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