Struct rustc_mir_dataflow::impls::MaybeLiveLocals

pub struct MaybeLiveLocals;

A live-variable dataflow analysis.

This analysis considers references as being used only at the point of the borrow. In other words, this analysis does not track uses because of references that already exist. See this mir-dataflow test for an example. You almost never want to use this analysis without also looking at the results of MaybeBorrowedLocals.

Field-(in)sensitivity

As the name suggests, this analysis is field insensitive. If a projection of a variable x is assigned to (e.g. x.0 = 42), it does not “define” x as far as liveness is concerned. In fact, such an assignment is currently marked as a “use” of x in an attempt to be maximally conservative.

Enums and SetDiscriminant

Assigning a literal value to an enum (e.g. Option<i32>), does not result in a simple assignment of the form _1 = /*...*/ in the MIR. For example, the following assignment to x:

x = Some(4);

compiles to this MIR

((_1 as Some).0: i32) = const 4_i32;
discriminant(_1) = 1;

However, MaybeLiveLocals does mark x (_1) as “killed” after a statement like this. That’s because it treats the SetDiscriminant operation as a definition of x, even though the writes that actually initialized the locals happened earlier.

This makes MaybeLiveLocals unsuitable for certain classes of optimization normally associated with a live variables analysis, notably dead-store elimination. It’s a dirty hack, but it works okay for the generator state transform (currently the main consumuer of this analysis).

Implementations

impl MaybeLiveLocals

fn transfer_function<'a, T>(&self, trans: &'a mut T) -> TransferFunction<'a, T>

Trait Implementations

impl<'tcx> AnalysisDomain<'tcx> for MaybeLiveLocals

type Domain = BitSet<Local>

The type that holds the dataflow state at any given point in the program.

type Direction = Backward

The direction of this analysis. Either Forward or Backward.

const NAME: &'static str

A descriptive name for this analysis. Used only for debugging.

fn bottom_value(&self, body: &Body<'tcx>) -> Self::Domain

The initial value of the dataflow state upon entry to each basic block.

fn initialize_start_block(&self, _: &Body<'tcx>, _: &mut Self::Domain)

Mutates the initial value of the dataflow state upon entry to the START_BLOCK.

impl<'tcx> GenKillAnalysis<'tcx> for MaybeLiveLocals

type Idx = Local

fn statement_effect(
    &self,
    trans: &mut impl GenKill<Self::Idx>,
    statement: &Statement<'tcx>,
    location: Location
)

See Analysis::apply_statement_effect.

fn terminator_effect(
    &self,
    trans: &mut impl GenKill<Self::Idx>,
    terminator: &Terminator<'tcx>,
    location: Location
)

See Analysis::apply_terminator_effect.

fn call_return_effect(
    &self,
    trans: &mut impl GenKill<Self::Idx>,
    _block: BasicBlock,
    return_places: CallReturnPlaces<'_, 'tcx>
)

See Analysis::apply_call_return_effect.

fn yield_resume_effect(
    &self,
    trans: &mut impl GenKill<Self::Idx>,
    _resume_block: BasicBlock,
    resume_place: Place<'tcx>
)

See Analysis::apply_yield_resume_effect.

fn before_statement_effect(
    &self,
    _trans: &mut impl GenKill<Self::Idx>,
    _statement: &Statement<'tcx>,
    _location: Location
)

See Analysis::apply_before_statement_effect.

fn before_terminator_effect(
    &self,
    _trans: &mut impl GenKill<Self::Idx>,
    _terminator: &Terminator<'tcx>,
    _location: Location
)

See Analysis::apply_before_terminator_effect.

fn switch_int_edge_effects<G: GenKill<Self::Idx>>(
    &self,
    _block: BasicBlock,
    _discr: &Operand<'tcx>,
    _edge_effects: &mut impl SwitchIntEdgeEffects<G>
)

See Analysis::apply_switch_int_edge_effects.

impl<'tcx> RustcPeekAt<'tcx> for MaybeLiveLocals

fn peek_at(
    &self,
    tcx: TyCtxt<'tcx>,
    place: Place<'tcx>,
    flow_state: &BitSet<Local>,
    call: PeekCall
)

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.

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