[turbofan] Fix select lowering.

Select lowering must not merge Select nodes that depend on each other,
because the resulting graph is not schedulable.

TEST=unittests
R=jarin@chromium.org

Review URL: https://codereview.chromium.org/717473002

Cr-Commit-Position: refs/heads/master@{#25236}
git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25236 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/compiler/node.h

@@ -68,6 +68,8 @@ typedef std::set<Node*, std::less<Node*>, zone_allocator<Node*> > NodeSet;
 typedef NodeSet::iterator NodeSetIter;
 typedef NodeSet::reverse_iterator NodeSetRIter;
 
+typedef ZoneDeque<Node*> NodeDeque;
+
 typedef ZoneVector<Node*> NodeVector;
 typedef NodeVector::iterator NodeVectorIter;
 typedef NodeVector::const_iterator NodeVectorConstIter;

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

src/compiler/select-lowering.h

@@ -28,8 +28,10 @@ class SelectLowering FINAL : public Reducer {
   Reduction Reduce(Node* node) OVERRIDE;
 
  private:
-  typedef std::map<Node*, Node*, std::less<Node*>,
-                   zone_allocator<std::pair<Node* const, Node*>>> Merges;
+  typedef std::multimap<Node*, Node*, std::less<Node*>,
+                        zone_allocator<std::pair<Node* const, Node*>>> Merges;
+
+  bool ReachableFrom(Node* const sink, Node* const source);
 
   CommonOperatorBuilder* common() const { return common_; }
   Graph* graph() const { return graph_; }

test/unittests/compiler/select-lowering-unittest.cc

@@ -10,6 +10,7 @@
 using testing::AllOf;
 using testing::Capture;
 using testing::CaptureEq;
+using testing::Not;
 
 namespace v8 {
 namespace internal {
@@ -33,12 +34,12 @@ TEST_F(SelectLoweringTest, SelectWithSameConditions) {
   Node* const p2 = Parameter(2);
   Node* const p3 = Parameter(3);
   Node* const p4 = Parameter(4);
+  Node* const s0 = graph()->NewNode(common()->Select(kMachInt32), p0, p1, p2);
 
   Capture<Node*> branch;
   Capture<Node*> merge;
   {
-    Reduction const r =
-        Reduce(graph()->NewNode(common()->Select(kMachInt32), p0, p1, p2));
+    Reduction const r = Reduce(s0);
     ASSERT_TRUE(r.Changed());
     EXPECT_THAT(
         r.replacement(),
@@ -55,6 +56,15 @@ TEST_F(SelectLoweringTest, SelectWithSameConditions) {
     ASSERT_TRUE(r.Changed());
     EXPECT_THAT(r.replacement(), IsPhi(kMachInt32, p3, p4, CaptureEq(&merge)));
   }
+  {
+    // We must not reuse the diamond if it is reachable from either else/then
+    // values of the Select, because the resulting graph can not be scheduled.
+    Reduction const r =
+        Reduce(graph()->NewNode(common()->Select(kMachInt32), p0, s0, p0));
+    ASSERT_TRUE(r.Changed());
+    EXPECT_THAT(r.replacement(),
+                IsPhi(kMachInt32, s0, p0, Not(CaptureEq(&merge))));
+  }
 }
 
 }  // namespace compiler