pub struct ReferencePropagation;
Expand description
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
andy
may be dead, we allow replacement and mark storage statements ony
for removal; - if we want to replace
*x
by non-reborrowy
andy
must be live, we allow replacement; - if we want to replace
*x
by non-reborrowy
andy
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§
source§impl<'tcx> MirPass<'tcx> for ReferencePropagation
impl<'tcx> MirPass<'tcx> for ReferencePropagation
source§fn is_enabled(&self, sess: &Session) -> bool
fn is_enabled(&self, sess: &Session) -> bool
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
Auto Trait Implementations§
impl Freeze for ReferencePropagation
impl RefUnwindSafe for ReferencePropagation
impl Send for ReferencePropagation
impl Sync for ReferencePropagation
impl Unpin for ReferencePropagation
impl UnwindSafe for ReferencePropagation
Blanket Implementations§
source§impl<T> BorrowMut<T> for Twhere
T: ?Sized,
impl<T> BorrowMut<T> for Twhere
T: ?Sized,
source§fn borrow_mut(&mut self) -> &mut T
fn borrow_mut(&mut self) -> &mut T
source§impl<T, R> CollectAndApply<T, R> for T
impl<T, R> CollectAndApply<T, R> for T
source§impl<T> Filterable for T
impl<T> Filterable for T
source§fn filterable(
self,
filter_name: &'static str,
) -> RequestFilterDataProvider<T, fn(_: DataRequest<'_>) -> bool>
fn filterable( self, filter_name: &'static str, ) -> RequestFilterDataProvider<T, fn(_: DataRequest<'_>) -> bool>
source§impl<T> Instrument for T
impl<T> Instrument for T
source§fn instrument(self, span: Span) -> Instrumented<Self>
fn instrument(self, span: Span) -> Instrumented<Self>
source§fn in_current_span(self) -> Instrumented<Self>
fn in_current_span(self) -> Instrumented<Self>
source§impl<T> IntoEither for T
impl<T> IntoEither for T
source§fn into_either(self, into_left: bool) -> Either<Self, Self>
fn into_either(self, into_left: bool) -> Either<Self, Self>
self
into a Left
variant of Either<Self, Self>
if into_left
is true
.
Converts self
into a Right
variant of Either<Self, Self>
otherwise. Read moresource§fn into_either_with<F>(self, into_left: F) -> Either<Self, Self>
fn into_either_with<F>(self, into_left: F) -> Either<Self, Self>
self
into a Left
variant of Either<Self, Self>
if into_left(&self)
returns true
.
Converts self
into a Right
variant of Either<Self, Self>
otherwise. Read moresource§impl<P> IntoQueryParam<P> for P
impl<P> IntoQueryParam<P> for P
fn into_query_param(self) -> P
source§impl<T> MaybeResult<T> for T
impl<T> MaybeResult<T> for T
source§impl<I, T, U> Upcast<I, U> for Twhere
U: UpcastFrom<I, T>,
impl<I, T, U> Upcast<I, U> for Twhere
U: UpcastFrom<I, T>,
source§impl<I, T> UpcastFrom<I, T> for T
impl<I, T> UpcastFrom<I, T> for T
fn upcast_from(from: T, _tcx: I) -> T
source§impl<Tcx, T> Value<Tcx> for Twhere
Tcx: DepContext,
impl<Tcx, T> Value<Tcx> for Twhere
Tcx: DepContext,
default fn from_cycle_error( tcx: Tcx, cycle_error: &CycleError, _guar: ErrorGuaranteed, ) -> T
source§impl<T> WithSubscriber for T
impl<T> WithSubscriber for T
source§fn with_subscriber<S>(self, subscriber: S) -> WithDispatch<Self>
fn with_subscriber<S>(self, subscriber: S) -> WithDispatch<Self>
source§fn with_current_subscriber(self) -> WithDispatch<Self>
fn with_current_subscriber(self) -> WithDispatch<Self>
impl<'a, T> Captures<'a> for Twhere
T: ?Sized,
impl<'a, T> Captures<'a> for Twhere
T: ?Sized,
impl<T> ErasedDestructor for Twhere
T: 'static,
impl<T> MaybeSendSync for T
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