[ignition] Add a reverse bytecode iterator

This pre-calculates and stores a vector of bytecode offsets, and then allows
one to iterate over it backwards. This could probably be adapted to a
bidirectional/random access iterator if we wanted to, but for now reverse
is all we need.

Review-Url: https://codereview.chromium.org/2518003002
Cr-Commit-Position: refs/heads/master@{#41153}

BUILD.gn

@@ -1446,6 +1446,8 @@ v8_source_set("v8_base") {
     "src/interpreter/bytecode-array-builder.h",
     "src/interpreter/bytecode-array-iterator.cc",
     "src/interpreter/bytecode-array-iterator.h",
+    "src/interpreter/bytecode-array-reverse-iterator.cc",
+    "src/interpreter/bytecode-array-reverse-iterator.h",
     "src/interpreter/bytecode-array-writer.cc",
     "src/interpreter/bytecode-array-writer.h",
     "src/interpreter/bytecode-dead-code-optimizer.cc",

src/interpreter/bytecode-array-reverse-iterator.cc

+// Copyright 2015 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-array-reverse-iterator.h"
+#include "src/objects-inl.h"
+
+namespace v8 {
+namespace internal {
+namespace interpreter {
+
+BytecodeArrayReverseIterator::BytecodeArrayReverseIterator(
+    Handle<BytecodeArray> bytecode_array, Zone* zone)
+    : BytecodeArrayAccessor(bytecode_array, 0), offsets_(zone) {
+  // Run forwards through the bytecode array to determine the offset of each
+  // bytecode.
+  while (current_offset() < bytecode_array->length()) {
+    offsets_.push_back(current_offset());
+    SetOffset(current_offset() + current_bytecode_size());
+  }
+  Reset();
+}
+
+void BytecodeArrayReverseIterator::Advance() {
+  it_offsets_++;
+  UpdateOffsetFromIterator();
+}
+
+void BytecodeArrayReverseIterator::Reset() {
+  it_offsets_ = offsets_.rbegin();
+  UpdateOffsetFromIterator();
+}
+
+bool BytecodeArrayReverseIterator::done() const {
+  return it_offsets_ == offsets_.rend();
+}
+
+void BytecodeArrayReverseIterator::UpdateOffsetFromIterator() {
+  if (it_offsets_ != offsets_.rend()) {
+    SetOffset(*it_offsets_);
+  }
+}
+
+}  // namespace interpreter
+}  // namespace internal
+}  // namespace v8

src/interpreter/bytecode-array-reverse-iterator.h

+// 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.
+
+#ifndef V8_INTERPRETER_BYTECODE_ARRAY_REVERSE_ITERATOR_H_
+#define V8_INTERPRETER_BYTECODE_ARRAY_REVERSE_ITERATOR_H_
+
+#include "src/interpreter/bytecode-array-accessor.h"
+#include "src/zone/zone-containers.h"
+#include "src/zone/zone.h"
+
+namespace v8 {
+namespace internal {
+namespace interpreter {
+
+class V8_EXPORT_PRIVATE BytecodeArrayReverseIterator final
+    : public BytecodeArrayAccessor {
+ public:
+  explicit BytecodeArrayReverseIterator(Handle<BytecodeArray> bytecode_array,
+                                        Zone* zone);
+
+  void Advance();
+  void Reset();
+  bool done() const;
+
+ private:
+  ZoneVector<int> offsets_;
+  ZoneVector<int>::const_reverse_iterator it_offsets_;
+
+  void UpdateOffsetFromIterator();
+
+  DISALLOW_COPY_AND_ASSIGN(BytecodeArrayReverseIterator);
+};
+
+}  // namespace interpreter
+}  // namespace internal
+}  // namespace v8
+
+#endif  // V8_INTERPRETER_BYTECODE_ARRAY_REVERSE_ITERATOR_H_

src/v8.gyp

@@ -982,6 +982,8 @@
         'interpreter/bytecode-array-builder.h',
         'interpreter/bytecode-array-iterator.cc',
         'interpreter/bytecode-array-iterator.h',
+        'interpreter/bytecode-array-reverse-iterator.cc',
+        'interpreter/bytecode-array-reverse-iterator.h',
         'interpreter/bytecode-array-writer.cc',
         'interpreter/bytecode-array-writer.h',
         'interpreter/bytecode-dead-code-optimizer.cc',

test/unittests/BUILD.gn

@@ -98,6 +98,7 @@ v8_executable("unittests") {
     "heap/slot-set-unittest.cc",
     "interpreter/bytecode-array-builder-unittest.cc",
     "interpreter/bytecode-array-iterator-unittest.cc",
+    "interpreter/bytecode-array-reverse-iterator-unittest.cc",
     "interpreter/bytecode-array-writer-unittest.cc",
     "interpreter/bytecode-dead-code-optimizer-unittest.cc",
     "interpreter/bytecode-decoder-unittest.cc",

test/unittests/unittests.gyp

@@ -89,6 +89,7 @@
       'interpreter/bytecodes-unittest.cc',
       'interpreter/bytecode-array-builder-unittest.cc',
       'interpreter/bytecode-array-iterator-unittest.cc',
+      'interpreter/bytecode-array-reverse-iterator-unittest.cc',
       'interpreter/bytecode-array-writer-unittest.cc',
       'interpreter/bytecode-dead-code-optimizer-unittest.cc',
       'interpreter/bytecode-decoder-unittest.cc',