rustc_mir_transform/
dest_prop.rs

1//! Propagates assignment destinations backwards in the CFG to eliminate redundant assignments.
2//!
3//! # Motivation
4//!
5//! MIR building can insert a lot of redundant copies, and Rust code in general often tends to move
6//! values around a lot. The result is a lot of assignments of the form `dest = {move} src;` in MIR.
7//! MIR building for constants in particular tends to create additional locals that are only used
8//! inside a single block to shuffle a value around unnecessarily.
9//!
10//! LLVM by itself is not good enough at eliminating these redundant copies (eg. see
11//! <https://github.com/rust-lang/rust/issues/32966>), so this leaves some performance on the table
12//! that we can regain by implementing an optimization for removing these assign statements in rustc
13//! itself. When this optimization runs fast enough, it can also speed up the constant evaluation
14//! and code generation phases of rustc due to the reduced number of statements and locals.
15//!
16//! # The Optimization
17//!
18//! Conceptually, this optimization is "destination propagation". It is similar to the Named Return
19//! Value Optimization, or NRVO, known from the C++ world, except that it isn't limited to return
20//! values or the return place `_0`. On a very high level, independent of the actual implementation
21//! details, it does the following:
22//!
23//! 1) Identify `dest = src;` statements with values for `dest` and `src` whose storage can soundly
24//!    be merged.
25//! 2) Replace all mentions of `src` with `dest` ("unifying" them and propagating the destination
26//!    backwards).
27//! 3) Delete the `dest = src;` statement (by making it a `nop`).
28//!
29//! Step 1) is by far the hardest, so it is explained in more detail below.
30//!
31//! ## Soundness
32//!
33//! We have a pair of places `p` and `q`, whose memory we would like to merge. In order for this to
34//! be sound, we need to check a number of conditions:
35//!
36//! * `p` and `q` must both be *constant* - it does not make much sense to talk about merging them
37//!   if they do not consistently refer to the same place in memory. This is satisfied if they do
38//!   not contain any indirection through a pointer or any indexing projections.
39//!
40//! * `p` and `q` must have the **same type**. If we replace a local with a subtype or supertype,
41//!   we may end up with a different vtable for that local. See the `subtyping-impacts-selection`
42//!   tests for an example where that causes issues.
43//!
44//! * We need to make sure that the goal of "merging the memory" is actually structurally possible
45//!   in MIR. For example, even if all the other conditions are satisfied, there is no way to
46//!   "merge" `_5.foo` and `_6.bar`. For now, we ensure this by requiring that both `p` and `q` are
47//!   locals with no further projections. Future iterations of this pass should improve on this.
48//!
49//! * Finally, we want `p` and `q` to use the same memory - however, we still need to make sure that
50//!   each of them has enough "ownership" of that memory to continue "doing its job." More
51//!   precisely, what we will check is that whenever the program performs a write to `p`, then it
52//!   does not currently care about what the value in `q` is (and vice versa). We formalize the
53//!   notion of "does not care what the value in `q` is" by checking the *liveness* of `q`.
54//!
55//!   Because of the difficulty of computing liveness of places that have their address taken, we do
56//!   not even attempt to do it. Any places that are in a local that has its address taken is
57//!   excluded from the optimization.
58//!
59//! The first two conditions are simple structural requirements on the `Assign` statements that can
60//! be trivially checked. The third requirement however is more difficult and costly to check.
61//!
62//! ## Current implementation
63//!
64//! The current implementation relies on live range computation to check for conflicts. We only
65//! allow to merge locals that have disjoint live ranges. The live range are defined with
66//! half-statement granularity, so as to make all writes be live for at least a half statement.
67//!
68//! ## Future Improvements
69//!
70//! There are a number of ways in which this pass could be improved in the future:
71//!
72//! * Merging storage liveness ranges instead of removing storage statements completely. This may
73//!   improve stack usage.
74//!
75//! * Allow merging locals into places with projections, eg `_5` into `_6.foo`.
76//!
77//! * Liveness analysis with more precision than whole locals at a time. The smaller benefit of this
78//!   is that it would allow us to dest prop at "sub-local" levels in some cases. The bigger benefit
79//!   of this is that such liveness analysis can report more accurate results about whole locals at
80//!   a time. For example, consider:
81//!
82//!   ```ignore (syntax-highlighting-only)
83//!   _1 = u;
84//!   // unrelated code
85//!   _1.f1 = v;
86//!   _2 = _1.f1;
87//!   ```
88//!
89//!   Because the current analysis only thinks in terms of locals, it does not have enough
90//!   information to report that `_1` is dead in the "unrelated code" section.
91//!
92//! * Liveness analysis enabled by alias analysis. This would allow us to not just bail on locals
93//!   that ever have their address taken. Of course that requires actually having alias analysis
94//!   (and a model to build it on), so this might be a bit of a ways off.
95//!
96//! * Various perf improvements. There are a bunch of comments in here marked `PERF` with ideas for
97//!   how to do things more efficiently. However, the complexity of the pass as a whole should be
98//!   kept in mind.
99//!
100//! ## Previous Work
101//!
102//! A [previous attempt][attempt 1] at implementing an optimization like this turned out to be a
103//! significant regression in compiler performance. Fixing the regressions introduced a lot of
104//! undesirable complexity to the implementation.
105//!
106//! A [subsequent approach][attempt 2] tried to avoid the costly computation by limiting itself to
107//! acyclic CFGs, but still turned out to be far too costly to run due to suboptimal performance
108//! within individual basic blocks, requiring a walk across the entire block for every assignment
109//! found within the block. For the `tuple-stress` benchmark, which has 458745 statements in a
110//! single block, this proved to be far too costly.
111//!
112//! [Another approach after that][attempt 3] was much closer to correct, but had some soundness
113//! issues - it was failing to consider stores outside live ranges, and failed to uphold some of the
114//! requirements that MIR has for non-overlapping places within statements. However, it also had
115//! performance issues caused by `O(l² * s)` runtime, where `l` is the number of locals and `s` is
116//! the number of statements and terminators.
117//!
118//! Since the first attempt at this, the compiler has improved dramatically, and new analysis
119//! frameworks have been added that should make this approach viable without requiring a limited
120//! approach that only works for some classes of CFGs:
121//! - rustc now has a powerful dataflow analysis framework that can handle forwards and backwards
122//!   analyses efficiently.
123//! - Layout optimizations for coroutines have been added to improve code generation for
124//!   async/await, which are very similar in spirit to what this optimization does.
125//!
126//! [The next approach][attempt 4] computes a conflict matrix between locals by forbidding merging
127//! locals with competing writes or with one write while the other is live.
128//!
129//! ## Pre/Post Optimization
130//!
131//! It is recommended to run `SimplifyCfg` and then `SimplifyLocals` some time after this pass, as
132//! it replaces the eliminated assign statements with `nop`s and leaves unused locals behind.
133//!
134//! [liveness]: https://en.wikipedia.org/wiki/Live_variable_analysis
135//! [attempt 1]: https://github.com/rust-lang/rust/pull/47954
136//! [attempt 2]: https://github.com/rust-lang/rust/pull/71003
137//! [attempt 3]: https://github.com/rust-lang/rust/pull/72632
138//! [attempt 4]: https://github.com/rust-lang/rust/pull/96451
139
140use rustc_data_structures::union_find::UnionFind;
141use rustc_index::bit_set::DenseBitSet;
142use rustc_index::interval::SparseIntervalMatrix;
143use rustc_index::{IndexVec, newtype_index};
144use rustc_middle::mir::visit::{MutVisitor, PlaceContext, VisitPlacesWith, Visitor};
145use rustc_middle::mir::*;
146use rustc_middle::ty::TyCtxt;
147use rustc_mir_dataflow::impls::{DefUse, MaybeLiveLocals};
148use rustc_mir_dataflow::points::DenseLocationMap;
149use rustc_mir_dataflow::{Analysis, Results};
150use tracing::{debug, trace};
151
152pub(super) struct DestinationPropagation;
153
154impl<'tcx> crate::MirPass<'tcx> for DestinationPropagation {
155    fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
156        sess.mir_opt_level() >= 2
157    }
158
159    #[tracing::instrument(level = "trace", skip(self, tcx, body))]
160    fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
161        let def_id = body.source.def_id();
162        trace!(?def_id);
163
164        let borrowed = rustc_mir_dataflow::impls::borrowed_locals(body);
165
166        let candidates = Candidates::find(body, &borrowed);
167        trace!(?candidates);
168        if candidates.c.is_empty() {
169            return;
170        }
171
172        let live = MaybeLiveLocals.iterate_to_fixpoint(tcx, body, Some("MaybeLiveLocals-DestProp"));
173
174        let points = DenseLocationMap::new(body);
175        let mut relevant = RelevantLocals::compute(&candidates, body.local_decls.len());
176        let mut live = save_as_intervals(&points, body, &relevant, live.results);
177
178        dest_prop_mir_dump(tcx, body, &points, &live, &relevant);
179
180        let mut merged_locals = DenseBitSet::new_empty(body.local_decls.len());
181
182        for (src, dst) in candidates.c.into_iter() {
183            trace!(?src, ?dst);
184
185            let Some(mut src) = relevant.find(src) else { continue };
186            let Some(mut dst) = relevant.find(dst) else { continue };
187            if src == dst {
188                continue;
189            }
190
191            let Some(src_live_ranges) = live.row(src) else { continue };
192            let Some(dst_live_ranges) = live.row(dst) else { continue };
193            trace!(?src, ?src_live_ranges);
194            trace!(?dst, ?dst_live_ranges);
195
196            if src_live_ranges.disjoint(dst_live_ranges) {
197                // We want to replace `src` by `dst`.
198                let mut orig_src = relevant.original[src];
199                let mut orig_dst = relevant.original[dst];
200
201                // The return place and function arguments are required and cannot be renamed.
202                // This check cannot be made during candidate collection, as we may want to
203                // unify the same non-required local with several required locals.
204                match (is_local_required(orig_src, body), is_local_required(orig_dst, body)) {
205                    // Renaming `src` is ok.
206                    (false, _) => {}
207                    // Renaming `src` is wrong, but renaming `dst` is ok.
208                    (true, false) => {
209                        std::mem::swap(&mut src, &mut dst);
210                        std::mem::swap(&mut orig_src, &mut orig_dst);
211                    }
212                    // Neither local can be renamed, so skip this case.
213                    (true, true) => continue,
214                }
215
216                trace!(?src, ?dst, "merge");
217                merged_locals.insert(orig_src);
218                merged_locals.insert(orig_dst);
219
220                // Replace `src` by `dst`.
221                let head = relevant.union(src, dst);
222                live.union_rows(/* read */ src, /* write */ head);
223                live.union_rows(/* read */ dst, /* write */ head);
224            }
225        }
226        trace!(?merged_locals);
227        trace!(?relevant.renames);
228
229        if merged_locals.is_empty() {
230            return;
231        }
232
233        apply_merges(body, tcx, relevant, merged_locals);
234    }
235
236    fn is_required(&self) -> bool {
237        false
238    }
239}
240
241//////////////////////////////////////////////////////////
242// Merging
243//
244// Applies the actual optimization
245
246fn apply_merges<'tcx>(
247    body: &mut Body<'tcx>,
248    tcx: TyCtxt<'tcx>,
249    relevant: RelevantLocals,
250    merged_locals: DenseBitSet<Local>,
251) {
252    let mut merger = Merger { tcx, relevant, merged_locals };
253    merger.visit_body_preserves_cfg(body);
254}
255
256struct Merger<'tcx> {
257    tcx: TyCtxt<'tcx>,
258    relevant: RelevantLocals,
259    merged_locals: DenseBitSet<Local>,
260}
261
262impl<'tcx> MutVisitor<'tcx> for Merger<'tcx> {
263    fn tcx(&self) -> TyCtxt<'tcx> {
264        self.tcx
265    }
266
267    fn visit_local(&mut self, local: &mut Local, _: PlaceContext, _location: Location) {
268        if let Some(relevant) = self.relevant.find(*local) {
269            *local = self.relevant.original[relevant];
270        }
271    }
272
273    fn visit_statement(&mut self, statement: &mut Statement<'tcx>, location: Location) {
274        match &statement.kind {
275            // FIXME: Don't delete storage statements, but "merge" the storage ranges instead.
276            StatementKind::StorageDead(local) | StatementKind::StorageLive(local)
277                if self.merged_locals.contains(*local) =>
278            {
279                statement.make_nop();
280                return;
281            }
282            _ => (),
283        };
284        self.super_statement(statement, location);
285        match &statement.kind {
286            StatementKind::Assign(box (dest, rvalue)) => {
287                match rvalue {
288                    Rvalue::CopyForDeref(place)
289                    | Rvalue::Use(Operand::Copy(place) | Operand::Move(place)) => {
290                        // These might've been turned into self-assignments by the replacement
291                        // (this includes the original statement we wanted to eliminate).
292                        if dest == place {
293                            debug!("{:?} turned into self-assignment, deleting", location);
294                            statement.make_nop();
295                        }
296                    }
297                    _ => {}
298                }
299            }
300
301            _ => {}
302        }
303    }
304}
305
306//////////////////////////////////////////////////////////
307// Relevant locals
308//
309// Small utility to reduce size of the conflict matrix by only considering locals that appear in
310// the candidates
311
312newtype_index! {
313    /// Represent a subset of locals which appear in candidates.
314    struct RelevantLocal {}
315}
316
317#[derive(Debug)]
318struct RelevantLocals {
319    original: IndexVec<RelevantLocal, Local>,
320    shrink: IndexVec<Local, Option<RelevantLocal>>,
321    renames: UnionFind<RelevantLocal>,
322}
323
324impl RelevantLocals {
325    #[tracing::instrument(level = "trace", skip(candidates, num_locals), ret)]
326    fn compute(candidates: &Candidates, num_locals: usize) -> RelevantLocals {
327        let mut original = IndexVec::with_capacity(candidates.c.len());
328        let mut shrink = IndexVec::from_elem_n(None, num_locals);
329
330        // Mark a local as relevant and record it into the maps.
331        let mut declare = |local| {
332            shrink.get_or_insert_with(local, || original.push(local));
333        };
334
335        for &(src, dest) in candidates.c.iter() {
336            declare(src);
337            declare(dest)
338        }
339
340        let renames = UnionFind::new(original.len());
341        RelevantLocals { original, shrink, renames }
342    }
343
344    fn find(&mut self, src: Local) -> Option<RelevantLocal> {
345        let src = self.shrink[src]?;
346        let src = self.renames.find(src);
347        Some(src)
348    }
349
350    fn union(&mut self, lhs: RelevantLocal, rhs: RelevantLocal) -> RelevantLocal {
351        let head = self.renames.unify(lhs, rhs);
352        // We need to ensure we keep the original local of the RHS, as it may be a required local.
353        self.original[head] = self.original[rhs];
354        head
355    }
356}
357
358/////////////////////////////////////////////////////
359// Candidate accumulation
360
361#[derive(Debug, Default)]
362struct Candidates {
363    /// The set of candidates we are considering in this optimization.
364    ///
365    /// Whether a place ends up in the key or the value does not correspond to whether it appears as
366    /// the lhs or rhs of any assignment. As a matter of fact, the places in here might never appear
367    /// in an assignment at all. This happens because if we see an assignment like this:
368    ///
369    /// ```ignore (syntax-highlighting-only)
370    /// _1.0 = _2.0
371    /// ```
372    ///
373    /// We will still report that we would like to merge `_1` and `_2` in an attempt to allow us to
374    /// remove that assignment.
375    c: Vec<(Local, Local)>,
376}
377
378// We first implement some utility functions which we will expose removing candidates according to
379// different needs. Throughout the liveness filtering, the `candidates` are only ever accessed
380// through these methods, and not directly.
381impl Candidates {
382    /// Collects the candidates for merging.
383    ///
384    /// This is responsible for enforcing the first and third bullet point.
385    fn find(body: &Body<'_>, borrowed: &DenseBitSet<Local>) -> Candidates {
386        let mut visitor = FindAssignments { body, candidates: Default::default(), borrowed };
387        visitor.visit_body(body);
388
389        Candidates { c: visitor.candidates }
390    }
391}
392
393struct FindAssignments<'a, 'tcx> {
394    body: &'a Body<'tcx>,
395    candidates: Vec<(Local, Local)>,
396    borrowed: &'a DenseBitSet<Local>,
397}
398
399impl<'tcx> Visitor<'tcx> for FindAssignments<'_, 'tcx> {
400    fn visit_statement(&mut self, statement: &Statement<'tcx>, _: Location) {
401        if let StatementKind::Assign(box (
402            lhs,
403            Rvalue::CopyForDeref(rhs) | Rvalue::Use(Operand::Copy(rhs) | Operand::Move(rhs)),
404        )) = &statement.kind
405            && let Some(src) = lhs.as_local()
406            && let Some(dest) = rhs.as_local()
407        {
408            // As described at the top of the file, we do not go near things that have
409            // their address taken.
410            if self.borrowed.contains(src) || self.borrowed.contains(dest) {
411                return;
412            }
413
414            // As described at the top of this file, we do not touch locals which have
415            // different types.
416            let src_ty = self.body.local_decls()[src].ty;
417            let dest_ty = self.body.local_decls()[dest].ty;
418            if src_ty != dest_ty {
419                // FIXME(#112651): This can be removed afterwards. Also update the module description.
420                trace!("skipped `{src:?} = {dest:?}` due to subtyping: {src_ty} != {dest_ty}");
421                return;
422            }
423
424            // We may insert duplicates here, but that's fine
425            self.candidates.push((src, dest));
426        }
427    }
428}
429
430/// Some locals are part of the function's interface and can not be removed.
431///
432/// Note that these locals *can* still be merged with non-required locals by removing that other
433/// local.
434fn is_local_required(local: Local, body: &Body<'_>) -> bool {
435    match body.local_kind(local) {
436        LocalKind::Arg | LocalKind::ReturnPointer => true,
437        LocalKind::Temp => false,
438    }
439}
440
441/////////////////////////////////////////////////////////
442// MIR Dump
443
444fn dest_prop_mir_dump<'tcx>(
445    tcx: TyCtxt<'tcx>,
446    body: &Body<'tcx>,
447    points: &DenseLocationMap,
448    live: &SparseIntervalMatrix<RelevantLocal, TwoStepIndex>,
449    relevant: &RelevantLocals,
450) {
451    let locals_live_at = |location| {
452        live.rows()
453            .filter(|&r| live.contains(r, location))
454            .map(|rl| relevant.original[rl])
455            .collect::<Vec<_>>()
456    };
457
458    if let Some(dumper) = MirDumper::new(tcx, "DestinationPropagation-dataflow", body) {
459        let extra_data = &|pass_where, w: &mut dyn std::io::Write| {
460            if let PassWhere::BeforeLocation(loc) = pass_where {
461                let location = TwoStepIndex::new(points, loc, Effect::Before);
462                let live = locals_live_at(location);
463                writeln!(w, "        // before: {:?} => {:?}", location, live)?;
464            }
465            if let PassWhere::AfterLocation(loc) = pass_where {
466                let location = TwoStepIndex::new(points, loc, Effect::After);
467                let live = locals_live_at(location);
468                writeln!(w, "        // after: {:?} => {:?}", location, live)?;
469            }
470            Ok(())
471        };
472
473        dumper.set_extra_data(extra_data).dump_mir(body)
474    }
475}
476
477#[derive(Copy, Clone, Debug)]
478enum Effect {
479    Before,
480    After,
481}
482
483rustc_index::newtype_index! {
484    /// A reversed `PointIndex` but with the lower bit encoding early/late inside the statement.
485    /// The reversed order allows to use the more efficient `IntervalSet::append` method while we
486    /// iterate on the statements in reverse order.
487    #[orderable]
488    #[debug_format = "TwoStepIndex({})"]
489    struct TwoStepIndex {}
490}
491
492impl TwoStepIndex {
493    fn new(elements: &DenseLocationMap, location: Location, effect: Effect) -> TwoStepIndex {
494        let point = elements.point_from_location(location);
495        let effect = match effect {
496            Effect::Before => 0,
497            Effect::After => 1,
498        };
499        let max_index = 2 * elements.num_points() as u32 - 1;
500        let index = 2 * point.as_u32() + (effect as u32);
501        // Reverse the indexing to use more efficient `IntervalSet::append`.
502        TwoStepIndex::from_u32(max_index - index)
503    }
504}
505
506/// Add points depending on the result of the given dataflow analysis.
507fn save_as_intervals<'tcx>(
508    elements: &DenseLocationMap,
509    body: &Body<'tcx>,
510    relevant: &RelevantLocals,
511    results: Results<DenseBitSet<Local>>,
512) -> SparseIntervalMatrix<RelevantLocal, TwoStepIndex> {
513    let mut values = SparseIntervalMatrix::new(2 * elements.num_points());
514    let mut state = MaybeLiveLocals.bottom_value(body);
515    let reachable_blocks = traversal::reachable_as_bitset(body);
516
517    let two_step_loc = |location, effect| TwoStepIndex::new(elements, location, effect);
518    let append_at =
519        |values: &mut SparseIntervalMatrix<_, _>, state: &DenseBitSet<Local>, twostep| {
520            for (relevant, &original) in relevant.original.iter_enumerated() {
521                if state.contains(original) {
522                    values.append(relevant, twostep);
523                }
524            }
525        };
526
527    // Iterate blocks in decreasing order, to visit locations in decreasing order. This
528    // allows to use the more efficient `append` method to interval sets.
529    for block in body.basic_blocks.indices().rev() {
530        if !reachable_blocks.contains(block) {
531            continue;
532        }
533
534        state.clone_from(&results[block]);
535
536        let block_data = &body.basic_blocks[block];
537        let loc = Location { block, statement_index: block_data.statements.len() };
538
539        let term = block_data.terminator();
540        let mut twostep = two_step_loc(loc, Effect::After);
541        append_at(&mut values, &state, twostep);
542        // Ensure we have a non-zero live range even for dead stores. This is done by marking all
543        // the written-to locals as live in the second half of the statement.
544        // We also ensure that operands read by terminators conflict with writes by that terminator.
545        // For instance a function call may read args after having written to the destination.
546        VisitPlacesWith(|place: Place<'tcx>, ctxt| {
547            if let Some(relevant) = relevant.shrink[place.local] {
548                match DefUse::for_place(place, ctxt) {
549                    DefUse::Def | DefUse::Use | DefUse::PartialWrite => {
550                        values.insert(relevant, twostep);
551                    }
552                    DefUse::NonUse => {}
553                }
554            }
555        })
556        .visit_terminator(term, loc);
557
558        twostep = TwoStepIndex::from_u32(twostep.as_u32() + 1);
559        debug_assert_eq!(twostep, two_step_loc(loc, Effect::Before));
560        MaybeLiveLocals.apply_early_terminator_effect(&mut state, term, loc);
561        MaybeLiveLocals.apply_primary_terminator_effect(&mut state, term, loc);
562        append_at(&mut values, &state, twostep);
563
564        for (statement_index, stmt) in block_data.statements.iter().enumerate().rev() {
565            let loc = Location { block, statement_index };
566            twostep = TwoStepIndex::from_u32(twostep.as_u32() + 1);
567            debug_assert_eq!(twostep, two_step_loc(loc, Effect::After));
568            append_at(&mut values, &state, twostep);
569            // Like terminators, ensure we have a non-zero live range even for dead stores.
570            // Some rvalues interleave reads and writes, for instance `Rvalue::Aggregate`, see
571            // https://github.com/rust-lang/rust/issues/146383. By precaution, treat statements
572            // as behaving so by default.
573            // We make an exception for simple assignments `_a.stuff = {copy|move} _b.stuff`,
574            // as marking `_b` live here would prevent unification.
575            let is_simple_assignment = match stmt.kind {
576                StatementKind::Assign(box (
577                    lhs,
578                    Rvalue::CopyForDeref(rhs)
579                    | Rvalue::Use(Operand::Copy(rhs) | Operand::Move(rhs)),
580                )) => lhs.projection == rhs.projection,
581                _ => false,
582            };
583            VisitPlacesWith(|place: Place<'tcx>, ctxt| {
584                if let Some(relevant) = relevant.shrink[place.local] {
585                    match DefUse::for_place(place, ctxt) {
586                        DefUse::Def | DefUse::PartialWrite => {
587                            values.insert(relevant, twostep);
588                        }
589                        DefUse::Use if !is_simple_assignment => {
590                            values.insert(relevant, twostep);
591                        }
592                        DefUse::Use | DefUse::NonUse => {}
593                    }
594                }
595            })
596            .visit_statement(stmt, loc);
597
598            twostep = TwoStepIndex::from_u32(twostep.as_u32() + 1);
599            debug_assert_eq!(twostep, two_step_loc(loc, Effect::Before));
600            MaybeLiveLocals.apply_early_statement_effect(&mut state, stmt, loc);
601            MaybeLiveLocals.apply_primary_statement_effect(&mut state, stmt, loc);
602            // ... but reads from operands are marked as live here so they do not conflict with
603            // the all the writes we manually marked as live in the second half of the statement.
604            append_at(&mut values, &state, twostep);
605        }
606    }
607
608    values
609}