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use itertools::{Either, Itertools};
use rustc_data_structures::fx::FxHashSet;
use rustc_middle::mir::{Body, Local};
use rustc_middle::ty::{RegionVid, TyCtxt};
use rustc_mir_dataflow::impls::MaybeInitializedPlaces;
use rustc_mir_dataflow::move_paths::MoveData;
use rustc_mir_dataflow::ResultsCursor;
use std::rc::Rc;
use crate::{
constraints::OutlivesConstraintSet,
facts::{AllFacts, AllFactsExt},
location::LocationTable,
nll::ToRegionVid,
region_infer::values::RegionValueElements,
universal_regions::UniversalRegions,
};
use super::TypeChecker;
mod local_use_map;
mod polonius;
mod trace;
/// Combines liveness analysis with initialization analysis to
/// determine which variables are live at which points, both due to
/// ordinary uses and drops. Returns a set of (ty, location) pairs
/// that indicate which types must be live at which point in the CFG.
/// This vector is consumed by `constraint_generation`.
///
/// N.B., this computation requires normalization; therefore, it must be
/// performed before
pub(super) fn generate<'mir, 'tcx>(
typeck: &mut TypeChecker<'_, 'tcx>,
body: &Body<'tcx>,
elements: &Rc<RegionValueElements>,
flow_inits: &mut ResultsCursor<'mir, 'tcx, MaybeInitializedPlaces<'mir, 'tcx>>,
move_data: &MoveData<'tcx>,
location_table: &LocationTable,
use_polonius: bool,
) {
debug!("liveness::generate");
let free_regions = regions_that_outlive_free_regions(
typeck.infcx.num_region_vars(),
&typeck.borrowck_context.universal_regions,
&typeck.borrowck_context.constraints.outlives_constraints,
);
let (relevant_live_locals, boring_locals) =
compute_relevant_live_locals(typeck.tcx(), &free_regions, &body);
let facts_enabled = use_polonius || AllFacts::enabled(typeck.tcx());
let polonius_drop_used = facts_enabled.then(|| {
let mut drop_used = Vec::new();
polonius::populate_access_facts(typeck, body, location_table, move_data, &mut drop_used);
drop_used
});
trace::trace(
typeck,
body,
elements,
flow_inits,
move_data,
relevant_live_locals,
boring_locals,
polonius_drop_used,
);
}
// The purpose of `compute_relevant_live_locals` is to define the subset of `Local`
// variables for which we need to do a liveness computation. We only need
// to compute whether a variable `X` is live if that variable contains
// some region `R` in its type where `R` is not known to outlive a free
// region (i.e., where `R` may be valid for just a subset of the fn body).
fn compute_relevant_live_locals<'tcx>(
tcx: TyCtxt<'tcx>,
free_regions: &FxHashSet<RegionVid>,
body: &Body<'tcx>,
) -> (Vec<Local>, Vec<Local>) {
let (boring_locals, relevant_live_locals): (Vec<_>, Vec<_>) =
body.local_decls.iter_enumerated().partition_map(|(local, local_decl)| {
if tcx.all_free_regions_meet(&local_decl.ty, |r| {
free_regions.contains(&r.to_region_vid())
}) {
Either::Left(local)
} else {
Either::Right(local)
}
});
debug!("{} total variables", body.local_decls.len());
debug!("{} variables need liveness", relevant_live_locals.len());
debug!("{} regions outlive free regions", free_regions.len());
(relevant_live_locals, boring_locals)
}
/// Computes all regions that are (currently) known to outlive free
/// regions. For these regions, we do not need to compute
/// liveness, since the outlives constraints will ensure that they
/// are live over the whole fn body anyhow.
fn regions_that_outlive_free_regions<'tcx>(
num_region_vars: usize,
universal_regions: &UniversalRegions<'tcx>,
constraint_set: &OutlivesConstraintSet<'tcx>,
) -> FxHashSet<RegionVid> {
// Build a graph of the outlives constraints thus far. This is
// a reverse graph, so for each constraint `R1: R2` we have an
// edge `R2 -> R1`. Therefore, if we find all regions
// reachable from each free region, we will have all the
// regions that are forced to outlive some free region.
let rev_constraint_graph = constraint_set.reverse_graph(num_region_vars);
let fr_static = universal_regions.fr_static;
let rev_region_graph = rev_constraint_graph.region_graph(constraint_set, fr_static);
// Stack for the depth-first search. Start out with all the free regions.
let mut stack: Vec<_> = universal_regions.universal_regions().collect();
// Set of all free regions, plus anything that outlives them. Initially
// just contains the free regions.
let mut outlives_free_region: FxHashSet<_> = stack.iter().cloned().collect();
// Do the DFS -- for each thing in the stack, find all things
// that outlive it and add them to the set. If they are not,
// push them onto the stack for later.
while let Some(sub_region) = stack.pop() {
stack.extend(
rev_region_graph
.outgoing_regions(sub_region)
.filter(|&r| outlives_free_region.insert(r)),
);
}
// Return the final set of things we visited.
outlives_free_region
}
|
