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

src/compiler/common-operator-reducer.cc

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/compiler/common-operator-reducer.h"
 
 #include <algorithm>
 
 #include "src/compiler/common-operator.h"
 #include "src/compiler/graph.h"
@@ skipping 285 matching lines @@
   }
   // We might now be able to further reduce the {merge} node.
   Revisit(merge);
   return Replace(value);
 }
 
 
 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 &&
       NodeProperties::GetControlInput(effect) == control &&
       control->opcode() == IrOpcode::kMerge) {
     int const control_input_count = control->InputCount();
     DCHECK_NE(0, control_input_count);
     DCHECK_EQ(control_input_count, value->InputCount() - 1);
     DCHECK_EQ(control_input_count, effect->InputCount() - 1);
     DCHECK_EQ(IrOpcode::kEnd, graph()->end()->opcode());
@@ skipping 87 matching lines @@
   node->ReplaceInput(0, a);
   node->ReplaceInput(1, b);
   node->TrimInputCount(2);
   NodeProperties::ChangeOp(node, op);
   return Changed(node);
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8