graph) {
  GRAPH_DUMP("Before CSE", graph);
  CommonSubexpressionEliminator cse(graph);
  return cse.run([](Node*) { return nullptr; });
}
} // namespace jit
} // namespace torch