[base] Introduce base::Optional, cloned from chromium

base::Optional is a replacement for std::optional, until we switch to
C++17 and can use std::optional directly.
The implementation is copied from chromium's base::Optional, but put in
the {v8::base} namespace instead of just {base}. Also, the
specialization of std::hash for base::Optional is omitted, since it's
disallowed in the style guide.

A first use in the AsmJsParser is introduced, if that one sticks, I
will refactor more uses of std::unique_ptr to use base::Optional
instead, avoiding the heap allocation.

R=mstarzinger@chromium.org
BUG=v8:6474

Change-Id: I019599d4bf9ff0105bf592dfb96d6050feba18ae
Reviewed-on: https://chromium-review.googlesource.com/528884
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45810}

BUILD.gn

@@ -2513,6 +2513,7 @@ v8_component("v8_libbase") {
     "src/base/macros.h",
     "src/base/once.cc",
     "src/base/once.h",
+    "src/base/optional.h",
     "src/base/platform/condition-variable.cc",
     "src/base/platform/condition-variable.h",
     "src/base/platform/elapsed-timer.h",

src/asmjs/asm-parser.cc

@@ -11,6 +11,7 @@
 
 #include "src/asmjs/asm-js.h"
 #include "src/asmjs/asm-types.h"
+#include "src/base/optional.h"
 #include "src/objects-inl.h"  // TODO(mstarzinger): Temporary cycle breaker.
 #include "src/parsing/scanner.h"
 #include "src/wasm/wasm-opcodes.h"
@@ -2014,8 +2015,7 @@ AsmType* AsmJsParser::ValidateCall() {
   // both cases we might be seeing the {function_name} for the first time and
   // hence allocate a {VarInfo} here, all subsequent uses of the same name then
   // need to match the information stored at this point.
-  // TODO(mstarzinger): Consider using Chromiums base::Optional instead.
-  std::unique_ptr<TemporaryVariableScope> tmp;
+  base::Optional<TemporaryVariableScope> tmp;
   if (Check('[')) {
     RECURSEn(EqualityExpression());
     EXPECT_TOKENn('&');
@@ -2050,8 +2050,8 @@ AsmType* AsmJsParser::ValidateCall() {
     current_function_builder_->EmitI32Const(function_info->index);
     current_function_builder_->Emit(kExprI32Add);
     // We have to use a temporary for the correct order of evaluation.
-    tmp.reset(new TemporaryVariableScope(this));
-    current_function_builder_->EmitSetLocal(tmp.get()->get());
+    tmp.emplace(this);
+    current_function_builder_->EmitSetLocal(tmp->get());
     // The position of function table calls is after the table lookup.
     call_pos = static_cast<int>(scanner_.Position());
   } else {
@@ -2285,7 +2285,7 @@ AsmType* AsmJsParser::ValidateCall() {
       }
     }
     if (function_info->kind == VarKind::kTable) {
-      current_function_builder_->EmitGetLocal(tmp.get()->get());
+      current_function_builder_->EmitGetLocal(tmp->get());
       current_function_builder_->AddAsmWasmOffset(call_pos, to_number_pos);
       current_function_builder_->Emit(kExprCallIndirect);
       current_function_builder_->EmitU32V(signature_index);

src/base/optional.h

@@ -0,0 +1,493 @@
+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// This file is a clone of "base/optional.h" in chromium.
+// Keep in sync, especially when fixing bugs.
+// Copyright 2017 the V8 project authors. All rights reserved.
+
+#ifndef V8_BASE_OPTIONAL_H_
+#define V8_BASE_OPTIONAL_H_
+
+#include <type_traits>
+
+#include "src/base/logging.h"
+
+namespace v8 {
+namespace base {
+
+// Specification:
+// http://en.cppreference.com/w/cpp/utility/optional/in_place_t
+struct in_place_t {};
+
+// Specification:
+// http://en.cppreference.com/w/cpp/utility/optional/nullopt_t
+struct nullopt_t {
+  constexpr explicit nullopt_t(int) {}
+};
+
+// Specification:
+// http://en.cppreference.com/w/cpp/utility/optional/in_place
+constexpr in_place_t in_place = {};
+
+// Specification:
+// http://en.cppreference.com/w/cpp/utility/optional/nullopt
+constexpr nullopt_t nullopt(0);
+
+namespace internal {
+
+template <typename T, bool = std::is_trivially_destructible<T>::value>
+struct OptionalStorage {
+  // Initializing |empty_| here instead of using default member initializing
+  // to avoid errors in g++ 4.8.
+  constexpr OptionalStorage() : empty_('\0') {}
+
+  constexpr explicit OptionalStorage(const T& value)
+      : is_null_(false), value_(value) {}
+
+  // TODO(alshabalin): Can't use 'constexpr' with std::move until C++14.
+  explicit OptionalStorage(T&& value)
+      : is_null_(false), value_(std::move(value)) {}
+
+  // TODO(alshabalin): Can't use 'constexpr' with std::forward until C++14.
+  template <class... Args>
+  explicit OptionalStorage(base::in_place_t, Args&&... args)
+      : is_null_(false), value_(std::forward<Args>(args)...) {}
+
+  // When T is not trivially destructible we must call its
+  // destructor before deallocating its memory.
+  ~OptionalStorage() {
+    if (!is_null_) value_.~T();
+  }
+
+  bool is_null_ = true;
+  union {
+    // |empty_| exists so that the union will always be initialized, even when
+    // it doesn't contain a value. Union members must be initialized for the
+    // constructor to be 'constexpr'.
+    char empty_;
+    T value_;
+  };
+};
+
+template <typename T>
+struct OptionalStorage<T, true> {
+  // Initializing |empty_| here instead of using default member initializing
+  // to avoid errors in g++ 4.8.
+  constexpr OptionalStorage() : empty_('\0') {}
+
+  constexpr explicit OptionalStorage(const T& value)
+      : is_null_(false), value_(value) {}
+
+  // TODO(alshabalin): Can't use 'constexpr' with std::move until C++14.
+  explicit OptionalStorage(T&& value)
+      : is_null_(false), value_(std::move(value)) {}
+
+  // TODO(alshabalin): Can't use 'constexpr' with std::forward until C++14.
+  template <class... Args>
+  explicit OptionalStorage(base::in_place_t, Args&&... args)
+      : is_null_(false), value_(std::forward<Args>(args)...) {}
+
+  // When T is trivially destructible (i.e. its destructor does nothing) there
+  // is no need to call it. Explicitly defaulting the destructor means it's not
+  // user-provided. Those two together make this destructor trivial.
+  ~OptionalStorage() = default;
+
+  bool is_null_ = true;
+  union {
+    // |empty_| exists so that the union will always be initialized, even when
+    // it doesn't contain a value. Union members must be initialized for the
+    // constructor to be 'constexpr'.
+    char empty_;
+    T value_;
+  };
+};
+
+}  // namespace internal
+
+// base::Optional is a Chromium version of the C++17 optional class:
+// std::optional documentation:
+// http://en.cppreference.com/w/cpp/utility/optional
+// Chromium documentation:
+// https://chromium.googlesource.com/chromium/src/+/master/docs/optional.md
+//
+// These are the differences between the specification and the implementation:
+// - The constructor and emplace method using initializer_list are not
+//   implemented because 'initializer_list' is banned from Chromium.
+// - Constructors do not use 'constexpr' as it is a C++14 extension.
+// - 'constexpr' might be missing in some places for reasons specified locally.
+// - No exceptions are thrown, because they are banned from Chromium.
+// - All the non-members are in the 'base' namespace instead of 'std'.
+template <typename T>
+class Optional {
+ public:
+  using value_type = T;
+
+  constexpr Optional() {}
+
+  explicit constexpr Optional(base::nullopt_t) {}
+
+  Optional(const Optional& other) {
+    if (!other.storage_.is_null_) Init(other.value());
+  }
+
+  Optional(Optional&& other) {
+    if (!other.storage_.is_null_) Init(std::move(other.value()));
+  }
+
+  explicit constexpr Optional(const T& value) : storage_(value) {}
+
+  // TODO(alshabalin): Can't use 'constexpr' with std::move until C++14.
+  explicit Optional(T&& value) : storage_(std::move(value)) {}
+
+  // TODO(alshabalin): Can't use 'constexpr' with std::forward until C++14.
+  template <class... Args>
+  explicit Optional(base::in_place_t, Args&&... args)
+      : storage_(base::in_place, std::forward<Args>(args)...) {}
+
+  ~Optional() = default;
+
+  Optional& operator=(base::nullopt_t) {
+    FreeIfNeeded();
+    return *this;
+  }
+
+  Optional& operator=(const Optional& other) {
+    if (other.storage_.is_null_) {
+      FreeIfNeeded();
+      return *this;
+    }
+
+    InitOrAssign(other.value());
+    return *this;
+  }
+
+  Optional& operator=(Optional&& other) {
+    if (other.storage_.is_null_) {
+      FreeIfNeeded();
+      return *this;
+    }
+
+    InitOrAssign(std::move(other.value()));
+    return *this;
+  }
+
+  template <class U>
+  typename std::enable_if<std::is_same<std::decay<U>, T>::value,
+                          Optional&>::type
+  operator=(U&& value) {
+    InitOrAssign(std::forward<U>(value));
+    return *this;
+  }
+
+  // TODO(mlamouri): can't use 'constexpr' with DCHECK.
+  const T* operator->() const {
+    DCHECK(!storage_.is_null_);
+    return &value();
+  }
+
+  // TODO(mlamouri): using 'constexpr' here breaks compiler that assume it was
+  // meant to be 'constexpr const'.
+  T* operator->() {
+    DCHECK(!storage_.is_null_);
+    return &value();
+  }
+
+  constexpr const T& operator*() const & { return value(); }
+
+  // TODO(mlamouri): using 'constexpr' here breaks compiler that assume it was
+  // meant to be 'constexpr const'.
+  T& operator*() & { return value(); }
+
+  constexpr const T&& operator*() const && { return std::move(value()); }
+
+  // TODO(mlamouri): using 'constexpr' here breaks compiler that assume it was
+  // meant to be 'constexpr const'.
+  T&& operator*() && { return std::move(value()); }
+
+  constexpr explicit operator bool() const { return !storage_.is_null_; }
+
+  constexpr bool has_value() const { return !storage_.is_null_; }
+
+  // TODO(mlamouri): using 'constexpr' here breaks compiler that assume it was
+  // meant to be 'constexpr const'.
+  T& value() & {
+    DCHECK(!storage_.is_null_);
+    return storage_.value_;
+  }
+
+  // TODO(mlamouri): can't use 'constexpr' with DCHECK.
+  const T& value() const & {
+    DCHECK(!storage_.is_null_);
+    return storage_.value_;
+  }
+
+  // TODO(mlamouri): using 'constexpr' here breaks compiler that assume it was
+  // meant to be 'constexpr const'.
+  T&& value() && {
+    DCHECK(!storage_.is_null_);
+    return std::move(storage_.value_);
+  }
+
+  // TODO(mlamouri): can't use 'constexpr' with DCHECK.
+  const T&& value() const && {
+    DCHECK(!storage_.is_null_);
+    return std::move(storage_.value_);
+  }
+
+  template <class U>
+  constexpr T value_or(U&& default_value) const & {
+    // TODO(mlamouri): add the following assert when possible:
+    // static_assert(std::is_copy_constructible<T>::value,
+    //               "T must be copy constructible");
+    static_assert(std::is_convertible<U, T>::value,
+                  "U must be convertible to T");
+    return storage_.is_null_ ? static_cast<T>(std::forward<U>(default_value))
+                             : value();
+  }
+
+  template <class U>
+  T value_or(U&& default_value) && {
+    // TODO(mlamouri): add the following assert when possible:
+    // static_assert(std::is_move_constructible<T>::value,
+    //               "T must be move constructible");
+    static_assert(std::is_convertible<U, T>::value,
+                  "U must be convertible to T");
+    return storage_.is_null_ ? static_cast<T>(std::forward<U>(default_value))
+                             : std::move(value());
+  }
+
+  void swap(Optional& other) {
+    if (storage_.is_null_ && other.storage_.is_null_) return;
+
+    if (storage_.is_null_ != other.storage_.is_null_) {
+      if (storage_.is_null_) {
+        Init(std::move(other.storage_.value_));
+        other.FreeIfNeeded();
+      } else {
+        other.Init(std::move(storage_.value_));
+        FreeIfNeeded();
+      }
+      return;
+    }
+
+    DCHECK(!storage_.is_null_ && !other.storage_.is_null_);
+    using std::swap;
+    swap(**this, *other);
+  }
+
+  void reset() { FreeIfNeeded(); }
+
+  template <class... Args>
+  void emplace(Args&&... args) {
+    FreeIfNeeded();
+    Init(std::forward<Args>(args)...);
+  }
+
+ private:
+  void Init(const T& value) {
+    DCHECK(storage_.is_null_);
+    new (&storage_.value_) T(value);
+    storage_.is_null_ = false;
+  }
+
+  void Init(T&& value) {
+    DCHECK(storage_.is_null_);
+    new (&storage_.value_) T(std::move(value));
+    storage_.is_null_ = false;
+  }
+
+  template <class... Args>
+  void Init(Args&&... args) {
+    DCHECK(storage_.is_null_);
+    new (&storage_.value_) T(std::forward<Args>(args)...);
+    storage_.is_null_ = false;
+  }
+
+  void InitOrAssign(const T& value) {
+    if (storage_.is_null_)
+      Init(value);
+    else
+      storage_.value_ = value;
+  }
+
+  void InitOrAssign(T&& value) {
+    if (storage_.is_null_)
+      Init(std::move(value));
+    else
+      storage_.value_ = std::move(value);
+  }
+
+  void FreeIfNeeded() {
+    if (storage_.is_null_) return;
+    storage_.value_.~T();
+    storage_.is_null_ = true;
+  }
+
+  internal::OptionalStorage<T> storage_;
+};
+
+template <class T>
+constexpr bool operator==(const Optional<T>& lhs, const Optional<T>& rhs) {
+  return !!lhs != !!rhs ? false : lhs == nullopt || (*lhs == *rhs);
+}
+
+template <class T>
+constexpr bool operator!=(const Optional<T>& lhs, const Optional<T>& rhs) {
+  return !(lhs == rhs);
+}
+
+template <class T>
+constexpr bool operator<(const Optional<T>& lhs, const Optional<T>& rhs) {
+  return rhs == nullopt ? false : (lhs == nullopt ? true : *lhs < *rhs);
+}
+
+template <class T>
+constexpr bool operator<=(const Optional<T>& lhs, const Optional<T>& rhs) {
+  return !(rhs < lhs);
+}
+
+template <class T>
+constexpr bool operator>(const Optional<T>& lhs, const Optional<T>& rhs) {
+  return rhs < lhs;
+}
+
+template <class T>
+constexpr bool operator>=(const Optional<T>& lhs, const Optional<T>& rhs) {
+  return !(lhs < rhs);
+}
+
+template <class T>
+constexpr bool operator==(const Optional<T>& opt, base::nullopt_t) {
+  return !opt;
+}
+
+template <class T>
+constexpr bool operator==(base::nullopt_t, const Optional<T>& opt) {
+  return !opt;
+}
+
+template <class T>
+constexpr bool operator!=(const Optional<T>& opt, base::nullopt_t) {
+  return !!opt;
+}
+
+template <class T>
+constexpr bool operator!=(base::nullopt_t, const Optional<T>& opt) {
+  return !!opt;
+}
+
+template <class T>
+constexpr bool operator<(const Optional<T>& opt, base::nullopt_t) {
+  return false;
+}
+
+template <class T>
+constexpr bool operator<(base::nullopt_t, const Optional<T>& opt) {
+  return !!opt;
+}
+
+template <class T>
+constexpr bool operator<=(const Optional<T>& opt, base::nullopt_t) {
+  return !opt;
+}
+
+template <class T>
+constexpr bool operator<=(base::nullopt_t, const Optional<T>& opt) {
+  return true;
+}
+
+template <class T>
+constexpr bool operator>(const Optional<T>& opt, base::nullopt_t) {
+  return !!opt;
+}
+
+template <class T>
+constexpr bool operator>(base::nullopt_t, const Optional<T>& opt) {
+  return false;
+}
+
+template <class T>
+constexpr bool operator>=(const Optional<T>& opt, base::nullopt_t) {
+  return true;
+}
+
+template <class T>
+constexpr bool operator>=(base::nullopt_t, const Optional<T>& opt) {
+  return !opt;
+}
+
+template <class T>
+constexpr bool operator==(const Optional<T>& opt, const T& value) {
+  return opt != nullopt ? *opt == value : false;
+}
+
+template <class T>
+constexpr bool operator==(const T& value, const Optional<T>& opt) {
+  return opt == value;
+}
+
+template <class T>
+constexpr bool operator!=(const Optional<T>& opt, const T& value) {
+  return !(opt == value);
+}
+
+template <class T>
+constexpr bool operator!=(const T& value, const Optional<T>& opt) {
+  return !(opt == value);
+}
+
+template <class T>
+constexpr bool operator<(const Optional<T>& opt, const T& value) {
+  return opt != nullopt ? *opt < value : true;
+}
+
+template <class T>
+constexpr bool operator<(const T& value, const Optional<T>& opt) {
+  return opt != nullopt ? value < *opt : false;
+}
+
+template <class T>
+constexpr bool operator<=(const Optional<T>& opt, const T& value) {
+  return !(opt > value);
+}
+
+template <class T>
+constexpr bool operator<=(const T& value, const Optional<T>& opt) {
+  return !(value > opt);
+}
+
+template <class T>
+constexpr bool operator>(const Optional<T>& opt, const T& value) {
+  return value < opt;
+}
+
+template <class T>
+constexpr bool operator>(const T& value, const Optional<T>& opt) {
+  return opt < value;
+}
+
+template <class T>
+constexpr bool operator>=(const Optional<T>& opt, const T& value) {
+  return !(opt < value);
+}
+
+template <class T>
+constexpr bool operator>=(const T& value, const Optional<T>& opt) {
+  return !(value < opt);
+}
+
+template <class T>
+constexpr Optional<typename std::decay<T>::type> make_optional(T&& value) {
+  return Optional<typename std::decay<T>::type>(std::forward<T>(value));
+}
+
+template <class T>
+void swap(Optional<T>& lhs, Optional<T>& rhs) {
+  lhs.swap(rhs);
+}
+
+}  // namespace base
+}  // namespace v8
+
+#endif  // V8_BASE_OPTIONAL_H_

src/v8.gyp

@@ -2011,6 +2011,7 @@
         'base/macros.h',
         'base/once.cc',
         'base/once.h',
+        'base/optional.h',
         'base/platform/elapsed-timer.h',
         'base/platform/time.cc',
         'base/platform/time.h',