[turbofan] Fix corner case in loop analysis.

src/compiler/loop-analysis.cc

Index: src/compiler/loop-analysis.cc
diff --git a/src/compiler/loop-analysis.cc b/src/compiler/loop-analysis.cc
index 01b9659e6bd0752fb2a765eb1bbc0660eb160e82..1b5f5288f4d2157585cf8f99be7b09ad49ffd957 100644
--- a/src/compiler/loop-analysis.cc
+++ b/src/compiler/loop-analysis.cc
@@ -152,18 +152,20 @@ class LoopFinderImpl {
       queued_.Set(node, false);
 
       // Setup loop headers first.
+      NodeInfo& ni = info(node);
       if (node->opcode() == IrOpcode::kLoop) {
         // found the loop node first.
-        CreateLoopInfo(node);
+        CreateLoopInfo(node, ni);
       } else if (node->opcode() == IrOpcode::kPhi ||
                  node->opcode() == IrOpcode::kEffectPhi) {
         // found a phi first.
         Node* merge = node->InputAt(node->InputCount() - 1);
-        if (merge->opcode() == IrOpcode::kLoop) CreateLoopInfo(merge);
+        if (merge->opcode() == IrOpcode::kLoop) {
+          ni.MarkBackward(CreateLoopInfo(merge, info(merge)).loop_mark);
+        }
       }
 
       // Propagate reachability marks backwards from this node.
-      NodeInfo& ni = info(node);
       if (ni.IsLoopHeader()) {
         // Handle edges from loop header nodes specially.
         for (int i = 0; i < node->InputCount(); i++) {
@@ -186,9 +188,8 @@ class LoopFinderImpl {
   }
 
   // Make a new loop header for the given node.
-  void CreateLoopInfo(Node* node) {
-    NodeInfo& ni = info(node);
-    if (ni.IsLoopHeader()) return;  // loop already set up.
+  NodeInfo& CreateLoopInfo(Node* node, NodeInfo& ni) {

[Michael Starzinger, 2015/01/14 16:28:32]

nit: Instead of returning the NodeInfo here, we could just keep this "void" and
store the NodeInfo in a local variable at the call-site in line 164. I think
that would be easier to read.

+    if (ni.IsLoopHeader()) return ni;  // loop already set up.
 
     loops_found_++;
     size_t loop_num = loops_.size() + 1;
@@ -197,7 +198,6 @@ class LoopFinderImpl {
     loops_.push_back({node, nullptr, nullptr, nullptr});
     loop_tree_->NewLoop();
     LoopMarks loop_mark = kVisited | (1 << loop_num);
-    ni.node = node;
     ni.loop_mark = loop_mark;
 
     // Setup loop mark for phis attached to loop header.
@@ -207,6 +207,7 @@ class LoopFinderImpl {
         info(use).loop_mark = loop_mark;
       }
     }
+    return ni;
   }
 
   // Propagate marks forward from loops.