Fix a bug in block reordering/block compaction.

runtime/vm/flow_graph_compiler.h

 // Copyright (c) 2012, 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.
 
 #ifndef VM_FLOW_GRAPH_COMPILER_H_
 #define VM_FLOW_GRAPH_COMPILER_H_
 
 #include "vm/allocation.h"
 #include "vm/assembler.h"
 #include "vm/code_descriptors.h"
@@ skipping 184 matching lines @@
             intptr_t count_arg)
       : cid(cid_arg), target(target_arg), count(count_arg) {}
 };
 
 
 class FlowGraphCompiler : public ValueObject {
  private:
   class BlockInfo : public ZoneAllocated {
    public:
     BlockInfo()
-        : jump_label_(&block_label_),
-          block_label_(),
-          fallthrough_label_(NULL),
+        : block_label_(),
+          jump_label_(&block_label_),
+          next_nonempty_label_(NULL),
           is_marked_(false) {}
 
+    // The label to jump to when control is transferred to this block.  For
+    // nonempty blocks it is the label of the block itself.  For empty
+    // blocks it is the label of the first nonempty successor block.
     Label* jump_label() const { return jump_label_; }
     void set_jump_label(Label* label) { jump_label_ = label; }
 
-    // Label of the block that will follow this block in the generated code.
-    // Can be NULL if the block is the last block.
-    Label* fallthrough_label() const { return fallthrough_label_; }
-    void set_fallthrough_label(Label* fallthrough_label) {
-      fallthrough_label_ = fallthrough_label;
-    }
+    // The label of the first nonempty block after this one in the block
+    // order, or NULL if there is no nonempty block following this one.
+    Label* next_nonempty_label() const { return next_nonempty_label_; }
+    void set_next_nonempty_label(Label* label) { next_nonempty_label_ = label; }
 
     bool WasCompacted() const {
       return jump_label_ != &block_label_;
     }
 
+    // Block compaction is recursive.  Block info for already-compacted
+    // blocks is marked so as to avoid cycles in the graph.
     bool is_marked() const { return is_marked_; }
     void mark() { is_marked_ = true; }
 
    private:
-    Label* jump_label_;
     Label block_label_;
 
-    Label* fallthrough_label_;
+    Label* jump_label_;
+    Label* next_nonempty_label_;
 
     bool is_marked_;
   };
 
  public:
   FlowGraphCompiler(Assembler* assembler,
                     FlowGraph* flow_graph,
                     bool is_optimizing);
 
   ~FlowGraphCompiler();
@@ skipping 358 matching lines @@
   // that should be used when deoptimizing we store it in this variable.
   // In future AddDeoptStub should be moved out of the instruction template.
   Environment* pending_deoptimization_env_;
 
   DISALLOW_COPY_AND_ASSIGN(FlowGraphCompiler);
 };
 
 }  // namespace dart
 
 #endif  // VM_FLOW_GRAPH_COMPILER_H_