// Copyright 2018 the V8 project authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file.
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#ifndef V8_TORQUE_CONTEXTUAL_H_
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#define V8_TORQUE_CONTEXTUAL_H_
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#include <type_traits>
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#include "src/base/macros.h"
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#include "src/base/platform/platform.h"
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namespace v8 {
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namespace internal {
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namespace torque {
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// {ContextualVariable} provides a clean alternative to a global variable.
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// The contextual variable is mutable, and supports managing the value of
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// a variable in a well-nested fashion via the {Scope} class.
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// {ContextualVariable} only stores a pointer to the current value, which
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// is stored in a {Scope} object. The most recent value can be retrieved
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// via Get(). Because only {Scope} has actual storage, there must be at
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// least one active {Scope} (i.e. in a surrounding C++ scope), whenever Get()
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// is called.
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// Note that contextual variables must only be used from the same thread,
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// i.e. {Scope} and Get() have to be in the same thread.
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template <class Derived, class VarType>
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class ContextualVariable {
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public:
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using VariableType = VarType;
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// A {Scope} contains a new object of type {VarType} and gives
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// ContextualVariable::Get() access to it. Upon destruction, the contextual
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// variable is restored to the state before the {Scope} was created. Scopes
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// have to follow a stack discipline: A {Scope} has to be destructed before
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// any older scope is destructed.
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class Scope {
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public:
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template <class... Args>
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explicit Scope(Args&&... args)
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: current_(std::forward<Args>(args)...), previous_(Top()) {
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Top() = ¤t_;
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}
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~Scope() {
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// Ensure stack discipline.
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DCHECK_EQ(¤t_, Top());
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Top() = previous_;
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}
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private:
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VarType current_;
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VarType* previous_;
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static_assert(std::is_base_of<ContextualVariable, Derived>::value,
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"Curiously Recurring Template Pattern");
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DISALLOW_NEW_AND_DELETE();
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DISALLOW_COPY_AND_ASSIGN(Scope);
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};
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// Access the most recent active {Scope}. There has to be an active {Scope}
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// for this contextual variable.
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static VarType& Get() {
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DCHECK_NOT_NULL(Top());
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return *Top();
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}
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private:
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V8_EXPORT_PRIVATE static VarType*& Top();
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};
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// Usage: DECLARE_CONTEXTUAL_VARIABLE(VarName, VarType)
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#define DECLARE_CONTEXTUAL_VARIABLE(VarName, ...) \
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struct VarName \
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: v8::internal::torque::ContextualVariable<VarName, __VA_ARGS__> {};
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#define DEFINE_CONTEXTUAL_VARIABLE(VarName) \
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template <> \
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V8_EXPORT_PRIVATE VarName::VariableType*& \
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ContextualVariable<VarName, VarName::VariableType>::Top() { \
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static thread_local VarName::VariableType* top = nullptr; \
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return top; \
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}
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// By inheriting from {ContextualClass} a class can become a contextual variable
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// of itself, which is very similar to a singleton.
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template <class T>
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using ContextualClass = ContextualVariable<T, T>;
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} // namespace torque
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} // namespace internal
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} // namespace v8
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#endif // V8_TORQUE_CONTEXTUAL_H_
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