CODEWITHSUNDEEP
dynamicrustvtable
CodeSandwich
CodeSandwich

Reputation: 1751

Why do trait object vtables contain size and alignment?

Rust's trait objects are fat pointers that contain 2 regular pointers: to data and to a vtable. The vtable is a structure containing a destructor function pointer, all trait method pointers and finally the size and alignment of the data.

What are the size and alignment fields for?

I couldn't find much:

Upvotes: 10

Views: 1264

Answers (2)

July Tikhonov
July Tikhonov

Reputation: 11

To demonstrate why the vtable must store the alignment of the value, we can look at Rc::<dyn Trait>::as_ref associated function (i.e. dereference of Rc::<dyn Trait> into &dyn Trait).

A value of type Rc<dyn Trait> is essentially two addresses: the address of RcBox (which is a struct that contains reference counts and the object value) and the address of vtable corresponding to the object value. To obtain the address of the object value, we need to offset the address of RcBox past the reference counts (two 8-byte numbers, assuming 64-bit machine). However, if the object value has alignment greater than 16, it must be offset further (to satisfy its alignment). This means that to obtain the address of the object value, we must know the alignment of the value.

Upvotes: 1

Wesley Wiser
Wesley Wiser

Reputation: 9851

Here's what I've found so far:

The size & alignment properties in a vtable are loaded in the librustc_codegen_llvm::glue::size_and_align_of_dst() function which returns the size and alignment of a dynamically sized type. For ty::Dynamic(..) values (the compiler's internal way of describing trait objects), the size and alignment are read from the vtable:

match t.sty {
    ty::Dynamic(..) => {
        // load size/align from vtable
        let vtable = info.unwrap();
        (meth::SIZE.get_usize(bx, vtable), meth::ALIGN.get_usize(bx, vtable))
    }
    ...
}

This function in turn is used in several places:

I didn't spot any places where these values are currently fed into the Rust deallocation function (__rust_dealloc()), but they could certainly be used for that in the future.

Upvotes: 11

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