Lunderberg commented on pull request #42:
URL: https://github.com/apache/tvm-rfcs/pull/42#issuecomment-963314748
A slight modification to the example of using a helper function to simplify
implementation of the visitors for `BufferStore` and `BufferLoad`. Use of
`CopyOnWrite` to pass a non-const reference to the helper function would cause
unnecessary copies in the case where no change is necessary. The copy in
`CopyOnWrite` occurs when `CopyOnWrite()` is called, not when the write itself
happens. Therefore, the helper function should be rewritten in order to avoid
this copy. This doesn't change the conclusion, but would change the
recommended way to write the helper function.
<details>
<summary>BufferLoad/BufferStore, using templated helper
function</summary>
```c++
Stmt VisitStmt_(const BufferStoreNode* op) final {
return
ModifyBuffer(Downcast<BufferStore>(StmtExprMutator::VisitStmt_(op)));
}
PrimExpr VisitStmt_(const BufferLoadNode* op) final {
return ModifyBuffer(Downcast<BufferLoad>(StmtExprMutator::VisitExpr_(op)));
}
template<typename Node>
Node ModifyBuffer(Node node) {
auto it = buffer_map_.find(node->buffer);
if (it != buffer_map_.end()) {
auto writer = node.CopyOnWrite();
writer->buffer = it->second;
}
return node;
}
```
</details>
(I think the original helper function could still work if
`obj.CopyOnWrite()->field` returned a proxy object that was convertible to the
underlying type, and whose `operator=` performed the copy itself. However,
that's probably more trouble/magic than it's worth.)
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