Change labeled statement to use visitSubGraph for its body instead of marking its "exit block" spec…

frog/leg/ssa/codegen.dart

Index: frog/leg/ssa/codegen.dart
diff --git a/frog/leg/ssa/codegen.dart b/frog/leg/ssa/codegen.dart
index 80d3b6d3654504a44836b1147322b7590b3a8d7c..50b81773ea441245791cbc7781764b63b5acdf9b 100644
--- a/frog/leg/ssa/codegen.dart
+++ b/frog/leg/ssa/codegen.dart
@@ -101,6 +101,11 @@ class SsaCodeGenerator implements HVisitor {
   HGraph currentGraph;
   HBasicBlock currentBlock;
 
+  // Records a block-information that is being handled specially.
+  // Used to break bad recursion.

[ngeoffray, 2012/03/06 11:17:37]

What's a bad recursion?

[Lasse Reichstein Nielsen, 2012/03/08 09:02:02]

Infinite recursion, recursing on the same input again. Will reword.

+  HLabeledBlockInformation currentBlockInformation;
+  // The subgraph is used to delimit traversal for some constructions, e.g.,
+  // if branches.
   SubGraph subGraph;
 
   SsaCodeGenerator(this.compiler,
@@ -233,47 +238,48 @@ class SsaCodeGenerator implements HVisitor {
     this.expectedPrecedence = oldPrecedence;
   }
 
-  void handleLabeledBlock(HBasicBlock node) {
-    HLabeledBlockInformation labeledBlockInfo = node.labeledBlockInformation;
-    if (labeledBlockInfo.start === node) {
-      addIndentation();
-      for (SourceString label in labeledBlockInfo.labels) {
-        addLabel(label);
-        buffer.add(":");
-      }
-      buffer.add("{\n");
-      indent++;
-    } else {
-      assert(labeledBlockInfo.end === node);
-      assert((){
-        // Check that this block is (transitively) dominated by the start block.
-        HBasicBlock block = node;
-        while (block.dominator !== null) {
-          block = block.dominator;
-          if (block === labeledBlockInfo.start) return true;
-        }
-        return false;
-      });
-      indent--;
-      addIndentation();
-      buffer.add("}\n");
+  void handleLabeledBlock(HLabeledBlockInformation labeledBlockInfo) {
+    addIndentation();
+    for (SourceString label in labeledBlockInfo.labels) {
+      addLabel(label);
+      buffer.add(":");
     }
+    buffer.add("{\n");
+    indent++;
+
+    visitSubGraph(labeledBlockInfo.body);
+
+    indent--;
+    addIndentation();
+    buffer.add("}\n");
+
+    visitBasicBlock(labeledBlockInfo.joinBlock);
   }
 
 
   visitBasicBlock(HBasicBlock node) {
+    // Abort traversal if we are leaving the currently active sub-graph.
     if (!subGraph.contains(node)) return;
 
-    currentBlock = node;
+    // If this node has special behavior attached, handle it.
+    // If we reach here again while handling the attached information,
+    // e.g., because we call visitSubGraph on a subgraph starting here,
+    // don't handle it again.

[ngeoffray, 2012/03/06 11:17:37]

How can you end up doing this? Should we re-design the subgraph to make sure we
never encounter this case?

[Lasse Reichstein Nielsen, 2012/03/08 09:02:02]

For a labeled expression, the "HLabeledBlockInformation" is put on the first
block of the body, which is also the first block of the subgraph.
Any suggestions for redesign are appreciated.

VisitBasicBlock is called by visitSubGraph, and the label/loop tests are
necessary for that (they can be branches of an if, which also uses
visitSubGraph).

+    if (node.hasLabeledBlockInformation() &&
+        node.labeledBlockInformation !== currentBlockInformation) {
+      HLabeledBlockInformation oldBlockInformation = currentBlockInformation;
+      currentBlockInformation = node.labeledBlockInformation;
+      handleLabeledBlock(currentBlockInformation);
+      currentBlockInformation = oldBlockInformation;
+      return;
+    }
 
-    if (node.hasLabeledBlockInformation()) {
-      handleLabeledBlock(node);
-    } else if (currentBlock.isLoopHeader()) {
+    currentBlock = node;
+    if (node.isLoopHeader()) {
       // While loop will be closed by the conditional loop-branch.
       // TODO(floitsch): HACK HACK HACK.
       beginLoop(node);
     }
-
     HInstruction instruction = node.first;
     while (instruction != null) {
       if (instruction is HGoto || instruction is HExit || instruction is HTry) {