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125 lines
6.4 KiB
125 lines
6.4 KiB
# Design of Scope in Paddle
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## Overview
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Scope is an important concept in programming languages, which defines a program region that a set of bindings between names and entities applies. In a specific scope, a valid name is uniquely associated with an entity, such as a variable. And in another scope, this name may refer to other entity or nothing at all. It clearly restricts the visibility and validity of names in a program. Hence **Scope** is introduced to PaddlePaddle to manage variables in context. But different from the original abstract concept, Scope now becomes an object with two important attributes:
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- Scope is an association of a name to variable.
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- Variables in a parent scope can be retrieved from local scope.
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A detailed explanation of these two attributes goes as following.
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## Scope is an association of a name to variable.
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Scope is an association of a name to variable. All variables belong to `Scope`. You need to specify a scope to run a Net, i.e., `net.Run(&scope)`. One net can run in different scopes and update different variable in the scope.
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1. Scope only contains a map of a name to variable.
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All parameters, data, states in a Net should be variables and stored inside a scope. Each op should get inputs and outputs to do computation from a scope, such as data buffer, state (momentum) etc.
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1. Variable can only be created by Scope and a variable can only be got from Scope. User cannot create or get a variable outside a scope. This is a constraints of our framework, and will keep our framework simple and clear.
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1. Scope only contains methods that are used to Create and Get Variables. Scope do not contain Operators and have no information to run them.
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`Net` is designed to drive the computation and Scope only contains a map of variables. There is no computation logic inside a `Scope`. Scope just handles the lifetime management of variables.
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- `Create` is used to create a Variable by its name and add the mapping relation.
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- `Get` is used to find a Variable by name.
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1. Every variable only belongs to one certain Scope.
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Variable can not belong to many scopes. If you want to use variables from parent scope, you can use `parent scope`.
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1. Scope should destruct all Variables inside it when itself is destructed. User can never store `Variable` pointer somewhere else.
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Because Variable can only be got from Scope. When destroying Scope, we also need to destroy all the Variables in it. If user store `Variable` pointer to private data member or some global variable, the pointer will be an invalid pointer when associated `Scope` is destroyed.
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```cpp
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class Scope {
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public:
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Variable* Var(const std::string& name);
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const Variable* FindVar(const std::string& name) const;
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private:
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std::unordered_map<std::string, std::unique_ptr<Variable>> vars_;
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};
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```
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## Parent scope and local scope
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Just like [scope](https://en.wikipedia.org/wiki/Scope_(computer_science)) in programming languages, `Scope` in the neural network can also be a local scope. There are two attributes about local scope.
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1. We can create local variables in a local scope. When that local scope is destroyed, all local variables should also be destroyed.
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2. Variables in a parent scope can be retrieved from local scopes of that parent scope, i.e., when user get a variable from a scope, it will try to search this variable in current scope. If there is no such variable in the local scope, `scope` will keep searching from its parent, until the variable is found or there is no parent.
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```cpp
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class Scope {
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public:
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Scope(const std::shared_ptr<Scope>& scope): parent_(scope) {}
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Variable* FindVar(const std::string& name) const {
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auto it = vars_.find(name);
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if (it != vars_.end()) {
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return it->second.get();
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} else if (parent_ != nullptr) {
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return parent_->FindVar(name);
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} else {
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return nullptr;
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}
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}
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private:
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std::shared_ptr<Scope> parent_ {nullptr};
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};
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```
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In `Scope` class, there is a private data member called `parent_`. `parent_` is a smart pointer to its parent scope. When user `Get` a variable by its `name`, the `name` will be searched inside the current scope. If the variable cannot be found locally and parent scope is not a `nullptr`, the variable will be searched inside that parent scope. `parent_` pointer's default value is `nullptr`. It means that the scope is a global scope when `parent_` is nullptr.
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A local scope is very useful when we implement Recurrent Neural Network. Each timestep of an RNN should be a `Net`. Each `Net` of timestep (`StepNet` for short) should use an independent local scope. Just like variables in a while loop is inside a local scope in programming languages. By using a single `StepNet` and changing local scope, we can implement an RNN easily.
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# Interface Design
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```cpp
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class Variable {
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private:
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Variable() = default;
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friend class Scope;
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};
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class Scope {
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private:
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Scope(const std::shared_ptr<Scope>& parent = nullptr);
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public:
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static std::shared_ptr<Scope> Create(const std::shared_ptr<Scope>& parent = nullptr);
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// return nullptr if not found.
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Variable* FindVar(const std::string& name) const;
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// return if already contains same name variable.
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Variable* Var(const std::string& name);
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private:
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std::shared_ptr<Scope> parent_;
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std::unordered_map<std::string, std::unique_ptr<Variable>> vars_;
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};
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```
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## Only scope can create a variable
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To ensure `only scope can create a variable`, we should mark `Variable`'s constructor as a private member function, and Scope is a friend class of Variable. And then only `Var` can construct `Variable`.
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## When scope destroyed, all variables inside this scope should be destroyed together
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The scope hold unique pointers for all variables. User can `FindVar` from scope, but he should not hold this pointer as a member variable. Because when scope is destroyed, all variables inside this scope will be destroyed together.
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## Sharing a parent scope
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Local scope contains a `parent_` pointer. It is a linked-list for scopes. Using a `shared_ptr` because when a local scope is using, its parents cannot be destroyed.
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Also, as the parent scope is a `shared_ptr`, we can only `Create()` a scope shared pointer. We cannot construct a scope variable, because it cannot be passed to other scope as `parent` pointer.
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## Orthogonal interface
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`FindVar` will return `nullptr` when `name` is not found. It can be used as `Contains` method. `Var` will return an `Error` when there is a name conflict locally. Combine `FindVar` and `Var`, we can implement `Var` easily.
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