[turbofan] Do not reduce Return nodes with multiple value inputs.

src/compiler/common-operator-reducer.cc

Index: src/compiler/common-operator-reducer.cc
diff --git a/src/compiler/common-operator-reducer.cc b/src/compiler/common-operator-reducer.cc
index 12fb0dc77b84766afbeac71ccb30b9ef9ef4bf40..7f26a29f74e583e4d0524f0b39f194abc102e202 100644
--- a/src/compiler/common-operator-reducer.cc
+++ b/src/compiler/common-operator-reducer.cc
@@ -287,7 +287,6 @@ Reduction CommonOperatorReducer::ReducePhi(Node* node) {
 
 Reduction CommonOperatorReducer::ReduceReturn(Node* node) {
   DCHECK_EQ(IrOpcode::kReturn, node->opcode());
-  Node* const value = node->InputAt(1);
   Node* effect = NodeProperties::GetEffectInput(node);
   Node* const control = NodeProperties::GetControlInput(node);
   bool changed = false;
@@ -298,6 +297,10 @@ Reduction CommonOperatorReducer::ReduceReturn(Node* node) {
     NodeProperties::ReplaceEffectInput(node, effect);
     changed = true;
   }
+  if (NodeProperties::PastValueIndex(node) != 2) {

[titzer, 2017/01/17 16:08:22]

This is worth a comment and a TODO. This is doing an optimization that turns a
return of a phi into two returns, afaict. We can generalize that later to also
work with multiple return values.

[titzer, 2017/01/17 16:43:21]

Please also use the value input count to make this clearer.

[ahaas, 2017/01/17 17:35:54]

I added the TODO.

PTAL: I added a helper function for return operators now which returns the value
input count of a return node without the helper input at index 0.

+    return NoChange();
+  }
+  Node* const value = node->InputAt(1);
   if (value->opcode() == IrOpcode::kPhi &&
       NodeProperties::GetControlInput(value) == control &&
       effect->opcode() == IrOpcode::kEffectPhi &&