# Design Doc: InferVarType

## The Problem Posed

The variable in our design can hold variant types. Such as LoDTensor and SelectedRows. An operator should be able to inference the variable types of its output.

For example, a lookup table operator takes two LoDTensor; one is a float tensor as the embedding table, the other is an int tensor as word ID. The gradient operator of lookup table will generate a SelectedRows as its output. A sum operator can take both LoDTensor and SelectedRows as its inputs and will generate a LoDTensor if any of its inputs is LoDTensor, otherwise, the sum operator will generate SelectedRows as its output.

The variable type will be constant at runtime. Every variable's type can either be set by the user (input data and parameter) or be inferred by the operator in compile time.

## Proposed Solution

The InferVarType is a compile-time function which is registered to each operator. The inferface of that function is:

using InferVarTypeFN = std::function<
void (const OpDescBind& /*op_desc*/, BlockDescBind* /*block*/)>;


It takes an operator description as its input and will write the output variable type and store them in block description.

The InferVarTypeFN will be registered in OpInfo, to replace infer_var_type_ field. The OpInfo should be

struct OpInfo {
InferVarTypeFN infer_var_type_;
...
};


The default InferVarType will set output type as LoDTensor. It can be done by GetInferVarType().

void DefaultInferVarType(const OpDescBind& op_desc, BlockDescBind* block) {
// set the output type of variable as LoDTensor.
// ...
}

struct OpInfo {
InferVarTypeFN infer_var_type_;
InferVarTypeFN GetInferVarType() const {
if (infer_var_type_) {
return infer_var_type_;
} else {
return DefaultInferVarType;
}
}
};


## Register InferVarType

We provide a thin base class for registering an InferVarTypeFN. To use a base class will ease the implementation of registry since we can detect the registry entry is an InferVarTypeFN or not.

class VarTypeInferer {
public:
virtual void operator()(const OpDescBind& op_desc, BlockDescBind* block) const = 0;
}


Operator developers can write the specialize VarTypeInferer as follow.

class SpecialVarTypeInferer : public VarTypeInferer {
public:
virtual void operator()(const OpDescBind& op_desc, BlockDescBind* block) const {
// .. own logic
}
}


Then user can register the InferVarType just like GradOpDescMaker and OpInfoMaker.

REGISTER_OPERATOR(some_op, OpType, SpecialVarTypeInferer, ...);