Lunderberg opened a new pull request, #14522:
URL: https://github.com/apache/tvm/pull/14522
Prior to this PR, the `ObjectRef::as<T>()` method could be used for any `T`
that inherits from `tvm::Object`, and would return a `const T*` if the class
could be cast to the specified type, or `nullptr` otherwise. However, if the
caller needed a `ObjectRef`, they would then need to call `GetRef<MyObjRef>` to
convert from a `const T*`.
This PR extends `ObjectRef::as<T>` to operate on a `T` that inherits from
`tvm::ObjectRef` as well. In this case, the return type is `Optional<T>`,
returning either an instance of the specified subclass, or `NullOpt` if the
object was not an instance of the specified subclass. Example usage of this
new conversion, along with how it relates to existing functionality, is shown
below.
```c++
// Unconditionally convert, throwing an exception if the object isn't
// of the specified type. In contexts where the type of the object is
// unknown, this shouldn't be used.
PrimExpr expr = Downcast<PrimExpr>(obj);
// Protect the Downcast from throwing an exception using IsInstance.
// This avoids the error, but performs the type-checking twice. In
// addition, it requires the caller to specify both the ObjectRef
// subclass and the Object subclass, even though these usually have a
// 1:1 correspondence.
if (obj->IsInstance<PrimExprNode>()) {
PrimExpr expr = Downcast<PrimExpr>(obj);
}
// Perform both type-checking and downcasting with the ObjectRef::as()
// method, then use GetRef to convert to an ObjectRef. This avoids
// double-checking the type, but still requires the caller to
if (const PrimExprNode* ptr = obj.as<PrimExprNode>()) {
PrimExpr expr = GetRef<PrimExpr>(ptr);
}
// New method introduced by this PR. The type-checking is only
// performed once, and the Object subclass is inferred from the
// ObjectRef subclass.
if (Optional<PrimExpr> opt = obj.as<PrimExpr>()) {
PrimExpr expr = opt.value();
}
```
This PR is implemented as two commits. The first commit implements the new
functionality, but makes no further changes. The second commit looks for cases
where `object_ref.as<TNode>()` was immediately followed by `GetRef<T>()`,
replacing with either `object_ref.as<T>()` or with `Downcast<T>(object_ref)`.
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