Revert "Allow LetCont to bind multiple continuations."

pkg/compiler/lib/src/cps_ir/shrinking_reductions.dart

Index: pkg/compiler/lib/src/cps_ir/shrinking_reductions.dart
diff --git a/pkg/compiler/lib/src/cps_ir/shrinking_reductions.dart b/pkg/compiler/lib/src/cps_ir/shrinking_reductions.dart
index 6615aa5651e36d4f31715cde6b236958962ccf78..089921bf92b559355e2be941b59bb2516e56111f 100644
--- a/pkg/compiler/lib/src/cps_ir/shrinking_reductions.dart
+++ b/pkg/compiler/lib/src/cps_ir/shrinking_reductions.dart
@@ -46,27 +46,8 @@ class ShrinkingReducer extends PassMixin {
     node.parent = _DELETED;
   }
 
-  /// Remove a given continuation from the CPS graph.  The LetCont itself is
-  /// removed if the given continuation is the only binding.
-  void _removeContinuation(Continuation cont) {
-    LetCont parent = cont.parent;
-    if (parent.continuations.length == 1) {
-      assert(cont.parent_index == 0);
-      _removeNode(parent);
-    } else {
-      List<Continuation> continuations = parent.continuations;
-      for (int i = cont.parent_index; i < continuations.length - 1; ++i) {
-        Continuation current = continuations[i + 1];
-        continuations[i] = current;
-        current.parent_index = i;
-      }
-      continuations.removeLast();
-    }
-    cont.parent = _DELETED;
-  }
-
   void _processTask(_ReductionTask task) {
-    // Skip tasks for deleted nodes.
+    // Lazily skip tasks for deleted nodes.
     if (task.node.parent == _DELETED) {
       return;
     }
@@ -108,11 +89,11 @@ class ShrinkingReducer extends PassMixin {
     assert(_isDeadCont(task.node));
 
     // Remove dead continuation.
-    Continuation cont = task.node;
-    _removeContinuation(cont);
+    LetCont letCont = task.node;
+    _removeNode(letCont);
 
     // Perform bookkeeping on removed body and scan for new redexes.
-    new _RemovalRedexVisitor(_worklist).visit(cont);
+    new _RemovalRedexVisitor(_worklist).visit(letCont.continuation);
   }
 
   /// Applies the beta-cont-lin reduction:
@@ -130,8 +111,9 @@ class ShrinkingReducer extends PassMixin {
     }
 
     // Remove the continuation.
-    Continuation cont = task.node;
-    _removeContinuation(cont);
+    LetCont letCont = task.node;
+    Continuation cont = letCont.continuation;
+    _removeNode(letCont);
 
     // Replace its invocation with the continuation body.
     InvokeContinuation invoke = cont.firstRef.parent;
@@ -146,7 +128,7 @@ class ShrinkingReducer extends PassMixin {
       argRef.definition.substituteFor(cont.parameters[i]);
     }
 
-    // Perform bookkeeping on substituted body and scan for new redexes.
+    // Perform bookkeeping on removed body and scan for new redexes.
     new _RemovalRedexVisitor(_worklist).visit(invoke);
   }
 
@@ -165,8 +147,9 @@ class ShrinkingReducer extends PassMixin {
     }
 
     // Remove the continuation.
-    Continuation cont = task.node;
-    _removeContinuation(cont);
+    LetCont letCont   = task.node;
+    Continuation cont = letCont.continuation;
+    _removeNode(letCont);
 
     InvokeContinuation invoke = cont.body;
     Continuation wrappedCont = invoke.continuation.definition;
@@ -182,14 +165,13 @@ class ShrinkingReducer extends PassMixin {
 /// Returns true iff the bound primitive is unused.
 bool _isDeadVal(LetPrim node) => !node.primitive.hasAtLeastOneUse;
 
-/// Returns true iff the continuation is unused.
-bool _isDeadCont(Continuation cont) {
-  return !cont.hasAtLeastOneUse && !cont.isReturnContinuation;
-}
+/// Returns true iff the bound continuation is unused.
+bool _isDeadCont(LetCont node) => !node.continuation.hasAtLeastOneUse;
 
-/// Returns true iff the continuation is used exactly once, and that
-/// use is as the continuation of a continuation invocation.
-bool _isBetaContLin(Continuation cont) {
+/// Returns true iff the bound continuation is used exactly once, and that
+/// use is as the receiver of a continuation invocation.
+bool _isBetaContLin(LetCont node) {
+  Continuation cont = node.continuation;
   if (!cont.hasExactlyOneUse) {
     return false;
   }
@@ -200,12 +182,14 @@ bool _isBetaContLin(Continuation cont) {
   }
 
   return false;
+
 }
 
-/// Returns true iff the continuation consists of a continuation
+/// Returns true iff the bound continuation consists of a continuation
 /// invocation, passing on all parameters. Special cases exist (see below).
-bool _isEtaCont(Continuation cont) {
-  if (cont.body is! InvokeContinuation) {
+bool _isEtaCont(LetCont node) {
+  Continuation cont = node.continuation;
+  if (!(cont.body is InvokeContinuation)) {
     return false;
   }
 
@@ -258,16 +242,12 @@ class _RedexVisitor extends RecursiveVisitor {
   void processLetCont(LetCont node) {
     if (node.parent == ShrinkingReducer._DELETED) {
       return;
-    }
-    for (int i = 0; i < node.continuations.length; ++i) {
-      Continuation cont = node.continuations[i];
-      if (_isDeadCont(cont)) {
-        worklist.add(new _ReductionTask(_ReductionKind.DEAD_CONT, cont));
-      } else if (_isEtaCont(cont)) {
-        worklist.add(new _ReductionTask(_ReductionKind.ETA_CONT, cont));
-      } else if (_isBetaContLin(cont)){
-        worklist.add(new _ReductionTask(_ReductionKind.BETA_CONT_LIN, cont));
-      }
+    } else if (_isDeadCont(node)) {
+      worklist.add(new _ReductionTask(_ReductionKind.DEAD_CONT, node));
+    } else if (_isEtaCont(node)) {
+      worklist.add(new _ReductionTask(_ReductionKind.ETA_CONT, node));
+    } else if (_isBetaContLin(node)){
+      worklist.add(new _ReductionTask(_ReductionKind.BETA_CONT_LIN, node));
     }
   }
 }
@@ -286,10 +266,6 @@ class _RemovalRedexVisitor extends _RedexVisitor {
     node.parent = ShrinkingReducer._DELETED;
   }
 
-  void processContinuation(Continuation node) {
-    node.parent = ShrinkingReducer._DELETED;
-  }
-
   void processReference(Reference reference) {
     reference.unlink();
 
@@ -307,8 +283,9 @@ class _RemovalRedexVisitor extends _RedexVisitor {
         // removed, or it is called nonrecursively outside its body.
         cont.isRecursive = false;
       }
-      if (_isDeadCont(cont)) {
-        worklist.add(new _ReductionTask(_ReductionKind.DEAD_CONT, cont));
+      Node parent = cont.parent;
+      if (parent is LetCont && _isDeadCont(parent)) {
+        worklist.add(new _ReductionTask(_ReductionKind.DEAD_CONT, parent));
       }
     }
   }
@@ -316,6 +293,7 @@ class _RemovalRedexVisitor extends _RedexVisitor {
 
 /// Traverses the CPS term and sets node.parent for each visited node.
 class ParentVisitor extends RecursiveVisitor {
+
   processFunctionDefinition(FunctionDefinition node) {
     node.body.parent = node;
     node.parameters.forEach((Definition p) => p.parent = node);
@@ -348,17 +326,13 @@ class ParentVisitor extends RecursiveVisitor {
   }
 
   processLetCont(LetCont node) {
-    for (int i = 0; i < node.continuations.length; ++i) {
-      Continuation cont = node.continuations[i];
-      cont.parent = node;
-      cont.parent_index = i;
-    }
+    node.continuation.parent = node;
     node.body.parent = node;
   }
 
   processInvokeStatic(InvokeStatic node) {
-    node.arguments.forEach((Reference ref) => ref.parent = node);
     node.continuation.parent = node;
+    node.arguments.forEach((Reference ref) => ref.parent = node);
   }
 
   processInvokeContinuation(InvokeContinuation node) {