Trait mmtk::vm::edge_shape::Edge
source · pub trait Edge: Copy + Send + Debug + PartialEq + Eq + Hash {
// Required methods
fn load(&self) -> Option<ObjectReference>;
fn store(&self, object: ObjectReference);
// Provided methods
fn prefetch_load(&self) { ... }
fn prefetch_store(&self) { ... }
}
Expand description
An Edge
represents a slot in an object (a.k.a. a field), on the stack (i.e. a local variable)
or any other places (such as global variables). A slot may hold an object reference. We can
load the object reference from it, and we can store an ObjectReference into it. For some VMs,
a slot may sometimes not hold an object reference. For example, it can hold a special NULL
pointer which does not point to any object, or it can hold a tagged non-reference value, such
as small integers and special values such as true
, false
, null
(a.k.a. “none”, “nil”,
etc. for other VMs), undefined
, etc.
This intends to abstract out the differences of reference field representation among different
VMs. If the VM represent a reference field as a word that holds the pointer to the object, it
can use the default SimpleEdge
we provide. In some cases, the VM need to implement its own
Edge
instances.
For example:
- The VM uses compressed pointer (Compressed OOP in OpenJDK’s terminology), where the heap size is limited, and a 64-bit pointer is stored in a 32-bit slot.
- The VM uses tagged pointer, where some bits of a word are used as metadata while the rest are used as pointer.
- A field holds a pointer to the middle of an object (an object field, or an array element, or some arbitrary offset) for some reasons.
When loading, Edge::load
shall decode its internal representation to a “regular”
ObjectReference
. The implementation
can do this with any appropriate operations, usually shifting and masking bits or subtracting
offset from the address. By doing this conversion, MMTk can implement GC algorithms in a
VM-neutral way, knowing only ObjectReference
.
When GC moves object, Edge::store
shall convert the updated ObjectReference
back to the
edge-specific representation. Compressed pointers remain compressed; tagged pointers preserve
their tag bits; and offsetted pointers keep their offsets.
The methods of this trait are called on hot paths. Please ensure they have high performance. Use inlining when appropriate.
Note: this trait only concerns the representation (i.e. the shape) of the edge, not its
semantics, such as whether it holds strong or weak references. If a VM holds a weak reference
in a word as a pointer, it can also use SimpleEdge
for weak reference fields.
Required Methods§
sourcefn load(&self) -> Option<ObjectReference>
fn load(&self) -> Option<ObjectReference>
Load object reference from the slot.
If the slot is not holding an object reference (For example, if it is holding NULL or a
tagged non-reference value. See trait-level doc comment.), this method should return
None
.
If the slot holds an object reference with tag bits, the returned value shall be the object reference with the tag bits removed.
sourcefn store(&self, object: ObjectReference)
fn store(&self, object: ObjectReference)
Store the object reference object
into the slot.
If the slot holds an object reference with tag bits, this method must preserve the tag
bits while updating the object reference so that it points to the forwarded object given by
the parameter object
.
FIXME: This design is inefficient for handling object references with tag bits. Consider introducing a new updating function to do the load, trace and store in one function. See: https://github.com/mmtk/mmtk-core/issues/1033
FIXME: This method is currently used by both moving GC algorithms and the subsuming write
barrier (crate::memory_manager::object_reference_write
). The two reference writing
operations have different semantics, and need to be implemented differently if the VM
supports offsetted or tagged references.
See: https://github.com/mmtk/mmtk-core/issues/1038
Provided Methods§
sourcefn prefetch_load(&self)
fn prefetch_load(&self)
Prefetch the edge so that a subsequent load
will be faster.
sourcefn prefetch_store(&self)
fn prefetch_store(&self)
Prefetch the edge so that a subsequent store
will be faster.
Object Safety§
Implementors§
impl Edge for Address
For backword compatibility, we let Address
implement Edge
so that existing bindings that
use Address
to represent an edge can continue to work.
However, we should use SimpleEdge
directly instead of using Address
. The purpose of the
Address
type is to represent an address in memory. It is not directly related to fields
that hold references to other objects. Calling load()
and store()
on an Address
does
not indicate how many bytes to load or store, or how to interpret those bytes. On the other
hand, SimpleEdge
is all about how to access a field that holds a reference represented
simply as an ObjectReference
. The intention and the semantics are clearer with
SimpleEdge
.