Struct GcStatusWord 

pub(crate) struct GcStatusWord(AtomicUsize);

A lock-free, atomic encoding of GcStatus. This packs the variant tag into a usize so the whole status fits in a single machine word and can be updated with compare-and-swap instead of behind a Mutex<GcStatus>. The tag is kept to Self::TAG_BITS bits (rather than using the whole word) to leave room for a payload-carrying variant (e.g. a nesting depth) to be added later without needing to re-encode the rest.

GcStatus is a state machine: only a handful of transitions between its variants are legal. Every legal transition is exposed here as its own method, each performing its own compare-and-swap retry loop and asserting that the transition is legal for the status it finds. Do not add a generic “set the status to X” method: doing so would make it possible to bypass the state machine’s invariants.

Tuple Fields

0: AtomicUsize

Implementations

impl GcStatusWord

const TAG_BITS: u32 = 3u32

Number of bits used to encode the variant tag. 3 bits is enough to distinguish the 5 variants, leaving the rest of the word free for a future payload-carrying variant.

const TAG_MASK: usize = 7usize

fn encode(status: GcStatus) -> usize

fn decode(bits: usize) -> GcStatus

pub(crate) fn new(status: GcStatus) -> Self

pub(crate) fn load(&self) -> GcStatus

Read the current status.

fn transition<F: FnMut(GcStatus) -> GcStatus>(&self, f: F) -> GcStatus

Retry f (a pure function of the current status) via AtomicUsize::fetch_update until it succeeds, and return the status it transitioned from (not the new status). Returning the old status (rather than the new one) lets a caller tell whether it “won” the race when multiple threads concurrently drive the same transition: only the thread whose CAS actually moved the status away from a given old value can be sure it is the one responsible for that transition, so it is the one that should perform any side effect that must happen exactly once (e.g. notifying the scheduler). If transition returned the new status instead, every racing thread would observe the same new status and none could tell which of them caused it. f may be invoked more than once under contention.

pub(crate) fn is_initialized(&self) -> bool

pub(crate) fn set_initialized(&self)

Uninitialized -> NotInGC.

pub(crate) fn set_uninitialized(&self)

Any status other than Uninitialized -> Uninitialized.

pub(crate) fn set_in_pause(&self)

PauseRequested -> InPause.

pub(crate) fn set_in_concurrent_gc(&self)

InPause -> InConcurrentGC, e.g. once a GC pause has finished but concurrent work (such as concurrent marking) was scheduled to continue after mutators resume.

pub(crate) fn set_not_in_gc(&self)

InPause -> NotInGC, e.g. once a GC pause has finished and no concurrent work remains.

pub(crate) fn try_request_pause(&self) -> PauseRequestOutcome

NotInGC/InConcurrentGC -> PauseRequested, unless MMTk is not yet initialized, or a pause has already been requested. See PauseRequestOutcome.