[turbofan] Fix select lowering.

src/compiler/select-lowering.cc

Index: src/compiler/select-lowering.cc
diff --git a/src/compiler/select-lowering.cc b/src/compiler/select-lowering.cc
index 2e51d72024bb8f96e3f82850352b132c682209bd..44d040df046dbaf504706fa7cfa7b6490fa1d945 100644
--- a/src/compiler/select-lowering.cc
+++ b/src/compiler/select-lowering.cc
@@ -26,26 +26,58 @@ Reduction SelectLowering::Reduce(Node* node) {
   if (node->opcode() != IrOpcode::kSelect) return NoChange();
   SelectParameters const p = SelectParametersOf(node->op());
 
-  Node* const cond = node->InputAt(0);
+  Node* cond = node->InputAt(0);
+  Node* vthen = node->InputAt(1);
+  Node* velse = node->InputAt(2);
+  Node* merge = nullptr;
 
   // Check if we already have a diamond for this condition.
-  auto i = merges_.find(cond);
-  if (i == merges_.end()) {
-    // Create a new diamond for this condition and remember its merge node.
-    Diamond d(graph(), common(), cond, p.hint());
-    i = merges_.insert(std::make_pair(cond, d.merge)).first;
-  }
+  auto range = merges_.equal_range(cond);
+  for (auto i = range.first;; ++i) {
+    if (i == range.second) {
+      // Create a new diamond for this condition and remember its merge node.
+      Diamond d(graph(), common(), cond, p.hint());
+      merges_.insert(std::make_pair(cond, d.merge));
+      merge = d.merge;
+      break;
+    }
 
-  DCHECK_EQ(cond, i->first);
+    // If the diamond is reachable from the Select, merging them would result in
+    // an unschedulable graph, so we cannot reuse the diamond in that case.
+    merge = i->second;
+    if (!ReachableFrom(merge, node)) {
+      break;
+    }
+  }
 
   // Create a Phi hanging off the previously determined merge.
   node->set_op(common()->Phi(p.type(), 2));
-  node->ReplaceInput(0, node->InputAt(1));
-  node->ReplaceInput(1, node->InputAt(2));
-  node->ReplaceInput(2, i->second);
+  node->ReplaceInput(0, vthen);
+  node->ReplaceInput(1, velse);
+  node->ReplaceInput(2, merge);
   return Changed(node);
 }
 
+
+bool SelectLowering::ReachableFrom(Node* const sink, Node* const source) {
+  // TODO(turbofan): This is probably horribly expensive, and it should be moved
+  // into node.h or somewhere else?!
+  Zone zone(graph()->zone()->isolate());
+  std::queue<Node*, NodeDeque> pending((NodeDeque(&zone)));
+  BoolVector visited(graph()->NodeCount(), false, &zone);
+  pending.push(source);
+  while (!pending.empty()) {
+    Node* current = pending.front();
+    if (current == sink) return true;
+    pending.pop();
+    visited[current->id()] = true;
+    for (auto input : current->inputs()) {
+      if (!visited[input->id()]) pending.push(input);
+    }
+  }
+  return false;
+}
+
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8