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elaborate_drops.rs
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elaborate_drops.rs
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use crate::deref_separator::deref_finder;
use crate::MirPass;
use rustc_data_structures::fx::FxHashMap;
use rustc_index::bit_set::BitSet;
use rustc_middle::mir::patch::MirPatch;
use rustc_middle::mir::*;
use rustc_middle::ty::{self, TyCtxt};
use rustc_mir_dataflow::elaborate_drops::{elaborate_drop, DropFlagState, Unwind};
use rustc_mir_dataflow::elaborate_drops::{DropElaborator, DropFlagMode, DropStyle};
use rustc_mir_dataflow::impls::{MaybeInitializedPlaces, MaybeUninitializedPlaces};
use rustc_mir_dataflow::move_paths::{LookupResult, MoveData, MovePathIndex};
use rustc_mir_dataflow::on_lookup_result_bits;
use rustc_mir_dataflow::un_derefer::UnDerefer;
use rustc_mir_dataflow::MoveDataParamEnv;
use rustc_mir_dataflow::{on_all_children_bits, on_all_drop_children_bits};
use rustc_mir_dataflow::{Analysis, ResultsCursor};
use rustc_span::Span;
use rustc_target::abi::VariantIdx;
use std::fmt;
pub struct ElaborateDrops;
impl<'tcx> MirPass<'tcx> for ElaborateDrops {
fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
debug!("elaborate_drops({:?} @ {:?})", body.source, body.span);
let def_id = body.source.def_id();
let param_env = tcx.param_env_reveal_all_normalized(def_id);
let (side_table, move_data) = match MoveData::gather_moves(body, tcx, param_env) {
Ok(move_data) => move_data,
Err((move_data, _)) => {
tcx.sess.delay_span_bug(
body.span,
"No `move_errors` should be allowed in MIR borrowck",
);
(Default::default(), move_data)
}
};
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);
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);
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);
ElaborateDropsCtxt {
tcx,
body,
env: &env,
init_data: InitializationData { inits, uninits },
drop_flags: Default::default(),
patch: MirPatch::new(body),
un_derefer: un_derefer,
}
.elaborate()
};
elaborate_patch.apply(body);
deref_finder(tcx, body);
}
}
/// Returns the set of basic blocks whose unwind edges are known
/// to not be reachable, because they are `drop` terminators
/// that can't drop anything.
fn find_dead_unwinds<'tcx>(
tcx: TyCtxt<'tcx>,
body: &Body<'tcx>,
env: &MoveDataParamEnv<'tcx>,
und: &UnDerefer<'tcx>,
) -> BitSet<BasicBlock> {
debug!("find_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 flow_inits = MaybeInitializedPlaces::new(tcx, body, &env)
.into_engine(tcx, body)
.pass_name("find_dead_unwinds")
.iterate_to_fixpoint()
.into_results_cursor(body);
for (bb, bb_data) in body.basic_blocks.iter_enumerated() {
let place = match bb_data.terminator().kind {
TerminatorKind::Drop { ref place, unwind: Some(_), .. }
| TerminatorKind::DropAndReplace { ref place, unwind: Some(_), .. } => {
und.derefer(place.as_ref(), body).unwrap_or(*place)
}
_ => continue,
};
debug!("find_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");
continue;
};
flow_inits.seek_before_primary_effect(body.terminator_loc(bb));
debug!(
"find_dead_unwinds @ {:?}: path({:?})={:?}; init_data={:?}",
bb,
place,
path,
flow_inits.get()
);
let mut maybe_live = false;
on_all_drop_children_bits(tcx, body, &env, path, |child| {
maybe_live |= flow_inits.contains(child);
});
debug!("find_dead_unwinds @ {:?}: maybe_live={}", bb, maybe_live);
if !maybe_live {
dead_unwinds.insert(bb);
}
}
dead_unwinds
}
struct InitializationData<'mir, 'tcx> {
inits: ResultsCursor<'mir, 'tcx, MaybeInitializedPlaces<'mir, 'tcx>>,
uninits: ResultsCursor<'mir, 'tcx, MaybeUninitializedPlaces<'mir, 'tcx>>,
}
impl InitializationData<'_, '_> {
fn seek_before(&mut self, loc: Location) {
self.inits.seek_before_primary_effect(loc);
self.uninits.seek_before_primary_effect(loc);
}
fn maybe_live_dead(&self, path: MovePathIndex) -> (bool, bool) {
(self.inits.contains(path), self.uninits.contains(path))
}
}
struct Elaborator<'a, 'b, 'tcx> {
ctxt: &'a mut ElaborateDropsCtxt<'b, 'tcx>,
}
impl fmt::Debug for Elaborator<'_, '_, '_> {
fn fmt(&self, _f: &mut fmt::Formatter<'_>) -> fmt::Result {
Ok(())
}
}
impl<'a, 'tcx> DropElaborator<'a, 'tcx> for Elaborator<'a, '_, 'tcx> {
type Path = MovePathIndex;
fn patch(&mut self) -> &mut MirPatch<'tcx> {
&mut self.ctxt.patch
}
fn body(&self) -> &'a Body<'tcx> {
self.ctxt.body
}
fn tcx(&self) -> TyCtxt<'tcx> {
self.ctxt.tcx
}
fn param_env(&self) -> ty::ParamEnv<'tcx> {
self.ctxt.param_env()
}
fn drop_style(&self, path: Self::Path, mode: DropFlagMode) -> DropStyle {
let ((maybe_live, maybe_dead), multipart) = match mode {
DropFlagMode::Shallow => (self.ctxt.init_data.maybe_live_dead(path), false),
DropFlagMode::Deep => {
let mut some_live = false;
let mut some_dead = false;
let mut children_count = 0;
on_all_drop_children_bits(self.tcx(), self.body(), self.ctxt.env, path, |child| {
let (live, dead) = self.ctxt.init_data.maybe_live_dead(child);
debug!("elaborate_drop: state({:?}) = {:?}", child, (live, dead));
some_live |= live;
some_dead |= dead;
children_count += 1;
});
((some_live, some_dead), children_count != 1)
}
};
match (maybe_live, maybe_dead, multipart) {
(false, _, _) => DropStyle::Dead,
(true, false, _) => DropStyle::Static,
(true, true, false) => DropStyle::Conditional,
(true, true, true) => DropStyle::Open,
}
}
fn clear_drop_flag(&mut self, loc: Location, path: Self::Path, mode: DropFlagMode) {
match mode {
DropFlagMode::Shallow => {
self.ctxt.set_drop_flag(loc, path, DropFlagState::Absent);
}
DropFlagMode::Deep => {
on_all_children_bits(
self.tcx(),
self.body(),
self.ctxt.move_data(),
path,
|child| self.ctxt.set_drop_flag(loc, child, DropFlagState::Absent),
);
}
}
}
fn field_subpath(&self, path: Self::Path, field: Field) -> Option<Self::Path> {
rustc_mir_dataflow::move_path_children_matching(self.ctxt.move_data(), path, |e| match e {
ProjectionElem::Field(idx, _) => idx == field,
_ => false,
})
}
fn array_subpath(&self, path: Self::Path, index: u64, size: u64) -> Option<Self::Path> {
rustc_mir_dataflow::move_path_children_matching(self.ctxt.move_data(), path, |e| match e {
ProjectionElem::ConstantIndex { offset, min_length, from_end } => {
debug_assert!(size == min_length, "min_length should be exact for arrays");
assert!(!from_end, "from_end should not be used for array element ConstantIndex");
offset == index
}
_ => false,
})
}
fn deref_subpath(&self, path: Self::Path) -> Option<Self::Path> {
rustc_mir_dataflow::move_path_children_matching(self.ctxt.move_data(), path, |e| {
e == ProjectionElem::Deref
})
}
fn downcast_subpath(&self, path: Self::Path, variant: VariantIdx) -> Option<Self::Path> {
rustc_mir_dataflow::move_path_children_matching(self.ctxt.move_data(), path, |e| match e {
ProjectionElem::Downcast(_, idx) => idx == variant,
_ => false,
})
}
fn get_drop_flag(&mut self, path: Self::Path) -> Option<Operand<'tcx>> {
self.ctxt.drop_flag(path).map(Operand::Copy)
}
}
struct ElaborateDropsCtxt<'a, 'tcx> {
tcx: TyCtxt<'tcx>,
body: &'a Body<'tcx>,
env: &'a MoveDataParamEnv<'tcx>,
init_data: InitializationData<'a, 'tcx>,
drop_flags: FxHashMap<MovePathIndex, Local>,
patch: MirPatch<'tcx>,
un_derefer: UnDerefer<'tcx>,
}
impl<'b, 'tcx> ElaborateDropsCtxt<'b, 'tcx> {
fn move_data(&self) -> &'b MoveData<'tcx> {
&self.env.move_data
}
fn param_env(&self) -> ty::ParamEnv<'tcx> {
self.env.param_env
}
fn create_drop_flag(&mut self, index: MovePathIndex, span: Span) {
let tcx = self.tcx;
let patch = &mut self.patch;
debug!("create_drop_flag({:?})", self.body.span);
self.drop_flags.entry(index).or_insert_with(|| patch.new_internal(tcx.types.bool, span));
}
fn drop_flag(&mut self, index: MovePathIndex) -> Option<Place<'tcx>> {
self.drop_flags.get(&index).map(|t| Place::from(*t))
}
/// create a patch that elaborates all drops in the input
/// MIR.
fn elaborate(mut self) -> MirPatch<'tcx> {
self.collect_drop_flags();
self.elaborate_drops();
self.drop_flags_on_init();
self.drop_flags_for_fn_rets();
self.drop_flags_for_args();
self.drop_flags_for_locs();
self.patch
}
fn collect_drop_flags(&mut self) {
for (bb, data) in self.body.basic_blocks.iter_enumerated() {
let terminator = data.terminator();
let place = match terminator.kind {
TerminatorKind::Drop { ref place, .. }
| TerminatorKind::DropAndReplace { ref place, .. } => {
self.un_derefer.derefer(place.as_ref(), self.body).unwrap_or(*place)
}
_ => continue,
};
self.init_data.seek_before(self.body.terminator_loc(bb));
let path = self.move_data().rev_lookup.find(place.as_ref());
debug!("collect_drop_flags: {:?}, place {:?} ({:?})", bb, place, path);
let path = match path {
LookupResult::Exact(e) => e,
LookupResult::Parent(None) => continue,
LookupResult::Parent(Some(parent)) => {
let (_maybe_live, maybe_dead) = self.init_data.maybe_live_dead(parent);
if self.body.local_decls[place.local].is_deref_temp() {
continue;
}
if maybe_dead {
self.tcx.sess.delay_span_bug(
terminator.source_info.span,
&format!(
"drop of untracked, uninitialized value {:?}, place {:?} ({:?})",
bb, place, path
),
);
}
continue;
}
};
on_all_drop_children_bits(self.tcx, self.body, self.env, path, |child| {
let (maybe_live, maybe_dead) = self.init_data.maybe_live_dead(child);
debug!(
"collect_drop_flags: collecting {:?} from {:?}@{:?} - {:?}",
child,
place,
path,
(maybe_live, maybe_dead)
);
if maybe_live && maybe_dead {
self.create_drop_flag(child, terminator.source_info.span)
}
});
}
}
fn elaborate_drops(&mut self) {
for (bb, data) in self.body.basic_blocks.iter_enumerated() {
let loc = Location { block: bb, statement_index: data.statements.len() };
let terminator = data.terminator();
let resume_block = self.patch.resume_block();
match terminator.kind {
TerminatorKind::Drop { mut place, target, unwind } => {
if let Some(new_place) = self.un_derefer.derefer(place.as_ref(), self.body) {
place = new_place;
}
self.init_data.seek_before(loc);
match self.move_data().rev_lookup.find(place.as_ref()) {
LookupResult::Exact(path) => elaborate_drop(
&mut Elaborator { ctxt: self },
terminator.source_info,
place,
path,
target,
if data.is_cleanup {
Unwind::InCleanup
} else {
Unwind::To(Option::unwrap_or(unwind, resume_block))
},
bb,
),
LookupResult::Parent(..) => {
self.tcx.sess.delay_span_bug(
terminator.source_info.span,
&format!("drop of untracked value {:?}", bb),
);
}
}
}
TerminatorKind::DropAndReplace { mut place, ref value, target, unwind } => {
assert!(!data.is_cleanup);
if let Some(new_place) = self.un_derefer.derefer(place.as_ref(), self.body) {
place = new_place;
}
self.elaborate_replace(loc, place, value, target, unwind);
}
_ => continue,
}
}
}
/// Elaborate a MIR `replace` terminator. This instruction
/// is not directly handled by codegen, and therefore
/// must be desugared.
///
/// The desugaring drops the location if needed, and then writes
/// the value (including setting the drop flag) over it in *both* arms.
///
/// The `replace` terminator can also be called on places that
/// are not tracked by elaboration (for example,
/// `replace x[i] <- tmp0`). The borrow checker requires that
/// these locations are initialized before the assignment,
/// so we just generate an unconditional drop.
fn elaborate_replace(
&mut self,
loc: Location,
place: Place<'tcx>,
value: &Operand<'tcx>,
target: BasicBlock,
unwind: Option<BasicBlock>,
) {
let bb = loc.block;
let data = &self.body[bb];
let terminator = data.terminator();
assert!(!data.is_cleanup, "DropAndReplace in unwind path not supported");
let assign = Statement {
kind: StatementKind::Assign(Box::new((place, Rvalue::Use(value.clone())))),
source_info: terminator.source_info,
};
let unwind = unwind.unwrap_or_else(|| self.patch.resume_block());
let unwind = self.patch.new_block(BasicBlockData {
statements: vec![assign.clone()],
terminator: Some(Terminator {
kind: TerminatorKind::Goto { target: unwind },
..*terminator
}),
is_cleanup: true,
});
let target = self.patch.new_block(BasicBlockData {
statements: vec![assign],
terminator: Some(Terminator { kind: TerminatorKind::Goto { target }, ..*terminator }),
is_cleanup: false,
});
match self.move_data().rev_lookup.find(place.as_ref()) {
LookupResult::Exact(path) => {
debug!("elaborate_drop_and_replace({:?}) - tracked {:?}", terminator, path);
self.init_data.seek_before(loc);
elaborate_drop(
&mut Elaborator { ctxt: self },
terminator.source_info,
place,
path,
target,
Unwind::To(unwind),
bb,
);
on_all_children_bits(self.tcx, self.body, self.move_data(), path, |child| {
self.set_drop_flag(
Location { block: target, statement_index: 0 },
child,
DropFlagState::Present,
);
self.set_drop_flag(
Location { block: unwind, statement_index: 0 },
child,
DropFlagState::Present,
);
});
}
LookupResult::Parent(parent) => {
// drop and replace behind a pointer/array/whatever. The location
// must be initialized.
debug!("elaborate_drop_and_replace({:?}) - untracked {:?}", terminator, parent);
self.patch.patch_terminator(
bb,
TerminatorKind::Drop { place, target, unwind: Some(unwind) },
);
}
}
}
fn constant_bool(&self, span: Span, val: bool) -> Rvalue<'tcx> {
Rvalue::Use(Operand::Constant(Box::new(Constant {
span,
user_ty: None,
literal: ConstantKind::from_bool(self.tcx, val),
})))
}
fn set_drop_flag(&mut self, loc: Location, path: MovePathIndex, val: DropFlagState) {
if let Some(&flag) = self.drop_flags.get(&path) {
let span = self.patch.source_info_for_location(self.body, loc).span;
let val = self.constant_bool(span, val.value());
self.patch.add_assign(loc, Place::from(flag), val);
}
}
fn drop_flags_on_init(&mut self) {
let loc = Location::START;
let span = self.patch.source_info_for_location(self.body, loc).span;
let false_ = self.constant_bool(span, false);
for flag in self.drop_flags.values() {
self.patch.add_assign(loc, Place::from(*flag), false_.clone());
}
}
fn drop_flags_for_fn_rets(&mut self) {
for (bb, data) in self.body.basic_blocks.iter_enumerated() {
if let TerminatorKind::Call {
destination, target: Some(tgt), cleanup: Some(_), ..
} = data.terminator().kind
{
assert!(!self.patch.is_patched(bb));
let loc = Location { block: tgt, statement_index: 0 };
let path = self.move_data().rev_lookup.find(destination.as_ref());
on_lookup_result_bits(self.tcx, self.body, self.move_data(), path, |child| {
self.set_drop_flag(loc, child, DropFlagState::Present)
});
}
}
}
fn drop_flags_for_args(&mut self) {
let loc = Location::START;
rustc_mir_dataflow::drop_flag_effects_for_function_entry(
self.tcx,
self.body,
self.env,
|path, ds| {
self.set_drop_flag(loc, path, ds);
},
)
}
fn drop_flags_for_locs(&mut self) {
// We intentionally iterate only over the *old* basic blocks.
//
// Basic blocks created by drop elaboration update their
// drop flags by themselves, to avoid the drop flags being
// clobbered before they are read.
for (bb, data) in self.body.basic_blocks.iter_enumerated() {
debug!("drop_flags_for_locs({:?})", data);
for i in 0..(data.statements.len() + 1) {
debug!("drop_flag_for_locs: stmt {}", i);
let mut allow_initializations = true;
if i == data.statements.len() {
match data.terminator().kind {
TerminatorKind::Drop { .. } => {
// drop elaboration should handle that by itself
continue;
}
TerminatorKind::DropAndReplace { .. } => {
// this contains the move of the source and
// the initialization of the destination. We
// only want the former - the latter is handled
// by the elaboration code and must be done
// *after* the destination is dropped.
assert!(self.patch.is_patched(bb));
allow_initializations = false;
}
TerminatorKind::Resume => {
// It is possible for `Resume` to be patched
// (in particular it can be patched to be replaced with
// a Goto; see `MirPatch::new`).
}
_ => {
assert!(!self.patch.is_patched(bb));
}
}
}
let loc = Location { block: bb, statement_index: i };
rustc_mir_dataflow::drop_flag_effects_for_location(
self.tcx,
self.body,
self.env,
loc,
|path, ds| {
if ds == DropFlagState::Absent || allow_initializations {
self.set_drop_flag(loc, path, ds)
}
},
)
}
// There may be a critical edge after this call,
// so mark the return as initialized *before* the
// call.
if let TerminatorKind::Call { destination, target: Some(_), cleanup: None, .. } =
data.terminator().kind
{
assert!(!self.patch.is_patched(bb));
let loc = Location { block: bb, statement_index: data.statements.len() };
let path = self.move_data().rev_lookup.find(destination.as_ref());
on_lookup_result_bits(self.tcx, self.body, self.move_data(), path, |child| {
self.set_drop_flag(loc, child, DropFlagState::Present)
});
}
}
}
}