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

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 269 matching lines @@
     if (input != value) return NoChange();
   }
   // 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(1);
   Node* effect = NodeProperties::GetEffectInput(node);
   Node* const control = NodeProperties::GetControlInput(node);
   bool changed = false;
   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);
     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 &&
       NodeProperties::GetControlInput(effect) == control &&
       control->opcode() == IrOpcode::kMerge) {
     Node::Inputs control_inputs = control->inputs();
     Node::Inputs value_inputs = value->inputs();
     Node::Inputs effect_inputs = effect->inputs();
     DCHECK_NE(0, control_inputs.count());
     DCHECK_EQ(control_inputs.count(), value_inputs.count() - 1);
@@ skipping 76 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