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Diffstat (limited to 'compiler/rustc_mir_transform/src/elaborate_drops.rs')
| -rw-r--r-- | compiler/rustc_mir_transform/src/elaborate_drops.rs | 613 |
1 files changed, 613 insertions, 0 deletions
diff --git a/compiler/rustc_mir_transform/src/elaborate_drops.rs b/compiler/rustc_mir_transform/src/elaborate_drops.rs new file mode 100644 index 000000000..9c1fcbaa6 --- /dev/null +++ b/compiler/rustc_mir_transform/src/elaborate_drops.rs @@ -0,0 +1,613 @@ +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 phase_change(&self) -> Option<MirPhase> { + Some(MirPhase::DropsLowered) + } + + 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) + }); + } + } + } +} |