Struct RefMap 

pub struct RefMap { /* private fields */ }

Which slots of a frame are Repr::Ref, as one bit each.

A frame’s roots are a static fact here rather than a program-counter dependent one, and that is the point of the register machine: a stack machine’s live-reference set changes as operands are pushed and popped, so its map has to be indexed by pc. Here the answer does not change between the first instruction of a function and the last.

The lowering guarantees the one fact this relies on: a slot’s Repr is fixed for the whole function. A slot may be reused by a later value of the same Repr — that is what keeps a frame from growing with every temporary a long body mentions — but never by one of a different Repr, because then no single bit would be right at every program counter.

A static map says which slots the collector reads. It cannot say when the value in one stopped being needed, because that is a fact about a program point. The lowering answers that in the data instead: it emits Clear at a reference’s last use, so a dead slot holds null and the collector traces nothing from it.

Implementations

impl RefMap

pub fn of(reprs: &[Repr]) -> RefMap

The map for a frame of slots words, with reprs[i] at slot i.

pub fn slots(&self) -> u32

How many slots the map covers.

pub fn is_ref(&self, slot: u32) -> bool

Whether slot slot holds a reference.

Out of range answers false rather than panicking: the collector walks a frame whose size it took from the same crate::Function as this map, so a disagreement is a bug in the lowering, and a collection is the worst place to discover one by unwinding.

pub fn iter(&self) -> impl Iterator<Item = u32> + '_

Every reference slot, ascending.

Iterating the set bits rather than every slot is what makes a frame of mostly scalars cheap to scan: a frame with no references at all costs one word read per 64 slots.

pub fn is_empty(&self) -> bool

Whether the frame holds no references at all.

Trait Implementations

impl Clone for RefMap

fn clone(&self) -> RefMap

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for RefMap

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Default for RefMap

fn default() -> RefMap

Returns the “default value” for a type.

impl Eq for RefMap

impl PartialEq for RefMap

fn eq(&self, other: &RefMap) -> bool

Tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl StructuralPartialEq for RefMap