Fix a bug in block reordering/block compaction.

runtime/vm/flow_graph_compiler.cc

Index: runtime/vm/flow_graph_compiler.cc
diff --git a/runtime/vm/flow_graph_compiler.cc b/runtime/vm/flow_graph_compiler.cc
index 3eb4a4d6e14b653b64521056fb7c409a88be9939..fbe348454530df913347d9f8616048f5d51574b0 100644
--- a/runtime/vm/flow_graph_compiler.cc
+++ b/runtime/vm/flow_graph_compiler.cc
@@ -180,12 +180,15 @@ static bool IsEmptyBlock(BlockEntryInstr* block) {
 void FlowGraphCompiler::CompactBlock(BlockEntryInstr* block) {
   BlockInfo* block_info = block_info_[block->postorder_number()];
 
+  // Break out of cycles in the control flow graph.
   if (block_info->is_marked()) {
     return;
   }
   block_info->mark();
 
   if (IsEmptyBlock(block)) {
+    // For empty blocks, record a corresponding nonempty target as their
+    // jump label.
     BlockEntryInstr* target = block->next()->AsGoto()->successor();
     CompactBlock(target);
     block_info->set_jump_label(GetJumpLabel(target));
@@ -194,7 +197,10 @@ void FlowGraphCompiler::CompactBlock(BlockEntryInstr* block) {
 
 
 void FlowGraphCompiler::CompactBlocks() {
-  Label* fallthrough_label = NULL;
+  // This algorithm does not garbage collect blocks in place, but merely
+  // records forwarding label information.  In this way it avoids having to
+  // change join and target entries.
+  Label* nonempty_label = NULL;
   for (intptr_t i = block_order().length() - 1; i >= 1; --i) {
     BlockEntryInstr* block = block_order()[i];
 
@@ -203,16 +209,19 @@ void FlowGraphCompiler::CompactBlocks() {
       CompactBlock(block);
     }
 
+    // For nonempty blocks, record the next nonempty block in the block
+    // order.  Since no code is emitted for empty blocks, control flow is
+    // eligible to fall through to the next nonempty one.
     if (!WasCompacted(block)) {
       BlockInfo* block_info = block_info_[block->postorder_number()];
-      block_info->set_fallthrough_label(fallthrough_label);
-      fallthrough_label = GetJumpLabel(block);
+      block_info->set_next_nonempty_label(nonempty_label);
+      nonempty_label = GetJumpLabel(block);
     }
   }
 
   ASSERT(block_order()[0]->IsGraphEntry());
   BlockInfo* block_info = block_info_[block_order()[0]->postorder_number()];
-  block_info->set_fallthrough_label(fallthrough_label);
+  block_info->set_next_nonempty_label(nonempty_label);
 }
 
 
@@ -289,8 +298,8 @@ bool FlowGraphCompiler::WasCompacted(
 
 bool FlowGraphCompiler::CanFallThroughTo(BlockEntryInstr* block_entry) const {
   const intptr_t current_index = current_block()->postorder_number();
-  Label* fallthrough_label = block_info_[current_index]->fallthrough_label();
-  return fallthrough_label == GetJumpLabel(block_entry);
+  Label* next_nonempty = block_info_[current_index]->next_nonempty_label();
+  return next_nonempty == GetJumpLabel(block_entry);
 }