Rework flow graph construction.

src/flow-graph.h

Index: src/flow-graph.h
diff --git a/src/flow-graph.h b/src/flow-graph.h
index 183b71d5b08082f068365f47519e276d615820fb..81a883ca995a7dd91a25d3d42ce8c4c862b957e6 100644
--- a/src/flow-graph.h
+++ b/src/flow-graph.h
@@ -36,339 +36,140 @@
 namespace v8 {
 namespace internal {
 
-// Flow-graph nodes.
-class Node: public ZoneObject {
- public:
-  Node() : number_(-1), mark_(false) {}
-
-  virtual ~Node() {}
-
-  virtual bool IsExitNode() { return false; }
-  virtual bool IsBlockNode() { return false; }
-  virtual bool IsBranchNode() { return false; }
-  virtual bool IsJoinNode() { return false; }
-
-  virtual void AddPredecessor(Node* predecessor) = 0;
-  virtual void AddSuccessor(Node* successor) = 0;
-
-  bool IsMarkedWith(bool mark) { return mark_ == mark; }
-  void MarkWith(bool mark) { mark_ = mark; }
-
-  // Perform a depth first search and record preorder and postorder
-  // traversal orders.
-  virtual void Traverse(bool mark,
-                        ZoneList<Node*>* preorder,
-                        ZoneList<Node*>* postorder) = 0;
-
-  int number() { return number_; }
-  void set_number(int number) { number_ = number; }
-
-  // Functions used by data-flow analyses.
-  virtual void InitializeReachingDefinitions(int definition_count,
-                                             List<BitVector*>* variables,
-                                             WorkList<Node>* worklist,
-                                             bool mark);
-  virtual void ComputeRDOut(BitVector* result) = 0;
-  virtual void UpdateRDIn(WorkList<Node>* worklist, bool mark) = 0;
-  virtual void PropagateReachingDefinitions(List<BitVector*>* variables);
-
-  // Functions used by dead-code elimination.
-  virtual void MarkCriticalInstructions(
-      List<AstNode*>* stack,
-      ZoneList<Expression*>* body_definitions,
-      int variable_count);
-
-#ifdef DEBUG
-  void AssignNodeNumber();
-  void PrintReachingDefinitions();
-  virtual void PrintText() = 0;
-#endif
-
- protected:
-  ReachingDefinitionsData rd_;
-
- private:
-  int number_;
-  bool mark_;
-
-  DISALLOW_COPY_AND_ASSIGN(Node);
-};
-
-
-// An exit node has a arbitrarily many predecessors and no successors.
-class ExitNode: public Node {
+// The nodes of a flow graph are basic blocks.  Basic blocks consist of
+// instructions represented as pointers to AST nodes in the order that they
+// would be visited by the code generator.  A block can have arbitrarily many
+// (even zero) predecessors and up to two successors.  Blocks with multiple
+// predecessors are "join nodes" and blocks with multiple successors are
+// "branch nodes".  A block can be both a branch and a join node.
+//
+// Flow graphs are in edge split form: a branch node is never the
+// predecessor of a merge node.  Empty basic blocks are inserted to maintain
+// edge split form.
+class BasicBlock: public ZoneObject {
  public:
-  ExitNode() : predecessors_(4) {}
+  // Construct a basic block with a given predecessor.  NULL indicates no
+  // predecessor or that the predecessor will be set later.
+  BasicBlock(BasicBlock* predecessor)

[fschneider, 2010/03/29 13:49:59]

explicit for 1-argument constructors?

[Kevin Millikin (Chromium), 2010/03/29 14:22:53]

Done.

+      : predecessors_(2),
+        instructions_(8),
+        left_successor_(NULL),
+        right_successor_(NULL),
+        mark_(false) {
+    if (predecessor != NULL) AddPredecessor(predecessor);
+  }
 
-  virtual bool IsExitNode() { return true; }
+  bool HasPredecessor() { return !predecessors_.is_empty(); }
+  bool HasSuccessor() { return left_successor_ != NULL; }
 
-  virtual void AddPredecessor(Node* predecessor) {
+  // Add a given basic block as a predecessor of this block.  This function
+  // also adds this block as a successor of the given block.
+  void AddPredecessor(BasicBlock* predecessor) {
     ASSERT(predecessor != NULL);
     predecessors_.Add(predecessor);
+    predecessor->AddSuccessor(this);
   }
 
-  virtual void AddSuccessor(Node* successor) { UNREACHABLE(); }
-
-  virtual void Traverse(bool mark,
-                        ZoneList<Node*>* preorder,
-                        ZoneList<Node*>* postorder);
-
-  virtual void ComputeRDOut(BitVector* result);
-  virtual void UpdateRDIn(WorkList<Node>* worklist, bool mark);
-
-#ifdef DEBUG
-  virtual void PrintText();
-#endif
-
- private:
-  ZoneList<Node*> predecessors_;
-
-  DISALLOW_COPY_AND_ASSIGN(ExitNode);
-};
-
-
-// Block nodes have a single successor and predecessor and a list of
-// instructions.
-class BlockNode: public Node {
- public:
-  BlockNode() : predecessor_(NULL), successor_(NULL), instructions_(4) {}
-
-  static BlockNode* cast(Node* node) {
-    ASSERT(node->IsBlockNode());
-    return reinterpret_cast<BlockNode*>(node);
-  }
-
-  virtual bool IsBlockNode() { return true; }
-
-  bool is_empty() { return instructions_.is_empty(); }
-
-  ZoneList<AstNode*>* instructions() { return &instructions_; }
-
-  virtual void AddPredecessor(Node* predecessor) {
-    ASSERT(predecessor_ == NULL && predecessor != NULL);
-    predecessor_ = predecessor;
-  }
-
-  virtual void AddSuccessor(Node* successor) {
-    ASSERT(successor_ == NULL && successor != NULL);
-    successor_ = successor;
-  }
-
+  // Add an instruction to the end of this block.  The block must be "open"
+  // by not having a successor yet.
   void AddInstruction(AstNode* instruction) {
+    ASSERT(!HasSuccessor() && instruction != NULL);
     instructions_.Add(instruction);
   }
 
-  virtual void Traverse(bool mark,
-                        ZoneList<Node*>* preorder,
-                        ZoneList<Node*>* postorder);
-
-  virtual void InitializeReachingDefinitions(int definition_count,
-                                             List<BitVector*>* variables,
-                                             WorkList<Node>* worklist,
-                                             bool mark);
-  virtual void ComputeRDOut(BitVector* result);
-  virtual void UpdateRDIn(WorkList<Node>* worklist, bool mark);
-  virtual void PropagateReachingDefinitions(List<BitVector*>* variables);
-
-  virtual void MarkCriticalInstructions(
-      List<AstNode*>* stack,
-      ZoneList<Expression*>* body_definitions,
-      int variable_count);
+  // Perform a depth-first traversal of graph rooted at this node,
+  // accumulating pre- and postorder traversal orders.  Visited nodes are
+  // marked with mark.
+  void BuildTraversalOrder(ZoneList<BasicBlock*>* preorder,
+                           ZoneList<BasicBlock*>* postorder,
+                           bool mark);
+  bool GetMark() { return mark_; }
 
 #ifdef DEBUG
-  virtual void PrintText();
+  // In debug mode, blocks are numbered in reverse postorder to help with
+  // printing.
+  int number() { return number_; }
+  void set_number(int n) { number_ = n; }
+
+  // Print a basic block, given the number of the first instruction.
+  // Returns the next number after the number of the last instruction.
+  int PrintAsText(int instruction_number);
 #endif
 
  private:
-  Node* predecessor_;
-  Node* successor_;
-  ZoneList<AstNode*> instructions_;
-
-  DISALLOW_COPY_AND_ASSIGN(BlockNode);
-};
-
-
-// Branch nodes have a single predecessor and a pair of successors.
-class BranchNode: public Node {
- public:
-  BranchNode() : predecessor_(NULL), successor0_(NULL), successor1_(NULL) {}
-
-  virtual bool IsBranchNode() { return true; }
-
-  virtual void AddPredecessor(Node* predecessor) {
-    ASSERT(predecessor_ == NULL && predecessor != NULL);
-    predecessor_ = predecessor;
-  }
-
-  virtual void AddSuccessor(Node* successor) {
-    ASSERT(successor1_ == NULL && successor != NULL);
-    if (successor0_ == NULL) {
-      successor0_ = successor;
+  // Add a given basic block as successor to this block.  This function does
+  // not add this block as a predecessor of the given block so as to avoid
+  // circularity.
+  void AddSuccessor(BasicBlock* successor) {
+    ASSERT(right_successor_ == NULL && successor != NULL);
+    if (HasSuccessor()) {
+      right_successor_ = successor;
     } else {
-      successor1_ = successor;
+      left_successor_ = successor;
     }
   }
 
-  virtual void Traverse(bool mark,
-                        ZoneList<Node*>* preorder,
-                        ZoneList<Node*>* postorder);
-
-  virtual void ComputeRDOut(BitVector* result);
-  virtual void UpdateRDIn(WorkList<Node>* worklist, bool mark);
-
-#ifdef DEBUG
-  virtual void PrintText();
-#endif
-
- private:
-  Node* predecessor_;
-  Node* successor0_;
-  Node* successor1_;
-
-  DISALLOW_COPY_AND_ASSIGN(BranchNode);
-};
-
-
-// Join nodes have arbitrarily many predecessors and a single successor.
-class JoinNode: public Node {
- public:
-  JoinNode() : predecessors_(2), successor_(NULL) {}
-
-  static JoinNode* cast(Node* node) {
-    ASSERT(node->IsJoinNode());
-    return reinterpret_cast<JoinNode*>(node);
-  }
-
-  virtual bool IsJoinNode() { return true; }
-
-  virtual void AddPredecessor(Node* predecessor) {
-    ASSERT(predecessor != NULL);
-    predecessors_.Add(predecessor);
-  }
-
-  virtual void AddSuccessor(Node* successor) {
-    ASSERT(successor_ == NULL && successor != NULL);
-    successor_ = successor;
-  }
-
-  virtual void Traverse(bool mark,
-                        ZoneList<Node*>* preorder,
-                        ZoneList<Node*>* postorder);
+  ZoneList<BasicBlock*> predecessors_;
+  ZoneList<AstNode*> instructions_;

[fschneider, 2010/03/29 13:49:59]

We probably need an accessor for instructions_. But it's ok to add it on first
use in a separate change.

+  BasicBlock* left_successor_;
+  BasicBlock* right_successor_;
 
-  virtual void ComputeRDOut(BitVector* result);
-  virtual void UpdateRDIn(WorkList<Node>* worklist, bool mark);
+  // Support for graph traversal.  Before traversal, all nodes in the graph
+  // have the same mark (true or false).  Traversal marks already-visited
+  // nodes with the opposite mark.  After traversal, all nodes again have
+  // the same mark.  Traversal of the same graph is not reentrant.
+  bool mark_;
 
 #ifdef DEBUG
-  virtual void PrintText();
+  int number_;
 #endif
 
- private:
-  ZoneList<Node*> predecessors_;
-  Node* successor_;
-
-  DISALLOW_COPY_AND_ASSIGN(JoinNode);
+  DISALLOW_COPY_AND_ASSIGN(BasicBlock);
 };
 
 
-// Flow graphs have a single entry and single exit.  The empty flowgraph is
-// represented by both entry and exit being NULL.
-class FlowGraph BASE_EMBEDDED {
+// A flow graph has distinguished entry and exit blocks.  The entry block is
+// the only one with no predecessors and the exit block is the only one with
+// no successors.
+class FlowGraph: public ZoneObject {
  public:
-  static FlowGraph Empty() {
-    FlowGraph graph;
-    graph.entry_ = new BlockNode();
-    graph.exit_ = graph.entry_;
-    return graph;
+  FlowGraph(BasicBlock* entry, BasicBlock* exit)
+      : entry_(entry), exit_(exit), preorder_(8), postorder_(8) {
   }
 
-  bool is_empty() const {
-    return entry_ == exit_ && BlockNode::cast(entry_)->is_empty();
-  }
-  Node* entry() const { return entry_; }
-  Node* exit() const { return exit_; }
-
-  // Add a single instruction to the end of this flowgraph.
-  void AppendInstruction(AstNode* instruction);
-
-  // Add a single node to the end of this flow graph.
-  void AppendNode(Node* node);
-
-  // Add a flow graph fragment to the end of this one.
-  void AppendGraph(FlowGraph* graph);
-
-  // Concatenate an if-then-else flow-graph to this one.  Control is split
-  // and merged, so the graph remains single-entry, single-exit.
-  void Split(BranchNode* branch,
-             FlowGraph* left,
-             FlowGraph* right,
-             JoinNode* merge);
-
-  // Concatenate a forward loop (e.g., while or for loop) flow-graph to this
-  // one.  Control is split by the condition and merged back from the back
-  // edge at end of the body to the beginning of the condition.  The single
-  // (free) exit of the result graph is the right (false) arm of the branch
-  // node.
-  void Loop(JoinNode* merge,
-            FlowGraph* condition,
-            BranchNode* branch,
-            FlowGraph* body);
+  ZoneList<BasicBlock*>* preorder() { return &preorder_; }
+  ZoneList<BasicBlock*>* postorder() { return &postorder_; }
 
 #ifdef DEBUG
-  void PrintText(FunctionLiteral* fun, ZoneList<Node*>* postorder);
+  void PrintAsText(Handle<String> name);
 #endif
 
  private:
-  FlowGraph() : entry_(NULL), exit_(NULL) {}
-
-  Node* entry_;
-  Node* exit_;
+  BasicBlock* entry_;
+  BasicBlock* exit_;
+  ZoneList<BasicBlock*> preorder_;
+  ZoneList<BasicBlock*> postorder_;
 };
 
 
-// Construct a flow graph from a function literal.  Build pre- and postorder
-// traversal orders as a byproduct.
+// The flow graph builder walks the AST adding reachable AST nodes to the
+// flow graph as instructions.  It remembers the entry and exit nodes of the
+// graph, and keeps a pointer to the current block being constructed.
 class FlowGraphBuilder: public AstVisitor {
  public:
-  explicit FlowGraphBuilder(int variable_count)
-      : graph_(FlowGraph::Empty()),
-        global_exit_(NULL),
-        preorder_(4),
-        postorder_(4),
-        variable_count_(variable_count),
-        body_definitions_(4) {
-  }
-
-  void Build(FunctionLiteral* lit);
+  FlowGraphBuilder() {}
 
-  FlowGraph* graph() { return &graph_; }
-  ZoneList<Node*>* preorder() { return &preorder_; }
-  ZoneList<Node*>* postorder() { return &postorder_; }
-  ZoneList<Expression*>* body_definitions() { return &body_definitions_; }
+  FlowGraph* Build(FunctionLiteral* lit);
 
  private:
-  ExitNode* global_exit() { return global_exit_; }
-
-  // Helpers to allow tranforming the ast during flow graph construction.
-  void VisitStatements(ZoneList<Statement*>* stmts);
-  Statement* ProcessStatement(Statement* stmt);
-
   // AST node visit functions.
 #define DECLARE_VISIT(type) virtual void Visit##type(type* node);
   AST_NODE_LIST(DECLARE_VISIT)
 #undef DECLARE_VISIT
 
-  FlowGraph graph_;
-  ExitNode* global_exit_;
-  ZoneList<Node*> preorder_;
-  ZoneList<Node*> postorder_;
-
-  // The flow graph builder collects a list of explicit definitions
-  // (assignments and count operations) to stack-allocated variables to use
-  // for reaching definitions analysis.  It does not count the implicit
-  // definition at function entry.  AST node numbers in the AST are used to
-  // refer into this list.
-  int variable_count_;
-  ZoneList<Expression*> body_definitions_;
+  BasicBlock* entry_;
+  BasicBlock* exit_;
+  BasicBlock* current_;
 
   DISALLOW_COPY_AND_ASSIGN(FlowGraphBuilder);
 };