[Interpreter] Add a simple dead-code elimination bytecode optimizer.

src/interpreter/bytecode-dead-code-optimizer.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/interpreter/bytecode-dead-code-optimizer.h"
+
+namespace v8 {
+namespace internal {
+namespace interpreter {
+
+BytecodeDeadCodeOptimizer::BytecodeDeadCodeOptimizer(
+    BytecodePipelineStage* next_stage)
+    : next_stage_(next_stage), exit_seen_in_block_(false) {}
+
+// override
+Handle<BytecodeArray> BytecodeDeadCodeOptimizer::ToBytecodeArray(
+    int fixed_register_count, int parameter_count,
+    Handle<FixedArray> handler_table) {
+  return next_stage_->ToBytecodeArray(fixed_register_count, parameter_count,
+                                      handler_table);
+}
+
+// override
+void BytecodeDeadCodeOptimizer::Write(BytecodeNode* node) {
+  // Don't emit dead code.
+  if (exit_seen_in_block_) return;
+
+  switch (node->bytecode()) {
+    case Bytecode::kReturn:
+    case Bytecode::kThrow:
+    case Bytecode::kReThrow:
+      exit_seen_in_block_ = true;
+      break;
+    default:
+      break;
+  }
+
+  next_stage_->Write(node);
+}
+
+// override
+void BytecodeDeadCodeOptimizer::WriteJump(BytecodeNode* node,
+                                          BytecodeLabel* label) {
+  // Don't emit dead code.
+  // TODO(rmcilroy): For forward jumps we could mark the label as dead, thereby
+  // avoiding emitting dead code when we bind the label.
+  if (exit_seen_in_block_) return;
+
+  switch (node->bytecode()) {
+    case Bytecode::kJump:
+    case Bytecode::kJumpConstant:
+      exit_seen_in_block_ = true;
+      break;
+    default:
+      break;
+  }
+
+  next_stage_->WriteJump(node, label);
+}
+
+// override
+void BytecodeDeadCodeOptimizer::BindLabel(BytecodeLabel* label) {
+  next_stage_->BindLabel(label);
+  exit_seen_in_block_ = false;
+}
+
+// override
+void BytecodeDeadCodeOptimizer::BindLabel(const BytecodeLabel& target,
+                                          BytecodeLabel* label) {
+  next_stage_->BindLabel(target, label);
+  // exit_seen_in_block_ was reset when target was bound, so shouldn't be
+  // changed here.
+}
+
+}  // namespace interpreter
+}  // namespace internal
+}  // namespace v8