[turbofan] Eliminate checkpoints before return in common op reducer.

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 be4d802f818d06c140a4b2ac7b6b50679a3b7e99..1e91a0286b7561fd85b8ee648ba07949ee962e10 100644
--- a/src/compiler/common-operator-reducer.cc
+++ b/src/compiler/common-operator-reducer.cc
@@ -303,8 +303,14 @@ Reduction CommonOperatorReducer::ReducePhi(Node* node) {
 Reduction CommonOperatorReducer::ReduceReturn(Node* node) {
   DCHECK_EQ(IrOpcode::kReturn, node->opcode());
   Node* const value = node->InputAt(0);
-  Node* const effect = node->InputAt(1);
-  Node* const control = node->InputAt(2);
+  Node* effect = NodeProperties::GetEffectInput(node);
+  Node* const control = NodeProperties::GetControlInput(node);
+  if (effect->opcode() == IrOpcode::kCheckpoint) {
+    // Any {Return} node can never be used to insert a deoptimization point,
+    // hence checkpoints can be cut out of the effect chain flowing into it.
+    effect = NodeProperties::GetEffectInput(effect);
+    NodeProperties::ReplaceEffectInput(node, effect);

[Benedikt Meurer, 2016/07/18 11:29:54]

You need to say Changed(node) as you changed some input of the node (but you
also need to make sure to allow the optimization below).

[Jarin, 2016/07/18 11:48:53]

Good catch! Done.

+  }
   if (value->opcode() == IrOpcode::kPhi &&
       NodeProperties::GetControlInput(value) == control &&
       effect->opcode() == IrOpcode::kEffectPhi &&