Function mmtk::memory_manager::object_reference_write

pub fn object_reference_write<VM: VMBinding>(
    mutator: &mut Mutator<VM>,
    src: ObjectReference,
    slot: VM::VMSlot,
    target: ObjectReference
)

👎Deprecated: Use object_reference_write_pre and object_reference_write_post instead, until this function is redesigned

The subsuming write barrier by MMTk. For performance reasons, a VM should implement the write barrier fast-path on their side rather than just calling this function.

For a correct barrier implementation, a VM binding needs to choose one of the following options:

• Use subsuming barrier object_reference_write
• Use both object_reference_write_pre and object_reference_write_post, or both, if the binding has difficulty delegating the store to mmtk-core with the subsuming barrier.
• Implement fast-path on the VM side, and call the generic api object_reference_write_slow as barrier slow-path call.
• Implement fast-path on the VM side, and do a specialized slow-path call.

Arguments:

• mutator: The mutator for the current thread.
• src: The modified source object.
• slot: The location of the field to be modified.
• target: The target for the write operation.

Deprecated

This function needs to be redesigned. Its current form has multiple issues.

• It is only able to write non-null object references into the slot. But dynamic language VMs may write non-reference values, such as tagged small integers, special values such as null, undefined, true, false, etc. into a field that previous contains an object reference.
• It relies on slot.store to write target into the slot, but slot.store is designed for forwarding references when an object is moved by GC, and is supposed to preserve tagged type information, the offset (if it is an interior pointer), etc. A write barrier is associated to an assignment operation, which usually updates such information instead.

We will redesign a more general subsuming write barrier to address those problems and replace the current object_reference_write. Before that happens, VM bindings should use object_reference_write_pre and object_reference_write_post instead.