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Auto merge of #81132 - bugadani:map-prealloc, r=matthewjasper
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Borrowck: refactor visited map to a bitset

This PR refactors `Borrows`  and the `precompute_borrows_out_of_scope` function so that this initial phase has a much reduced memory pressure. This is achieved by reducing what is stored on the heap, and also reusing heap memory as much as possible.
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@bors
bors committed Feb 9, 2021
2 parents 097bc6a + 5271c62 commit 87bacf2
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Showing 3 changed files with 96 additions and 93 deletions.
2 changes: 1 addition & 1 deletion Cargo.lock
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Original file line number Diff line number Diff line change
Expand Up @@ -5395,7 +5395,7 @@ dependencies = [
"chrono",
"lazy_static",
"matchers",
"parking_lot 0.11.0",
"parking_lot 0.9.0",
"regex",
"serde",
"serde_json",
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8 changes: 4 additions & 4 deletions compiler/rustc_mir/src/borrow_check/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -243,7 +243,7 @@ fn do_mir_borrowck<'a, 'tcx>(

let regioncx = Rc::new(regioncx);

let flow_borrows = Borrows::new(tcx, &body, regioncx.clone(), &borrow_set)
let flow_borrows = Borrows::new(tcx, &body, &regioncx, &borrow_set)
.into_engine(tcx, &body)
.pass_name("borrowck")
.iterate_to_fixpoint();
Expand Down Expand Up @@ -287,7 +287,7 @@ fn do_mir_borrowck<'a, 'tcx>(
regioncx: regioncx.clone(),
used_mut: Default::default(),
used_mut_upvars: SmallVec::new(),
borrow_set: borrow_set.clone(),
borrow_set: Rc::clone(&borrow_set),
dominators,
upvars: Vec::new(),
local_names: IndexVec::from_elem(None, &promoted_body.local_decls),
Expand Down Expand Up @@ -317,10 +317,10 @@ fn do_mir_borrowck<'a, 'tcx>(
move_error_reported: BTreeMap::new(),
uninitialized_error_reported: Default::default(),
errors_buffer,
regioncx,
regioncx: Rc::clone(&regioncx),
used_mut: Default::default(),
used_mut_upvars: SmallVec::new(),
borrow_set,
borrow_set: Rc::clone(&borrow_set),
dominators,
upvars,
local_names,
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179 changes: 91 additions & 88 deletions compiler/rustc_mir/src/dataflow/impls/borrows.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -11,7 +11,6 @@ use crate::borrow_check::{
use crate::dataflow::{self, fmt::DebugWithContext, GenKill};

use std::fmt;
use std::rc::Rc;

rustc_index::newtype_index! {
pub struct BorrowIndex {
Expand All @@ -30,132 +29,136 @@ pub struct Borrows<'a, 'tcx> {
tcx: TyCtxt<'tcx>,
body: &'a Body<'tcx>,

borrow_set: Rc<BorrowSet<'tcx>>,
borrow_set: &'a BorrowSet<'tcx>,
borrows_out_of_scope_at_location: FxHashMap<Location, Vec<BorrowIndex>>,

/// NLL region inference context with which NLL queries should be resolved
_nonlexical_regioncx: Rc<RegionInferenceContext<'tcx>>,
}

struct StackEntry {
bb: mir::BasicBlock,
lo: usize,
hi: usize,
first_part_only: bool,
}

fn precompute_borrows_out_of_scope<'tcx>(
body: &Body<'tcx>,
regioncx: &Rc<RegionInferenceContext<'tcx>>,
borrows_out_of_scope_at_location: &mut FxHashMap<Location, Vec<BorrowIndex>>,
borrow_index: BorrowIndex,
borrow_region: RegionVid,
location: Location,
) {
// We visit one BB at a time. The complication is that we may start in the
// middle of the first BB visited (the one containing `location`), in which
// case we may have to later on process the first part of that BB if there
// is a path back to its start.

// For visited BBs, we record the index of the first statement processed.
// (In fully processed BBs this index is 0.) Note also that we add BBs to
// `visited` once they are added to `stack`, before they are actually
// processed, because this avoids the need to look them up again on
// completion.
let mut visited = FxHashMap::default();
visited.insert(location.block, location.statement_index);

let mut stack = vec![];
stack.push(StackEntry {
bb: location.block,
lo: location.statement_index,
hi: body[location.block].statements.len(),
first_part_only: false,
});

while let Some(StackEntry { bb, lo, hi, first_part_only }) = stack.pop() {
let mut finished_early = first_part_only;
for i in lo..=hi {
let location = Location { block: bb, statement_index: i };
// If region does not contain a point at the location, then add to list and skip
// successor locations.
if !regioncx.region_contains(borrow_region, location) {
debug!("borrow {:?} gets killed at {:?}", borrow_index, location);
borrows_out_of_scope_at_location.entry(location).or_default().push(borrow_index);
finished_early = true;
break;
}
struct OutOfScopePrecomputer<'a, 'tcx> {
visited: BitSet<mir::BasicBlock>,
visit_stack: Vec<StackEntry>,
body: &'a Body<'tcx>,
regioncx: &'a RegionInferenceContext<'tcx>,
borrows_out_of_scope_at_location: FxHashMap<Location, Vec<BorrowIndex>>,
}

impl<'a, 'tcx> OutOfScopePrecomputer<'a, 'tcx> {
fn new(body: &'a Body<'tcx>, regioncx: &'a RegionInferenceContext<'tcx>) -> Self {
OutOfScopePrecomputer {
visited: BitSet::new_empty(body.basic_blocks().len()),
visit_stack: vec![],
body,
regioncx,
borrows_out_of_scope_at_location: FxHashMap::default(),
}
}
}

if !finished_early {
// Add successor BBs to the work list, if necessary.
let bb_data = &body[bb];
assert!(hi == bb_data.statements.len());
for &succ_bb in bb_data.terminator().successors() {
visited
.entry(succ_bb)
.and_modify(|lo| {
// `succ_bb` has been seen before. If it wasn't
// fully processed, add its first part to `stack`
// for processing.
if *lo > 0 {
stack.push(StackEntry {
impl<'tcx> OutOfScopePrecomputer<'_, 'tcx> {
fn precompute_borrows_out_of_scope(
&mut self,
borrow_index: BorrowIndex,
borrow_region: RegionVid,
location: Location,
) {
// We visit one BB at a time. The complication is that we may start in the
// middle of the first BB visited (the one containing `location`), in which
// case we may have to later on process the first part of that BB if there
// is a path back to its start.

// For visited BBs, we record the index of the first statement processed.
// (In fully processed BBs this index is 0.) Note also that we add BBs to
// `visited` once they are added to `stack`, before they are actually
// processed, because this avoids the need to look them up again on
// completion.
self.visited.insert(location.block);

let mut first_lo = location.statement_index;
let first_hi = self.body[location.block].statements.len();

self.visit_stack.push(StackEntry { bb: location.block, lo: first_lo, hi: first_hi });

while let Some(StackEntry { bb, lo, hi }) = self.visit_stack.pop() {
// If we process the first part of the first basic block (i.e. we encounter that block
// for the second time), we no longer have to visit its successors again.
let mut finished_early = bb == location.block && hi != first_hi;
for i in lo..=hi {
let location = Location { block: bb, statement_index: i };
// If region does not contain a point at the location, then add to list and skip
// successor locations.
if !self.regioncx.region_contains(borrow_region, location) {
debug!("borrow {:?} gets killed at {:?}", borrow_index, location);
self.borrows_out_of_scope_at_location
.entry(location)
.or_default()
.push(borrow_index);
finished_early = true;
break;
}
}

if !finished_early {
// Add successor BBs to the work list, if necessary.
let bb_data = &self.body[bb];
debug_assert!(hi == bb_data.statements.len());
for &succ_bb in bb_data.terminator().successors() {
if self.visited.insert(succ_bb) == false {
if succ_bb == location.block && first_lo > 0 {
// `succ_bb` has been seen before. If it wasn't
// fully processed, add its first part to `stack`
// for processing.
self.visit_stack.push(StackEntry {
bb: succ_bb,
lo: 0,
hi: *lo - 1,
first_part_only: true,
hi: first_lo - 1,
});

// And update this entry with 0, to represent the
// whole BB being processed.
first_lo = 0;
}
// And update this entry with 0, to represent the
// whole BB being processed.
*lo = 0;
})
.or_insert_with(|| {
} else {
// succ_bb hasn't been seen before. Add it to
// `stack` for processing.
stack.push(StackEntry {
self.visit_stack.push(StackEntry {
bb: succ_bb,
lo: 0,
hi: body[succ_bb].statements.len(),
first_part_only: false,
hi: self.body[succ_bb].statements.len(),
});
// Insert 0 for this BB, to represent the whole BB
// being processed.
0
});
}
}
}
}

self.visited.clear();
}
}

impl<'a, 'tcx> Borrows<'a, 'tcx> {
crate fn new(
tcx: TyCtxt<'tcx>,
body: &'a Body<'tcx>,
nonlexical_regioncx: Rc<RegionInferenceContext<'tcx>>,
borrow_set: &Rc<BorrowSet<'tcx>>,
nonlexical_regioncx: &'a RegionInferenceContext<'tcx>,
borrow_set: &'a BorrowSet<'tcx>,
) -> Self {
let mut borrows_out_of_scope_at_location = FxHashMap::default();
let mut prec = OutOfScopePrecomputer::new(body, nonlexical_regioncx);
for (borrow_index, borrow_data) in borrow_set.iter_enumerated() {
let borrow_region = borrow_data.region.to_region_vid();
let location = borrow_data.reserve_location;

precompute_borrows_out_of_scope(
body,
&nonlexical_regioncx,
&mut borrows_out_of_scope_at_location,
borrow_index,
borrow_region,
location,
);
prec.precompute_borrows_out_of_scope(borrow_index, borrow_region, location);
}

Borrows {
tcx,
body,
borrow_set: borrow_set.clone(),
borrows_out_of_scope_at_location,
_nonlexical_regioncx: nonlexical_regioncx,
borrow_set,
borrows_out_of_scope_at_location: prec.borrows_out_of_scope_at_location,
}
}

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