runtime/vm/flow_graph.h

// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

#ifndef VM_FLOW_GRAPH_H_
#define VM_FLOW_GRAPH_H_

#include "vm/growable_array.h"
#include "vm/intermediate_language.h"
#include "vm/parser.h"

namespace dart {

class BlockEffects;
class FlowGraphBuilder;
class ValueInliningContext;
class VariableLivenessAnalysis;

class BlockIterator : public ValueObject {
 public:
  explicit BlockIterator(const GrowableArray<BlockEntryInstr*>& block_order)
      : block_order_(block_order), current_(0) { }

  BlockIterator(const BlockIterator& other)
      : ValueObject(),
        block_order_(other.block_order_),
        current_(other.current_) { }

  void Advance() {
    ASSERT(!Done());
    current_++;
  }

  bool Done() const { return current_ >= block_order_.length(); }

  BlockEntryInstr* Current() const { return block_order_[current_]; }

 private:
  const GrowableArray<BlockEntryInstr*>& block_order_;
  intptr_t current_;
};


// Class to incapsulate the construction and manipulation of the flow graph.
class FlowGraph : public ZoneAllocated {
 public:
  FlowGraph(const FlowGraphBuilder& builder,
            GraphEntryInstr* graph_entry,
            intptr_t max_block_id);

  // Function properties.
  const ParsedFunction& parsed_function() const {
    return parsed_function_;
  }
  intptr_t parameter_count() const {
    return num_copied_params_ + num_non_copied_params_;
  }
  intptr_t variable_count() const {
    return parameter_count() + num_stack_locals_;
  }
  intptr_t num_stack_locals() const {
    return num_stack_locals_;
  }
  intptr_t num_copied_params() const {
    return num_copied_params_;
  }
  intptr_t num_non_copied_params() const {
    return num_non_copied_params_;
  }

  // Flow graph orders.
  const GrowableArray<BlockEntryInstr*>& preorder() const {
    return preorder_;
  }
  const GrowableArray<BlockEntryInstr*>& postorder() const {
    return postorder_;
  }
  const GrowableArray<BlockEntryInstr*>& reverse_postorder() const {
    return reverse_postorder_;
  }

  // Iterators.
  BlockIterator reverse_postorder_iterator() const {
    return BlockIterator(reverse_postorder());
  }
  BlockIterator postorder_iterator() const {
    return BlockIterator(postorder());
  }

  intptr_t current_ssa_temp_index() const { return current_ssa_temp_index_; }
  void set_current_ssa_temp_index(intptr_t index) {
    current_ssa_temp_index_ = index;
  }

  intptr_t max_virtual_register_number() const {
    return current_ssa_temp_index();
  }

  intptr_t max_block_id() const { return max_block_id_; }
  void set_max_block_id(intptr_t id) { max_block_id_ = id; }
  intptr_t allocate_block_id() { return ++max_block_id_; }

  GraphEntryInstr* graph_entry() const {
    return graph_entry_;
  }

  ConstantInstr* constant_null() const {
    return constant_null_;
  }

  intptr_t alloc_ssa_temp_index() { return current_ssa_temp_index_++; }

  intptr_t InstructionCount() const;

  ConstantInstr* GetConstant(const Object& object);
  void AddToInitialDefinitions(Definition* defn);

  void InsertBefore(Instruction* next,
                    Instruction* instr,
                    Environment* env,
                    Definition::UseKind use_kind);
  void InsertAfter(Instruction* prev,
                   Instruction* instr,
                   Environment* env,
                   Definition::UseKind use_kind);

  // Operations on the flow graph.
  void ComputeSSA(intptr_t next_virtual_register_number,
                  ZoneGrowableArray<Definition*>* inlining_parameters);

  // TODO(zerny): Once the SSA is feature complete this should be removed.
  void Bailout(const char* reason) const;

#ifdef DEBUG
  // Verification methods for debugging.
  bool VerifyUseLists();
#endif  // DEBUG

  void DiscoverBlocks();

  // Compute information about effects occuring in different blocks and
  // discover side-effect free paths.
  void ComputeBlockEffects();
  BlockEffects* block_effects() const { return block_effects_; }

  // Remove the redefinition instructions inserted to inhibit code motion.
  void RemoveRedefinitions();

  // Copy deoptimization target from one instruction to another if we still
  // have to keep deoptimization environment at gotos for LICM purposes.
  void CopyDeoptTarget(Instruction* to, Instruction* from) {
    if (is_licm_allowed()) {
      to->InheritDeoptTarget(from);
    }
  }

  // Returns true if every Goto in the graph is expected to have a
  // deoptimization environment and can be used as deoptimization target
  // for hoisted instructions.
  bool is_licm_allowed() const { return licm_allowed_; }

  // Stop preserving environments on Goto instructions. LICM is not allowed
  // after this point.
  void disallow_licm() { licm_allowed_ = false; }

  const ZoneGrowableArray<BlockEntryInstr*>& loop_headers() {
    if (loop_headers_ == NULL) {
      loop_headers_ = ComputeLoops();
    }
    return *loop_headers_;
  }

  // Per loop header invariant loads sets. Each set contains load id for
  // those loads that are not affected by anything in the loop and can be
  // hoisted out. Sets are computed by LoadOptimizer.
  ZoneGrowableArray<BitVector*>* loop_invariant_loads() const {
    return loop_invariant_loads_;
  }
  void set_loop_invariant_loads(
      ZoneGrowableArray<BitVector*>* loop_invariant_loads) {
    loop_invariant_loads_ = loop_invariant_loads;
  }

 private:
  friend class IfConverter;
  friend class BranchSimplifier;
  friend class ConstantPropagator;

  // SSA transformation methods and fields.
  void ComputeDominators(GrowableArray<BitVector*>* dominance_frontier);

  void CompressPath(
      intptr_t start_index,
      intptr_t current_index,
      GrowableArray<intptr_t>* parent,
      GrowableArray<intptr_t>* label);

  void Rename(GrowableArray<PhiInstr*>* live_phis,
              VariableLivenessAnalysis* variable_liveness,
              ZoneGrowableArray<Definition*>* inlining_parameters);
  void RenameRecursive(
      BlockEntryInstr* block_entry,
      GrowableArray<Definition*>* env,
      GrowableArray<PhiInstr*>* live_phis,
      VariableLivenessAnalysis* variable_liveness);

  void AttachEnvironment(Instruction* instr, GrowableArray<Definition*>* env);

  void InsertPhis(
      const GrowableArray<BlockEntryInstr*>& preorder,
      const GrowableArray<BitVector*>& assigned_vars,
      const GrowableArray<BitVector*>& dom_frontier);

  void RemoveDeadPhis(GrowableArray<PhiInstr*>* live_phis);

  void ReplacePredecessor(BlockEntryInstr* old_block,
                          BlockEntryInstr* new_block);

  // Finds natural loops in the flow graph and attaches a list of loop
  // body blocks for each loop header.
  ZoneGrowableArray<BlockEntryInstr*>* ComputeLoops();

  // DiscoverBlocks computes parent_ and assigned_vars_ which are then used
  // if/when computing SSA.
  GrowableArray<intptr_t> parent_;
  GrowableArray<BitVector*> assigned_vars_;

  intptr_t current_ssa_temp_index_;
  intptr_t max_block_id_;

  // Flow graph fields.
  const ParsedFunction& parsed_function_;
  const intptr_t num_copied_params_;
  const intptr_t num_non_copied_params_;
  const intptr_t num_stack_locals_;
  GraphEntryInstr* graph_entry_;
  GrowableArray<BlockEntryInstr*> preorder_;
  GrowableArray<BlockEntryInstr*> postorder_;
  GrowableArray<BlockEntryInstr*> reverse_postorder_;
  ConstantInstr* constant_null_;

  BlockEffects* block_effects_;
  bool licm_allowed_;

  ZoneGrowableArray<BlockEntryInstr*>* loop_headers_;
  ZoneGrowableArray<BitVector*>* loop_invariant_loads_;
};


class LivenessAnalysis : public ValueObject {
 public:
  LivenessAnalysis(intptr_t variable_count,
                   const GrowableArray<BlockEntryInstr*>& postorder);

  void Analyze();

  virtual ~LivenessAnalysis() { }

  BitVector* GetLiveInSetAt(intptr_t postorder_number) const {
    return live_in_[postorder_number];
  }

  BitVector* GetLiveOutSetAt(intptr_t postorder_number) const {
    return live_out_[postorder_number];
  }

  BitVector* GetLiveInSet(BlockEntryInstr* block) const {
    return GetLiveInSetAt(block->postorder_number());
  }

  BitVector* GetKillSet(BlockEntryInstr* block) const {
    return kill_[block->postorder_number()];
  }

  BitVector* GetLiveOutSet(BlockEntryInstr* block) const {
    return GetLiveOutSetAt(block->postorder_number());
  }

  // Print results of liveness analysis.
  void Dump();

 protected:
  // Compute initial values for live-out, kill and live-in sets.
  virtual void ComputeInitialSets() = 0;

  // Update live-out set for the given block: live-out should contain
  // all values that are live-in for block's successors.
  // Returns true if live-out set was changed.
  bool UpdateLiveOut(const BlockEntryInstr& instr);

  // Update live-in set for the given block: live-in should contain
  // all values that are live-out from the block and are not defined
  // by this block.
  // Returns true if live-in set was changed.
  bool UpdateLiveIn(const BlockEntryInstr& instr);

  // Perform fix-point iteration updating live-out and live-in sets
  // for blocks until they stop changing.
  void ComputeLiveInAndLiveOutSets();

  const intptr_t variable_count_;

  const GrowableArray<BlockEntryInstr*>& postorder_;

  // Live-out sets for each block.  They contain indices of variables
  // that are live out from this block: that is values that were either
  // defined in this block or live into it and that are used in some
  // successor block.
  GrowableArray<BitVector*> live_out_;

  // Kill sets for each block.  They contain indices of variables that
  // are defined by this block.
  GrowableArray<BitVector*> kill_;

  // Live-in sets for each block.  They contain indices of variables
  // that are used by this block or its successors.
  GrowableArray<BitVector*> live_in_;
};


// Information about side effect free paths between blocks.
class BlockEffects : public ZoneAllocated {
 public:
  explicit BlockEffects(FlowGraph* flow_graph);

  // Return true if the given instruction is not affected by anything between
  // its current block and target block. Used by CSE to determine if
  // a computation is available in the given block.
  bool IsAvailableAt(Instruction* instr, BlockEntryInstr* block) const;

  // Return true if the given instruction is not affected by anything between
  // the given block and its current block. Used by LICM to determine if
  // a computation can be moved to loop's preheader and remain available at
  // its current location.
  bool CanBeMovedTo(Instruction* instr, BlockEntryInstr* block) const;

 private:
  // Returns true if from dominates to and all paths between from and to are
  // free of side effects.
  bool IsSideEffectFreePath(BlockEntryInstr* from, BlockEntryInstr* to) const;

  // Per block sets of available blocks. Block A is available at the block B if
  // and only if A dominates B and all paths from A to B are free of side
  // effects.
  GrowableArray<BitVector*> available_at_;
};


}  // namespace dart

#endif  // VM_FLOW_GRAPH_H_