Struct ReferencePropagation

pub(crate) struct ReferencePropagation;

Propagate references using SSA analysis.

MIR building may produce a lot of borrow-dereference patterns.

This pass aims to transform the following pattern: _1 = &raw? mut? PLACE; _3 = *_1; _4 = &raw? mut? *_1;

Into _1 = &raw? mut? PLACE; _3 = PLACE; _4 = &raw? mut? PLACE;

where PLACE is a direct or an indirect place expression.

There are 3 properties that need to be upheld for this transformation to be legal:

• place stability: PLACE must refer to the same memory wherever it appears;
• pointer liveness: we must not introduce dereferences of dangling pointers;
• &mut borrow uniqueness.

Stability

If PLACE is an indirect projection, if its of the form (*LOCAL).PROJECTIONS where:

• LOCAL is SSA;
• all projections in PROJECTIONS have a stable offset (no dereference and no indexing).

If PLACE is a direct projection of a local, we consider it as constant if:

• the local is always live, or it has a single StorageLive;
• all projections have a stable offset.

Liveness

When performing an instantiation, we must take care not to introduce uses of dangling locals. To ensure this, we walk the body with the MaybeStorageDead dataflow analysis:

• if we want to replace *x by reborrow *y and y may be dead, we allow replacement and mark storage statements on y for removal;
• if we want to replace *x by non-reborrow y and y must be live, we allow replacement;
• if we want to replace *x by non-reborrow y and y may be dead, we do not replace.

Uniqueness

For &mut borrows, we also need to preserve the uniqueness property: we must avoid creating a state where we interleave uses of *_1 and _2. To do it, we only perform full instantiation of mutable borrows: we replace either all or none of the occurrences of *_1.

Some care has to be taken when _1 is copied in other locals. _1 = &raw? mut? _2; _3 = *_1; _4 = _1 _5 = *_4 In such cases, fully instantiating _1 means fully instantiating all of the copies.

For immutable borrows, we do not need to preserve such uniqueness property, so we perform all the possible instantiations without removing the _1 = &_2 statement.

Trait Implementations

impl<'tcx> MirPass<'tcx> for ReferencePropagation

fn is_enabled(&self, sess: &Session) -> bool

Returns true if this pass is enabled with the current combination of compiler flags.

fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>)

fn name(&self) -> &'static str

fn profiler_name(&self) -> &'static str

fn is_mir_dump_enabled(&self) -> bool

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: 0 bytes