skeleton Kernel -> SSA builder

pkg/compiler/lib/src/ssa/graph_builder.dart

+// Copyright (c) 2016, 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.
+
+import 'nodes.dart';
+
+/// Base class for objects that build up an SSA graph.
+///
+/// This contains helpers for building the graph and tracking information about
+/// the current state of the graph being built.
+abstract class GraphBuilder {
+  /// Holds the resulting SSA graph.
+  final HGraph graph = new HGraph();
+
+  HBasicBlock _current;
+
+  /// The current block to add instructions to. Might be null, if we are
+  /// visiting dead code, but see [isReachable].
+  HBasicBlock get current => _current;
+
+  void set current(c) {
+    isReachable = c != null;
+    _current = c;
+  }
+
+  /// The most recently opened block. Has the same value as [current] while
+  /// the block is open, but unlike [current], it isn't cleared when the
+  /// current block is closed.
+  HBasicBlock lastOpenedBlock;
+
+  /// Indicates whether the current block is dead (because it has a throw or a
+  /// return further up). If this is false, then [current] may be null. If the
+  /// block is dead then it may also be aborted, but for simplicity we only
+  /// abort on statement boundaries, not in the middle of expressions. See
+  /// [isAborted].
+  bool isReachable = true;
+
+  HBasicBlock addNewBlock() {
+    HBasicBlock block = graph.addNewBlock();
+    // If adding a new block during building of an expression, it is due to
+    // conditional expressions or short-circuit logical operators.
+    return block;
+  }
+
+  void open(HBasicBlock block) {
+    block.open();
+    current = block;
+    lastOpenedBlock = block;
+  }
+
+  HBasicBlock close(HControlFlow end) {
+    HBasicBlock result = current;
+    current.close(end);
+    current = null;
+    return result;
+  }
+
+  HBasicBlock closeAndGotoExit(HControlFlow end) {
+    HBasicBlock result = current;
+    current.close(end);
+    current = null;
+    result.addSuccessor(graph.exit);
+    return result;
+  }
+
+  void goto(HBasicBlock from, HBasicBlock to) {
+    from.close(new HGoto());
+    from.addSuccessor(to);
+  }
+
+  bool isAborted() {
+    return current == null;
+  }
+
+  /// Creates a new block, transitions to it from any current block, and
+  /// opens the new block.
+  HBasicBlock openNewBlock() {
+    HBasicBlock newBlock = addNewBlock();
+    if (!isAborted()) goto(current, newBlock);
+    open(newBlock);
+    return newBlock;
+  }
+
+  void add(HInstruction instruction) {
+    current.add(instruction);
+  }
+}