diff options
Diffstat (limited to 'compiler/rustc_mir_transform/src/dest_prop.rs')
| -rw-r--r-- | compiler/rustc_mir_transform/src/dest_prop.rs | 843 |
1 files changed, 385 insertions, 458 deletions
diff --git a/compiler/rustc_mir_transform/src/dest_prop.rs b/compiler/rustc_mir_transform/src/dest_prop.rs index cf7425251e8..da1d339b526 100644 --- a/compiler/rustc_mir_transform/src/dest_prop.rs +++ b/compiler/rustc_mir_transform/src/dest_prop.rs @@ -59,6 +59,12 @@ //! The first two conditions are simple structural requirements on the `Assign` statements that can //! be trivially checked. The third requirement however is more difficult and costly to check. //! +//! ## Current implementation +//! +//! The current implementation relies on live range computation to check for conflicts. We only +//! allow to merge locals that have disjoint live ranges. The live range are defined with +//! half-statement granularity, so as to make all writes be live for at least a half statement. +//! //! ## Future Improvements //! //! There are a number of ways in which this pass could be improved in the future: @@ -117,9 +123,8 @@ //! - Layout optimizations for coroutines have been added to improve code generation for //! async/await, which are very similar in spirit to what this optimization does. //! -//! Also, rustc now has a simple NRVO pass (see `nrvo.rs`), which handles a subset of the cases that -//! this destination propagation pass handles, proving that similar optimizations can be performed -//! on MIR. +//! [The next approach][attempt 4] computes a conflict matrix between locals by forbidding merging +//! locals with competing writes or with one write while the other is live. //! //! ## Pre/Post Optimization //! @@ -130,20 +135,18 @@ //! [attempt 1]: https://github.com/rust-lang/rust/pull/47954 //! [attempt 2]: https://github.com/rust-lang/rust/pull/71003 //! [attempt 3]: https://github.com/rust-lang/rust/pull/72632 +//! [attempt 4]: https://github.com/rust-lang/rust/pull/96451 -use rustc_data_structures::fx::{FxIndexMap, IndexEntry, IndexOccupiedEntry}; +use rustc_data_structures::fx::FxIndexMap; use rustc_index::bit_set::DenseBitSet; use rustc_index::interval::SparseIntervalMatrix; -use rustc_middle::bug; -use rustc_middle::mir::visit::{MutVisitor, PlaceContext, Visitor}; -use rustc_middle::mir::{ - Body, HasLocalDecls, InlineAsmOperand, Local, LocalKind, Location, MirDumper, Operand, - PassWhere, Place, Rvalue, Statement, StatementKind, TerminatorKind, traversal, -}; +use rustc_index::{IndexSlice, IndexVec, newtype_index}; +use rustc_middle::mir::visit::{MutVisitor, NonMutatingUseContext, PlaceContext, Visitor}; +use rustc_middle::mir::*; use rustc_middle::ty::TyCtxt; -use rustc_mir_dataflow::Analysis; -use rustc_mir_dataflow::impls::MaybeLiveLocals; -use rustc_mir_dataflow::points::{DenseLocationMap, PointIndex, save_as_intervals}; +use rustc_mir_dataflow::impls::DefUse; +use rustc_mir_dataflow::points::{DenseLocationMap, PointIndex}; +use rustc_mir_dataflow::{Analysis, Backward, Results}; use tracing::{debug, trace}; pub(super) struct DestinationPropagation; @@ -161,84 +164,62 @@ impl<'tcx> crate::MirPass<'tcx> for DestinationPropagation { sess.mir_opt_level() >= 3 } + #[tracing::instrument(level = "trace", skip(self, tcx, body))] fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) { let def_id = body.source.def_id(); - let mut candidates = Candidates::default(); - let mut write_info = WriteInfo::default(); - trace!(func = ?tcx.def_path_str(def_id)); + trace!(?def_id); let borrowed = rustc_mir_dataflow::impls::borrowed_locals(body); - let live = MaybeLiveLocals.iterate_to_fixpoint(tcx, body, Some("MaybeLiveLocals-DestProp")); + let candidates = Candidates::find(body, &borrowed); + trace!(?candidates); + if candidates.c.is_empty() { + return; + } + + let live = + MaybeTwoStepLiveLocals.iterate_to_fixpoint(tcx, body, Some("MaybeLiveLocals-DestProp")); + let points = DenseLocationMap::new(body); - let mut live = save_as_intervals(&points, body, live.analysis, live.results); - - // In order to avoid having to collect data for every single pair of locals in the body, we - // do not allow doing more than one merge for places that are derived from the same local at - // once. To avoid missed opportunities, we instead iterate to a fixed point - we'll refer to - // each of these iterations as a "round." - // - // Reaching a fixed point could in theory take up to `min(l, s)` rounds - however, we do not - // expect to see MIR like that. To verify this, a test was run against `[rust-lang/regex]` - - // the average MIR body saw 1.32 full iterations of this loop. The most that was hit were 30 - // for a single function. Only 80/2801 (2.9%) of functions saw at least 5. - // - // [rust-lang/regex]: - // https://github.com/rust-lang/regex/tree/b5372864e2df6a2f5e543a556a62197f50ca3650 - let mut round_count = 0; - loop { - // PERF: Can we do something smarter than recalculating the candidates and liveness - // results? - candidates.reset_and_find(body, &borrowed); - trace!(?candidates); - dest_prop_mir_dump(tcx, body, &points, &live, round_count); - - FilterInformation::filter_liveness( - &mut candidates, - &points, - &live, - &mut write_info, - body, - ); - - // Because we only filter once per round, it is unsound to use a local for more than - // one merge operation within a single round of optimizations. We store here which ones - // we have already used. - let mut merged_locals: DenseBitSet<Local> = - DenseBitSet::new_empty(body.local_decls.len()); - - // This is the set of merges we will apply this round. It is a subset of the candidates. - let mut merges = FxIndexMap::default(); - - for (src, candidates) in candidates.c.iter() { - if merged_locals.contains(*src) { - continue; - } - let Some(dest) = candidates.iter().find(|dest| !merged_locals.contains(**dest)) - else { - continue; - }; - - // Replace `src` by `dest` everywhere. - merges.insert(*src, *dest); - merged_locals.insert(*src); - merged_locals.insert(*dest); - - // Update liveness information based on the merge we just performed. - // Every location where `src` was live, `dest` will be live. - live.union_rows(*src, *dest); - } - trace!(merging = ?merges); + let mut relevant = RelevantLocals::compute(&candidates, body.local_decls.len()); + let mut live = save_as_intervals(&points, body, &relevant.original, live.results); - if merges.is_empty() { - break; - } - round_count += 1; + dest_prop_mir_dump(tcx, body, &points, &live, &relevant); + + let mut merged_locals = DenseBitSet::new_empty(body.local_decls.len()); + + for (src, candidates) in candidates.c.into_iter() { + trace!(?src, ?candidates); + + let Some(src) = relevant.find(src) else { continue }; + let Some(src_live_ranges) = &live.row(src) else { continue }; + trace!(?src, ?src_live_ranges); + + let dst = candidates.into_iter().find_map(|dst| { + let dst = relevant.find(dst)?; + let dst_live_ranges = &live.row(dst)?; + trace!(?dst, ?dst_live_ranges); + + let disjoint = src_live_ranges.disjoint(dst_live_ranges); + disjoint.then_some(dst) + }); + let Some(dst) = dst else { continue }; + + merged_locals.insert(relevant.original[src]); + merged_locals.insert(relevant.original[dst]); - apply_merges(body, tcx, merges, merged_locals); + relevant.union(src, dst); + live.union_rows(src, dst); } + trace!(?merged_locals); - trace!(round_count); + relevant.make_idempotent(); + + if merged_locals.is_empty() { + return; + } + + apply_merges(body, tcx, relevant, merged_locals); } fn is_required(&self) -> bool { @@ -246,30 +227,6 @@ impl<'tcx> crate::MirPass<'tcx> for DestinationPropagation { } } -#[derive(Debug, Default)] -struct Candidates { - /// The set of candidates we are considering in this optimization. - /// - /// We will always merge the key into at most one of its values. - /// - /// Whether a place ends up in the key or the value does not correspond to whether it appears as - /// the lhs or rhs of any assignment. As a matter of fact, the places in here might never appear - /// in an assignment at all. This happens because if we see an assignment like this: - /// - /// ```ignore (syntax-highlighting-only) - /// _1.0 = _2.0 - /// ``` - /// - /// We will still report that we would like to merge `_1` and `_2` in an attempt to allow us to - /// remove that assignment. - c: FxIndexMap<Local, Vec<Local>>, - - /// A reverse index of the `c` set; if the `c` set contains `a => Place { local: b, proj }`, - /// then this contains `b => a`. - // PERF: Possibly these should be `SmallVec`s? - reverse: FxIndexMap<Local, Vec<Local>>, -} - ////////////////////////////////////////////////////////// // Merging // @@ -278,16 +235,16 @@ struct Candidates { fn apply_merges<'tcx>( body: &mut Body<'tcx>, tcx: TyCtxt<'tcx>, - merges: FxIndexMap<Local, Local>, + relevant: RelevantLocals, merged_locals: DenseBitSet<Local>, ) { - let mut merger = Merger { tcx, merges, merged_locals }; + let mut merger = Merger { tcx, relevant, merged_locals }; merger.visit_body_preserves_cfg(body); } struct Merger<'tcx> { tcx: TyCtxt<'tcx>, - merges: FxIndexMap<Local, Local>, + relevant: RelevantLocals, merged_locals: DenseBitSet<Local>, } @@ -297,8 +254,8 @@ impl<'tcx> MutVisitor<'tcx> for Merger<'tcx> { } fn visit_local(&mut self, local: &mut Local, _: PlaceContext, _location: Location) { - if let Some(dest) = self.merges.get(local) { - *local = *dest; + if let Some(relevant) = self.relevant.find(*local) { + *local = self.relevant.original[relevant]; } } @@ -336,376 +293,116 @@ impl<'tcx> MutVisitor<'tcx> for Merger<'tcx> { } ////////////////////////////////////////////////////////// -// Liveness filtering +// Relevant locals // -// This section enforces bullet point 2 +// Small utility to reduce size of the conflict matrix by only considering locals that appear in +// the candidates -struct FilterInformation<'a, 'tcx> { - body: &'a Body<'tcx>, - points: &'a DenseLocationMap, - live: &'a SparseIntervalMatrix<Local, PointIndex>, - candidates: &'a mut Candidates, - write_info: &'a mut WriteInfo, - at: Location, +newtype_index! { + /// Represent a subset of locals which appear in candidates. + struct RelevantLocal {} } -// We first implement some utility functions which we will expose removing candidates according to -// different needs. Throughout the liveness filtering, the `candidates` are only ever accessed -// through these methods, and not directly. -impl Candidates { - /// Collects the candidates for merging. - /// - /// This is responsible for enforcing the first and third bullet point. - fn reset_and_find<'tcx>(&mut self, body: &Body<'tcx>, borrowed: &DenseBitSet<Local>) { - self.c.clear(); - self.reverse.clear(); - let mut visitor = FindAssignments { body, candidates: &mut self.c, borrowed }; - visitor.visit_body(body); - // Deduplicate candidates. - for (_, cands) in self.c.iter_mut() { - cands.sort(); - cands.dedup(); - } - // Generate the reverse map. - for (src, cands) in self.c.iter() { - for dest in cands.iter().copied() { - self.reverse.entry(dest).or_default().push(*src); - } - } - } - - /// Just `Vec::retain`, but the condition is inverted and we add debugging output - fn vec_filter_candidates( - src: Local, - v: &mut Vec<Local>, - mut f: impl FnMut(Local) -> CandidateFilter, - at: Location, - ) { - v.retain(|dest| { - let remove = f(*dest); - if remove == CandidateFilter::Remove { - trace!("eliminating {:?} => {:?} due to conflict at {:?}", src, dest, at); - } - remove == CandidateFilter::Keep - }); - } - - /// `vec_filter_candidates` but for an `Entry` - fn entry_filter_candidates( - mut entry: IndexOccupiedEntry<'_, Local, Vec<Local>>, - p: Local, - f: impl FnMut(Local) -> CandidateFilter, - at: Location, - ) { - let candidates = entry.get_mut(); - Self::vec_filter_candidates(p, candidates, f, at); - if candidates.len() == 0 { - // FIXME(#120456) - is `swap_remove` correct? - entry.swap_remove(); - } - } - - /// For all candidates `(p, q)` or `(q, p)` removes the candidate if `f(q)` says to do so - fn filter_candidates_by( - &mut self, - p: Local, - mut f: impl FnMut(Local) -> CandidateFilter, - at: Location, - ) { - // Cover the cases where `p` appears as a `src` - if let IndexEntry::Occupied(entry) = self.c.entry(p) { - Self::entry_filter_candidates(entry, p, &mut f, at); - } - // And the cases where `p` appears as a `dest` - let Some(srcs) = self.reverse.get_mut(&p) else { - return; - }; - // We use `retain` here to remove the elements from the reverse set if we've removed the - // matching candidate in the forward set. - srcs.retain(|src| { - if f(*src) == CandidateFilter::Keep { - return true; - } - let IndexEntry::Occupied(entry) = self.c.entry(*src) else { - return false; - }; - Self::entry_filter_candidates( - entry, - *src, - |dest| { - if dest == p { CandidateFilter::Remove } else { CandidateFilter::Keep } - }, - at, - ); - false - }); - } +#[derive(Debug)] +struct RelevantLocals { + original: IndexVec<RelevantLocal, Local>, + shrink: IndexVec<Local, Option<RelevantLocal>>, + renames: IndexVec<RelevantLocal, RelevantLocal>, } -#[derive(Copy, Clone, PartialEq, Eq)] -enum CandidateFilter { - Keep, - Remove, -} +impl RelevantLocals { + #[tracing::instrument(level = "trace", skip(candidates, num_locals), ret)] + fn compute(candidates: &Candidates, num_locals: usize) -> RelevantLocals { + let mut original = IndexVec::with_capacity(candidates.c.len()); + let mut shrink = IndexVec::from_elem_n(None, num_locals); -impl<'a, 'tcx> FilterInformation<'a, 'tcx> { - /// Filters the set of candidates to remove those that conflict. - /// - /// The steps we take are exactly those that are outlined at the top of the file. For each - /// statement/terminator, we collect the set of locals that are written to in that - /// statement/terminator, and then we remove all pairs of candidates that contain one such local - /// and another one that is live. - /// - /// We need to be careful about the ordering of operations within each statement/terminator - /// here. Many statements might write and read from more than one place, and we need to consider - /// them all. The strategy for doing this is as follows: We first gather all the places that are - /// written to within the statement/terminator via `WriteInfo`. Then, we use the liveness - /// analysis from *before* the statement/terminator (in the control flow sense) to eliminate - /// candidates - this is because we want to conservatively treat a pair of locals that is both - /// read and written in the statement/terminator to be conflicting, and the liveness analysis - /// before the statement/terminator will correctly report locals that are read in the - /// statement/terminator to be live. We are additionally conservative by treating all written to - /// locals as also being read from. - fn filter_liveness( - candidates: &mut Candidates, - points: &DenseLocationMap, - live: &SparseIntervalMatrix<Local, PointIndex>, - write_info: &mut WriteInfo, - body: &Body<'tcx>, - ) { - let mut this = FilterInformation { - body, - points, - live, - candidates, - // We don't actually store anything at this scope, we just keep things here to be able - // to reuse the allocation. - write_info, - // Doesn't matter what we put here, will be overwritten before being used - at: Location::START, + // Mark a local as relevant and record it into the maps. + let mut declare = |local| { + shrink.get_or_insert_with(local, || original.push(local)); }; - this.internal_filter_liveness(); - } - - fn internal_filter_liveness(&mut self) { - for (block, data) in traversal::preorder(self.body) { - self.at = Location { block, statement_index: data.statements.len() }; - self.write_info.for_terminator(&data.terminator().kind); - self.apply_conflicts(); - for (i, statement) in data.statements.iter().enumerate().rev() { - self.at = Location { block, statement_index: i }; - self.write_info.for_statement(&statement.kind, self.body); - self.apply_conflicts(); + for (&src, destinations) in candidates.c.iter() { + declare(src); + for &dest in destinations { + declare(dest) } } - } - fn apply_conflicts(&mut self) { - let writes = &self.write_info.writes; - for p in writes { - let other_skip = self.write_info.skip_pair.and_then(|(a, b)| { - if a == *p { - Some(b) - } else if b == *p { - Some(a) - } else { - None - } - }); - let at = self.points.point_from_location(self.at); - self.candidates.filter_candidates_by( - *p, - |q| { - if Some(q) == other_skip { - return CandidateFilter::Keep; - } - // It is possible that a local may be live for less than the - // duration of a statement This happens in the case of function - // calls or inline asm. Because of this, we also mark locals as - // conflicting when both of them are written to in the same - // statement. - if self.live.contains(q, at) || writes.contains(&q) { - CandidateFilter::Remove - } else { - CandidateFilter::Keep - } - }, - self.at, - ); - } + let renames = IndexVec::from_fn_n(|l| l, original.len()); + RelevantLocals { original, shrink, renames } } -} -/// Describes where a statement/terminator writes to -#[derive(Default, Debug)] -struct WriteInfo { - writes: Vec<Local>, - /// If this pair of locals is a candidate pair, completely skip processing it during this - /// statement. All other candidates are unaffected. - skip_pair: Option<(Local, Local)>, -} - -impl WriteInfo { - fn for_statement<'tcx>(&mut self, statement: &StatementKind<'tcx>, body: &Body<'tcx>) { - self.reset(); - match statement { - StatementKind::Assign(box (lhs, rhs)) => { - self.add_place(*lhs); - match rhs { - Rvalue::Use(op) => { - self.add_operand(op); - self.consider_skipping_for_assign_use(*lhs, op, body); - } - Rvalue::Repeat(op, _) => { - self.add_operand(op); - } - Rvalue::Cast(_, op, _) - | Rvalue::UnaryOp(_, op) - | Rvalue::ShallowInitBox(op, _) => { - self.add_operand(op); - } - Rvalue::BinaryOp(_, ops) => { - for op in [&ops.0, &ops.1] { - self.add_operand(op); - } - } - Rvalue::Aggregate(_, ops) => { - for op in ops { - self.add_operand(op); - } - } - Rvalue::WrapUnsafeBinder(op, _) => { - self.add_operand(op); - } - Rvalue::ThreadLocalRef(_) - | Rvalue::NullaryOp(_, _) - | Rvalue::Ref(_, _, _) - | Rvalue::RawPtr(_, _) - | Rvalue::Len(_) - | Rvalue::Discriminant(_) - | Rvalue::CopyForDeref(_) => {} - } - } - // Retags are technically also reads, but reporting them as a write suffices - StatementKind::SetDiscriminant { place, .. } - | StatementKind::Deinit(place) - | StatementKind::Retag(_, place) => { - self.add_place(**place); - } - StatementKind::Intrinsic(_) - | StatementKind::ConstEvalCounter - | StatementKind::Nop - | StatementKind::Coverage(_) - | StatementKind::StorageLive(_) - | StatementKind::StorageDead(_) - | StatementKind::BackwardIncompatibleDropHint { .. } - | StatementKind::PlaceMention(_) => {} - StatementKind::FakeRead(_) | StatementKind::AscribeUserType(_, _) => { - bug!("{:?} not found in this MIR phase", statement) - } + fn find(&self, src: Local) -> Option<RelevantLocal> { + let mut src = self.shrink[src]?; + while let s2 = self.renames[src] + && src != s2 + { + src = s2 } + Some(src) } - fn consider_skipping_for_assign_use<'tcx>( - &mut self, - lhs: Place<'tcx>, - rhs: &Operand<'tcx>, - body: &Body<'tcx>, - ) { - let Some(rhs) = rhs.place() else { return }; - if let Some(pair) = places_to_candidate_pair(lhs, rhs, body) { - self.skip_pair = Some(pair); - } + fn union(&mut self, lhs: RelevantLocal, rhs: RelevantLocal) { + self.renames[lhs] = rhs; } - fn for_terminator<'tcx>(&mut self, terminator: &TerminatorKind<'tcx>) { - self.reset(); - match terminator { - TerminatorKind::SwitchInt { discr: op, .. } - | TerminatorKind::Assert { cond: op, .. } => { - self.add_operand(op); - } - TerminatorKind::Call { destination, func, args, .. } => { - self.add_place(*destination); - self.add_operand(func); - for arg in args { - self.add_operand(&arg.node); - } - } - TerminatorKind::TailCall { func, args, .. } => { - self.add_operand(func); - for arg in args { - self.add_operand(&arg.node); - } - } - TerminatorKind::InlineAsm { operands, .. } => { - for asm_operand in operands { - match asm_operand { - InlineAsmOperand::In { value, .. } => { - self.add_operand(value); - } - InlineAsmOperand::Out { place, .. } => { - if let Some(place) = place { - self.add_place(*place); - } - } - // Note that the `late` field in `InOut` is about whether the registers used - // for these things overlap, and is of absolutely no interest to us. - InlineAsmOperand::InOut { in_value, out_place, .. } => { - if let Some(place) = out_place { - self.add_place(*place); - } - self.add_operand(in_value); - } - InlineAsmOperand::Const { .. } - | InlineAsmOperand::SymFn { .. } - | InlineAsmOperand::SymStatic { .. } - | InlineAsmOperand::Label { .. } => {} - } - } - } - TerminatorKind::Goto { .. } - | TerminatorKind::UnwindResume - | TerminatorKind::UnwindTerminate(_) - | TerminatorKind::Return - | TerminatorKind::Unreachable { .. } => (), - TerminatorKind::Drop { .. } => { - // `Drop`s create a `&mut` and so are not considered - } - TerminatorKind::Yield { .. } - | TerminatorKind::CoroutineDrop - | TerminatorKind::FalseEdge { .. } - | TerminatorKind::FalseUnwind { .. } => { - bug!("{:?} not found in this MIR phase", terminator) + fn make_idempotent(&mut self) { + for l in self.renames.indices() { + let mut h = self.renames[l]; + while let h2 = self.renames[h] + && h != h2 + { + h = h2 } + self.renames[l] = h; + debug_assert_eq!(h, self.renames[h], "non-idempotent for {l:?}"); } } +} - fn add_place(&mut self, place: Place<'_>) { - self.writes.push(place.local); - } +///////////////////////////////////////////////////// +// Candidate accumulation + +#[derive(Debug, Default)] +struct Candidates { + /// The set of candidates we are considering in this optimization. + /// + /// We will always merge the key into at most one of its values. + /// + /// Whether a place ends up in the key or the value does not correspond to whether it appears as + /// the lhs or rhs of any assignment. As a matter of fact, the places in here might never appear + /// in an assignment at all. This happens because if we see an assignment like this: + /// + /// ```ignore (syntax-highlighting-only) + /// _1.0 = _2.0 + /// ``` + /// + /// We will still report that we would like to merge `_1` and `_2` in an attempt to allow us to + /// remove that assignment. + c: FxIndexMap<Local, Vec<Local>>, +} + +// We first implement some utility functions which we will expose removing candidates according to +// different needs. Throughout the liveness filtering, the `candidates` are only ever accessed +// through these methods, and not directly. +impl Candidates { + /// Collects the candidates for merging. + /// + /// This is responsible for enforcing the first and third bullet point. + fn find(body: &Body<'_>, borrowed: &DenseBitSet<Local>) -> Candidates { + let mut visitor = FindAssignments { body, candidates: Default::default(), borrowed }; + visitor.visit_body(body); - fn add_operand<'tcx>(&mut self, op: &Operand<'tcx>) { - match op { - // FIXME(JakobDegen): In a previous version, the `Move` case was incorrectly treated as - // being a read only. This was unsound, however we cannot add a regression test because - // it is not possible to set this off with current MIR. Once we have that ability, a - // regression test should be added. - Operand::Move(p) => self.add_place(*p), - Operand::Copy(_) | Operand::Constant(_) => (), + // Deduplicate candidates. + for (_, cands) in visitor.candidates.iter_mut() { + cands.sort(); + cands.dedup(); } - } - fn reset(&mut self) { - self.writes.clear(); - self.skip_pair = None; + Candidates { c: visitor.candidates } } } -///////////////////////////////////////////////////// -// Candidate accumulation - /// If the pair of places is being considered for merging, returns the candidate which would be /// merged in order to accomplish this. /// @@ -743,7 +440,7 @@ fn places_to_candidate_pair<'tcx>( struct FindAssignments<'a, 'tcx> { body: &'a Body<'tcx>, - candidates: &'a mut FxIndexMap<Local, Vec<Local>>, + candidates: FxIndexMap<Local, Vec<Local>>, borrowed: &'a DenseBitSet<Local>, } @@ -803,22 +500,252 @@ fn dest_prop_mir_dump<'tcx>( tcx: TyCtxt<'tcx>, body: &Body<'tcx>, points: &DenseLocationMap, - live: &SparseIntervalMatrix<Local, PointIndex>, - round: usize, + live: &SparseIntervalMatrix<RelevantLocal, TwoStepIndex>, + relevant: &RelevantLocals, ) { - let locals_live_at = |location| { + let locals_live_at = |location, effect| { let location = points.point_from_location(location); - live.rows().filter(|&r| live.contains(r, location)).collect::<Vec<_>>() + let location = TwoStepIndex::new(location, effect); + live.rows() + .filter(|&r| live.contains(r, location)) + .map(|rl| relevant.original[rl]) + .collect::<Vec<_>>() }; if let Some(dumper) = MirDumper::new(tcx, "DestinationPropagation-dataflow", body) { let extra_data = &|pass_where, w: &mut dyn std::io::Write| { if let PassWhere::BeforeLocation(loc) = pass_where { - writeln!(w, " // live: {:?}", locals_live_at(loc))?; + writeln!(w, " // before: {:?}", locals_live_at(loc, Effect::Before))?; + } + if let PassWhere::AfterLocation(loc) = pass_where { + writeln!(w, " // after: {:?}", locals_live_at(loc, Effect::After))?; } Ok(()) }; - dumper.set_disambiguator(&round).set_extra_data(extra_data).dump_mir(body) + dumper.set_extra_data(extra_data).dump_mir(body) + } +} + +struct MaybeTwoStepLiveLocals; + +#[derive(Copy, Clone, Debug)] +enum Effect { + Before, + After, +} + +rustc_index::newtype_index! { + /// A `PointIndex` but with the lower bit encoding early/late inside the statement. + #[orderable] + #[debug_format = "TwoStepIndex({})"] + struct TwoStepIndex {} +} + +impl TwoStepIndex { + fn new(point: PointIndex, effect: Effect) -> TwoStepIndex { + let effect = match effect { + Effect::Before => 0, + Effect::After => 1, + }; + TwoStepIndex::from_u32(2 * point.as_u32() + (effect as u32)) + } +} + +struct VisitPlacesWith<F>(F); + +impl<'tcx, F> Visitor<'tcx> for VisitPlacesWith<F> +where + F: FnMut(Place<'tcx>, PlaceContext), +{ + fn visit_place(&mut self, place: &Place<'tcx>, ctxt: PlaceContext, _: Location) { + (self.0)(*place, ctxt); + for proj in place.projection.iter() { + match proj { + ProjectionElem::Index(index) => (self.0)( + index.into(), + PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy), + ), + ProjectionElem::Deref + | ProjectionElem::Field(..) + | ProjectionElem::Downcast(..) + | ProjectionElem::ConstantIndex { .. } + | ProjectionElem::Subslice { .. } + | ProjectionElem::OpaqueCast(..) + | ProjectionElem::UnwrapUnsafeBinder(..) + | ProjectionElem::Subtype(..) => {} + }; + } + } +} + +impl<'tcx> Analysis<'tcx> for MaybeTwoStepLiveLocals { + type Domain = DenseBitSet<Local>; + type Direction = Backward; + + const NAME: &'static str = "transitive liveness"; + + fn bottom_value(&self, body: &Body<'tcx>) -> DenseBitSet<Local> { + // bottom = not live + DenseBitSet::new_empty(body.local_decls.len()) + } + + fn initialize_start_block(&self, _: &Body<'tcx>, _: &mut DenseBitSet<Local>) { + // No variables are live until we observe a use + } + + // This happens between the previous statement and this one. + #[tracing::instrument(level = "trace", skip(self, statement))] + fn apply_primary_statement_effect( + &mut self, + state: &mut DenseBitSet<Local>, + statement: &Statement<'tcx>, + location: Location, + ) { + VisitPlacesWith(|place, ctxt| match DefUse::for_place(place, ctxt) { + DefUse::Def => { + state.remove(place.local); + } + DefUse::Use => { + state.insert(place.local); + } + DefUse::PartialWrite | DefUse::NonUse => {} + }) + .visit_statement(statement, location); + } + + // This happens between this statement and the next one. + #[tracing::instrument(level = "trace", skip(self, statement))] + fn apply_early_statement_effect( + &mut self, + state: &mut DenseBitSet<Local>, + statement: &Statement<'tcx>, + location: Location, + ) { + // We need to ensure we have a non-zero live range even for dead stores. This is done by + // marking all the writes locals as live in the second half of the statement. + VisitPlacesWith(|place: Place<'tcx>, ctxt| match DefUse::for_place(place, ctxt) { + DefUse::Def | DefUse::PartialWrite => { + state.insert(place.local); + } + // We already perform the reads in the first part of the statement. As statements are + // not splittable, we do not need to re-read the same values. + DefUse::Use | DefUse::NonUse => {} + }) + .visit_statement(statement, location); + } + + // We model terminator as a special case in this two-step analysis. Consider the terminator + // `destination = func(arg0...)`. + // + // -- state at (location, Effect::Before) + // read(arg0)... + // write(destination) + // -- state at (location, Effect::After) + // read(arg0)... + + // This happens between the last statement and the terminator. + #[tracing::instrument(level = "trace", skip(self, terminator))] + fn apply_primary_terminator_effect<'mir>( + &mut self, + state: &mut DenseBitSet<Local>, + terminator: &'mir Terminator<'tcx>, + location: Location, + ) -> TerminatorEdges<'mir, 'tcx> { + // Consider that all writes in this terminator happen at the start of the execution of the + // terminator. For instance if we pass a return-pointer to a `Call` terminator. + VisitPlacesWith(|place: Place<'tcx>, ctxt| match DefUse::for_place(place, ctxt) { + DefUse::Def => { + state.remove(place.local); + } + DefUse::Use => { + state.insert(place.local); + } + DefUse::PartialWrite | DefUse::NonUse => {} + }) + .visit_terminator(terminator, location); + terminator.edges() + } + + // This happens between the terminator and the end of the block. + #[tracing::instrument(level = "trace", skip(self, terminator))] + fn apply_early_terminator_effect<'mir>( + &mut self, + state: &mut DenseBitSet<Local>, + terminator: &'mir Terminator<'tcx>, + location: Location, + ) { + // Consider that all reads in this terminator happen at the end of the execution of the + // terminator, even after it may have written to the destination local. For instance if we + // pass arguments as pointers to a `Call` terminator. + VisitPlacesWith(|place: Place<'tcx>, ctxt| match DefUse::for_place(place, ctxt) { + DefUse::Def | DefUse::Use | DefUse::PartialWrite => { + state.insert(place.local); + } + DefUse::NonUse => {} + }) + .visit_terminator(terminator, location); } } + +/// Add points depending on the result of the given dataflow analysis. +fn save_as_intervals<'tcx>( + elements: &DenseLocationMap, + body: &Body<'tcx>, + relevant: &IndexSlice<RelevantLocal, Local>, + results: Results<DenseBitSet<Local>>, +) -> SparseIntervalMatrix<RelevantLocal, TwoStepIndex> { + let mut values = SparseIntervalMatrix::new(2 * elements.num_points()); + let mut state = MaybeTwoStepLiveLocals.bottom_value(body); + let reachable_blocks = traversal::reachable_as_bitset(body); + + let two_step_loc = |location, effect| { + let point = elements.point_from_location(location); + TwoStepIndex::new(point, effect) + }; + let mut prepend_at = |state: &mut DenseBitSet<Local>, twostep| { + for (relevant, &original) in relevant.iter_enumerated() { + if state.contains(original) { + values.prepend(relevant, twostep); + } + } + }; + + // Iterate blocks in decreasing order, to visit locations in decreasing order. This + // allows to use the more efficient `prepend` method to interval sets. + for block in body.basic_blocks.indices().rev() { + if !reachable_blocks.contains(block) { + continue; + } + + state.clone_from(&results[block]); + + let block_data = &body.basic_blocks[block]; + let loc = Location { block, statement_index: block_data.statements.len() }; + + let term = block_data.terminator(); + let mut twostep = two_step_loc(loc, Effect::After); + MaybeTwoStepLiveLocals.apply_early_terminator_effect(&mut state, term, loc); + prepend_at(&mut state, twostep); + + twostep = TwoStepIndex::from_u32(twostep.as_u32() - 1); + debug_assert_eq!(twostep, two_step_loc(loc, Effect::Before)); + MaybeTwoStepLiveLocals.apply_primary_terminator_effect(&mut state, term, loc); + prepend_at(&mut state, twostep); + + for (statement_index, stmt) in block_data.statements.iter().enumerate().rev() { + let loc = Location { block, statement_index }; + twostep = TwoStepIndex::from_u32(twostep.as_u32() - 1); + debug_assert_eq!(twostep, two_step_loc(loc, Effect::After)); + MaybeTwoStepLiveLocals.apply_early_statement_effect(&mut state, stmt, loc); + prepend_at(&mut state, twostep); + + twostep = TwoStepIndex::from_u32(twostep.as_u32() - 1); + debug_assert_eq!(twostep, two_step_loc(loc, Effect::Before)); + MaybeTwoStepLiveLocals.apply_primary_statement_effect(&mut state, stmt, loc); + prepend_at(&mut state, twostep); + } + } + + values +} |