[turbofan] Implement simplistic checkpoint reducer.

src/compiler/checkpoint-elimination.cc

+// Copyright 2016 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/checkpoint-elimination.h"
+
+#include "src/compiler/node-properties.h"
+
+namespace v8 {
+namespace internal {
+namespace compiler {
+
+CheckpointElimination::CheckpointElimination(Editor* editor)
+    : AdvancedReducer(editor) {}
+
+namespace {
+
+// The given checkpoint is redundant if it is effect-wise dominated by another
+// checkpoint and there is no observable write in between. For now we consider
+// a linear effect chain only instead of true effect-wise dominance.
+bool IsRedundantCheckPoint(Node* node) {
+  Node* effect = NodeProperties::GetEffectInput(node);
+  while (effect->op()->HasProperty(Operator::kNoWrite) &&
+         effect->op()->EffectInputCount() == 1) {
+    if (effect->opcode() == IrOpcode::kCheckPoint) return true;
+    effect = NodeProperties::GetEffectInput(effect);
+  }
+  return false;
+}
+
+}  // namespace
+
+Reduction CheckpointElimination::Reduce(Node* node) {
+  if (node->opcode() != IrOpcode::kCheckPoint) return NoChange();
+  DCHECK_EQ(IrOpcode::kCheckPoint, node->opcode());

[Benedikt Meurer, 2016/06/01 04:05:22]

The DCHECK_EQ is exactly the same as the check in the line above, so let's drop
that.

[Michael Starzinger, 2016/06/01 08:11:10]

Done.

+  if (IsRedundantCheckPoint(node)) {
+    return Replace(NodeProperties::GetEffectInput(node));
+  }
+  return NoChange();
+}
+
+}  // namespace compiler
+}  // namespace internal
+}  // namespace v8