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Remove dead unwinds before drop elaboration

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As a part of drop elaboration, we identify dead unwinds, i.e., unwind
edges on a drop terminators which are known to be unreachable, because
there is no need to drop anything.

Previously, the data flow framework was informed about the dead unwinds,
and it assumed those edges are absent from MIR. Unfortunately, the data
flow framework wasn't consistent in maintaining this assumption.

In particular, if a block was reachable only through a dead unwind edge,
its state was propagated to other blocks still. This became an issue in
the context of change removes DropAndReplace terminator, since it
introduces initialization into cleanup blocks.

To avoid this issue, remove unreachable unwind edges before the drop
elaboration, and elaborate only blocks that remain reachable.
This commit is contained in:
Tomasz Miąsko 2023-01-05 10:15:33 +01:00
parent 00eec854f1
commit 23e3840014
3 changed files with 44 additions and 76 deletions
Download patch file Download diff file Expand all files Collapse all files

57
compiler/rustc_mir_transform/src/elaborate_drops.rs
View file

@ -67,13 +67,11 @@ impl<'tcx> MirPass<'tcx> for ElaborateDrops {
};
let un_derefer = UnDerefer { tcx: tcx, derefer_sidetable: side_table };
let elaborate_patch = {
let body = &*body;
let env = MoveDataParamEnv { move_data, param_env };
let dead_unwinds = find_dead_unwinds(tcx, body, &env, &un_derefer);
remove_dead_unwinds(tcx, body, &env, &un_derefer);
let inits = MaybeInitializedPlaces::new(tcx, body, &env)
.into_engine(tcx, body)
.dead_unwinds(&dead_unwinds)
.pass_name("elaborate_drops")
.iterate_to_fixpoint()
.into_results_cursor(body);
@ -81,11 +79,12 @@ impl<'tcx> MirPass<'tcx> for ElaborateDrops {
let uninits = MaybeUninitializedPlaces::new(tcx, body, &env)
.mark_inactive_variants_as_uninit()
.into_engine(tcx, body)
.dead_unwinds(&dead_unwinds)
.pass_name("elaborate_drops")
.iterate_to_fixpoint()
.into_results_cursor(body);
let reachable = traversal::reachable_as_bitset(body);
ElaborateDropsCtxt {
tcx,
body,
@ -94,6 +93,7 @@ impl<'tcx> MirPass<'tcx> for ElaborateDrops {
drop_flags: Default::default(),
patch: MirPatch::new(body),
un_derefer: un_derefer,
reachable,
}
.elaborate()
};
@ -102,22 +102,21 @@ impl<'tcx> MirPass<'tcx> for ElaborateDrops {
}
}
/// Returns the set of basic blocks whose unwind edges are known
/// to not be reachable, because they are `drop` terminators
/// Removes unwind edges which are known to be unreachable, because they are in `drop` terminators
/// that can't drop anything.
fn find_dead_unwinds<'tcx>(
fn remove_dead_unwinds<'tcx>(
tcx: TyCtxt<'tcx>,
body: &Body<'tcx>,
body: &mut Body<'tcx>,
env: &MoveDataParamEnv<'tcx>,
und: &UnDerefer<'tcx>,
) -> BitSet<BasicBlock> {
debug!("find_dead_unwinds({:?})", body.span);
) {
debug!("remove_dead_unwinds({:?})", body.span);
// We only need to do this pass once, because unwind edges can only
// reach cleanup blocks, which can't have unwind edges themselves.
let mut dead_unwinds = BitSet::new_empty(body.basic_blocks.len());
let mut dead_unwinds = Vec::new();
let mut flow_inits = MaybeInitializedPlaces::new(tcx, body, &env)
.into_engine(tcx, body)
.pass_name("find_dead_unwinds")
.pass_name("remove_dead_unwinds")
.iterate_to_fixpoint()
.into_results_cursor(body);
for (bb, bb_data) in body.basic_blocks.iter_enumerated() {
@ -129,16 +128,16 @@ fn find_dead_unwinds<'tcx>(
_ => continue,
};
debug!("find_dead_unwinds @ {:?}: {:?}", bb, bb_data);
debug!("remove_dead_unwinds @ {:?}: {:?}", bb, bb_data);
let LookupResult::Exact(path) = env.move_data.rev_lookup.find(place.as_ref()) else {
debug!("find_dead_unwinds: has parent; skipping");
debug!("remove_dead_unwinds: has parent; skipping");
continue;
};
flow_inits.seek_before_primary_effect(body.terminator_loc(bb));
debug!(
"find_dead_unwinds @ {:?}: path({:?})={:?}; init_data={:?}",
"remove_dead_unwinds @ {:?}: path({:?})={:?}; init_data={:?}",
bb,
place,
path,
@ -150,13 +149,22 @@ fn find_dead_unwinds<'tcx>(
maybe_live |= flow_inits.contains(child);
});
debug!("find_dead_unwinds @ {:?}: maybe_live={}", bb, maybe_live);
debug!("remove_dead_unwinds @ {:?}: maybe_live={}", bb, maybe_live);
if !maybe_live {
dead_unwinds.insert(bb);
dead_unwinds.push(bb);
}
}
dead_unwinds
if dead_unwinds.is_empty() {
return;
}
let basic_blocks = body.basic_blocks.as_mut();
for &bb in dead_unwinds.iter() {
if let Some(unwind) = basic_blocks[bb].terminator_mut().unwind_mut() {
*unwind = None;
}
}
}
struct InitializationData<'mir, 'tcx> {
@ -290,6 +298,7 @@ struct ElaborateDropsCtxt<'a, 'tcx> {
drop_flags: FxHashMap<MovePathIndex, Local>,
patch: MirPatch<'tcx>,
un_derefer: UnDerefer<'tcx>,
reachable: BitSet<BasicBlock>,
}
impl<'b, 'tcx> ElaborateDropsCtxt<'b, 'tcx> {
@ -329,6 +338,9 @@ impl<'b, 'tcx> ElaborateDropsCtxt<'b, 'tcx> {
fn collect_drop_flags(&mut self) {
for (bb, data) in self.body.basic_blocks.iter_enumerated() {
if !self.reachable.contains(bb) {
continue;
}
let terminator = data.terminator();
let place = match terminator.kind {
TerminatorKind::Drop { ref place, .. }
@ -384,6 +396,9 @@ impl<'b, 'tcx> ElaborateDropsCtxt<'b, 'tcx> {
fn elaborate_drops(&mut self) {
for (bb, data) in self.body.basic_blocks.iter_enumerated() {
if !self.reachable.contains(bb) {
continue;
}
let loc = Location { block: bb, statement_index: data.statements.len() };
let terminator = data.terminator();
@ -541,6 +556,9 @@ impl<'b, 'tcx> ElaborateDropsCtxt<'b, 'tcx> {
fn drop_flags_for_fn_rets(&mut self) {
for (bb, data) in self.body.basic_blocks.iter_enumerated() {
if !self.reachable.contains(bb) {
continue;
}
if let TerminatorKind::Call {
destination, target: Some(tgt), cleanup: Some(_), ..
} = data.terminator().kind
@ -576,6 +594,9 @@ impl<'b, 'tcx> ElaborateDropsCtxt<'b, 'tcx> {
// clobbered before they are read.
for (bb, data) in self.body.basic_blocks.iter_enumerated() {
if !self.reachable.contains(bb) {
continue;
}
debug!("drop_flags_for_locs({:?})", data);
for i in 0..(data.statements.len() + 1) {
debug!("drop_flag_for_locs: stmt {}", i);