Struct miri::stacked_borrows::stack::StackCache

struct StackCache {
    tags: [SbTag; 32],
    idx: [usize; 32],
}

A very small cache of searches of the borrow stack This maps tags to locations in the borrow stack. Any use of this still needs to do a probably-cold random access into the borrow stack to figure out what Permission an SbTag grants. We could avoid this by also storing the Permission in the cache, but most lookups into the cache are immediately followed by access of the full borrow stack anyway.

It may seem like maintaining this cache is a waste for small stacks, but (a) iterating over small fixed-size arrays is super fast, and (b) empirically this helps a lot, probably because runtime is dominated by large stacks.

Fields

tags: [SbTag; 32]

idx: [usize; 32]

Implementations

impl StackCache

fn add(&mut self, idx: usize, tag: SbTag)

When a tag is used, we call this function to add or refresh it in the cache.

We use the position in the cache to represent how recently a tag was used; the first position is the most recently used tag. So an add shifts every element towards the end, and inserts the new element at the start. We lose the last element. This strategy is effective at keeping the most-accessed tags in the cache, but it costs a linear shift across the entire cache when we add a new tag.

Trait Implementations

impl Clone for StackCache

fn clone(&self) -> StackCache

Returns a copy of the value.

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

Performs copy-assignment from source.

impl Debug for StackCache

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

Formats the value using the given formatter.

Layout

Note: Most layout information is completely unstable and may even differ between compilations. The only exception is types with certain repr(...) attributes. Please see the Rust Reference’s “Type Layout” chapter for details on type layout guarantees.

Size: 512 bytes