tree-ir: Move labeled statements outside loops.

pkg/compiler/lib/src/tree_ir/optimization/loop_rewriter.dart

Index: pkg/compiler/lib/src/tree_ir/optimization/loop_rewriter.dart
diff --git a/pkg/compiler/lib/src/tree_ir/optimization/loop_rewriter.dart b/pkg/compiler/lib/src/tree_ir/optimization/loop_rewriter.dart
index e2b70119a72044ba1435bf9f99c8926b99fb4c69..0129c0df5665451418042ff9789725d27866d54d 100644
--- a/pkg/compiler/lib/src/tree_ir/optimization/loop_rewriter.dart
+++ b/pkg/compiler/lib/src/tree_ir/optimization/loop_rewriter.dart
@@ -4,9 +4,31 @@
 
 part of tree_ir.optimization;
 
-/// Rewrites [WhileTrue] statements with an [If] body into a [WhileCondition],
-/// in situations where only one of the branches contains a [Continue] to the
-/// loop. Schematically:
+/// Rewrites [WhileTrue] statements.
+///
+/// Before this phase, loops usually contain a lot of "exit code", that is,
+/// code that happens at a point where a [Continue] can no longer be reached,
+/// and is therefore not really part of the loop.
+/// Exit code is moved down after the loop using the following rewrites rules:
+///
+/// EXTRACT LABELED STATEMENT:
+///
+///   L:
+///   while (true) {
+///     L2: {
+///       S1  (has references to L)
+///     }
+///     S2    (has no references to L)
+///   }
+///
+///     ==>
+///
+///   L2: {
+///     L: while (true) S1
+///   }
+///   S2
+///
+/// INTRODUCE CONDITIONAL LOOP:
 ///
 ///   L:
 ///   while (true) {
@@ -25,8 +47,10 @@ part of tree_ir.optimization;
 ///
 /// A similar transformation is used when S2 occurs in the 'then' position.
 ///
-/// Note that the above pattern needs no iteration since nested ifs
+/// Note that the last pattern above needs no iteration since nested ifs
 /// have been collapsed previously in the [StatementRewriter] phase.
+///
+/// [WhileCondition] statements exist only after this phase.
 class LoopRewriter extends RecursiveTransformer
                    implements Pass {
   String get passName => 'Loop rewriter';
@@ -49,6 +73,28 @@ class LoopRewriter extends RecursiveTransformer
 
   Statement visitWhileTrue(WhileTrue node) {
     assert(!usedContinueLabels.contains(node.label));
+
+    // Pull labeled statements outside the loop when possible.
+    // [head] and [tail] are the first and last labeled statements that were
+    // pulled out, and null when none have been pulled out.
+    LabeledStatement head, tail;
+    while (node.body is LabeledStatement) {
+      LabeledStatement inner = node.body;
+      inner.next = visitStatement(inner.next);
+      bool nextHasContinue = usedContinueLabels.remove(node.label);
+      if (nextHasContinue) break;
+      node.body = inner.body;
+      inner.body = node;
+      if (head == null) {
+        head = tail = inner;
+      } else {
+        tail.body = inner;
+        tail = inner;
+      }
+    }
+
+    // Rewrite while(true) to while(condition).
+    Statement loop = node;
     if (node.body is If) {
       If body = node.body;
       body.thenStatement = visitStatement(body.thenStatement);
@@ -57,23 +103,31 @@ class LoopRewriter extends RecursiveTransformer
       bool elseHasContinue = usedContinueLabels.remove(node.label);
       if (thenHasContinue && !elseHasContinue) {
         node.label.binding = null; // Prepare to rebind the label.
-        return new WhileCondition(
+        loop = new WhileCondition(
             node.label,
             body.condition,
             body.thenStatement,
             body.elseStatement);
       } else if (!thenHasContinue && elseHasContinue) {
         node.label.binding = null;
-        return new WhileCondition(
+        loop = new WhileCondition(
             node.label,
             new Not(body.condition),
             body.elseStatement,
             body.thenStatement);
       }
+    } else if (node.body is LabeledStatement) {
+      // If the body is a labeled statement, its .next has already been visited.
+      LabeledStatement body = node.body;
+      body.body = visitStatement(body.body);
+      usedContinueLabels.remove(node.label);
     } else {
       node.body = visitStatement(node.body);
       usedContinueLabels.remove(node.label);
     }
-    return node;
+
+    if (head == null) return loop;
+    tail.body = loop;
+    return head;
   }
 }