1 //! A number of passes which remove various redundancies in the CFG.
3 //! The `SimplifyCfg` pass gets rid of unnecessary blocks in the CFG, whereas the `SimplifyLocals`
4 //! gets rid of all the unnecessary local variable declarations.
6 //! The `SimplifyLocals` pass is kinda expensive and therefore not very suitable to be run often.
7 //! Most of the passes should not care or be impacted in meaningful ways due to extra locals
8 //! either, so running the pass once, right before codegen, should suffice.
10 //! On the other side of the spectrum, the `SimplifyCfg` pass is considerably cheap to run, thus
11 //! one should run it after every pass which may modify CFG in significant ways. This pass must
12 //! also be run before any analysis passes because it removes dead blocks, and some of these can be
15 //! The cause of this typing issue is typeck allowing most blocks whose end is not reachable have
16 //! an arbitrary return type, rather than having the usual () return type (as a note, typeck's
17 //! notion of reachability is in fact slightly weaker than MIR CFG reachability - see #31617). A
18 //! standard example of the situation is:
22 //! let _a: char = { return; };
26 //! Here the block (`{ return; }`) has the return type `char`, rather than `()`, but the MIR we
27 //! naively generate still contains the `_a = ()` write in the unreachable block "after" the
30 use crate::transform::{MirPass, MirSource};
31 use rustc_index::bit_set::BitSet;
32 use rustc_index::vec::{Idx, IndexVec};
33 use rustc_middle::mir::visit::{MutVisitor, MutatingUseContext, PlaceContext, Visitor};
34 use rustc_middle::mir::*;
35 use rustc_middle::ty::TyCtxt;
36 use smallvec::SmallVec;
39 pub struct SimplifyCfg {
44 pub fn new(label: &str) -> Self {
45 SimplifyCfg { label: format!("SimplifyCfg-{}", label) }
49 pub fn simplify_cfg(body: &mut Body<'_>) {
50 CfgSimplifier::new(body).simplify();
51 remove_dead_blocks(body);
53 // FIXME: Should probably be moved into some kind of pass manager
54 body.basic_blocks_mut().raw.shrink_to_fit();
57 impl<'tcx> MirPass<'tcx> for SimplifyCfg {
58 fn name(&self) -> Cow<'_, str> {
59 Cow::Borrowed(&self.label)
62 fn run_pass(&self, _tcx: TyCtxt<'tcx>, _src: MirSource<'tcx>, body: &mut Body<'tcx>) {
63 debug!("SimplifyCfg({:?}) - simplifying {:?}", self.label, body);
68 pub struct CfgSimplifier<'a, 'tcx> {
69 basic_blocks: &'a mut IndexVec<BasicBlock, BasicBlockData<'tcx>>,
70 pred_count: IndexVec<BasicBlock, u32>,
73 impl<'a, 'tcx> CfgSimplifier<'a, 'tcx> {
74 pub fn new(body: &'a mut Body<'tcx>) -> Self {
75 let mut pred_count = IndexVec::from_elem(0u32, body.basic_blocks());
77 // we can't use mir.predecessors() here because that counts
78 // dead blocks, which we don't want to.
79 pred_count[START_BLOCK] = 1;
81 for (_, data) in traversal::preorder(body) {
82 if let Some(ref term) = data.terminator {
83 for &tgt in term.successors() {
89 let basic_blocks = body.basic_blocks_mut();
91 CfgSimplifier { basic_blocks, pred_count }
94 pub fn simplify(mut self) {
97 let mut start = START_BLOCK;
99 // Vec of the blocks that should be merged. We store the indices here, instead of the
100 // statements itself to avoid moving the (relatively) large statements twice.
101 // We do not push the statements directly into the target block (`bb`) as that is slower
102 // due to additional reallocations
103 let mut merged_blocks = Vec::new();
105 let mut changed = false;
107 self.collapse_goto_chain(&mut start, &mut changed);
109 for bb in self.basic_blocks.indices() {
110 if self.pred_count[bb] == 0 {
114 debug!("simplifying {:?}", bb);
117 self.basic_blocks[bb].terminator.take().expect("invalid terminator state");
119 for successor in terminator.successors_mut() {
120 self.collapse_goto_chain(successor, &mut changed);
123 let mut inner_changed = true;
124 merged_blocks.clear();
125 while inner_changed {
126 inner_changed = false;
127 inner_changed |= self.simplify_branch(&mut terminator);
128 inner_changed |= self.merge_successor(&mut merged_blocks, &mut terminator);
129 changed |= inner_changed;
132 let statements_to_merge =
133 merged_blocks.iter().map(|&i| self.basic_blocks[i].statements.len()).sum();
135 if statements_to_merge > 0 {
136 let mut statements = std::mem::take(&mut self.basic_blocks[bb].statements);
137 statements.reserve(statements_to_merge);
138 for &from in &merged_blocks {
139 statements.append(&mut self.basic_blocks[from].statements);
141 self.basic_blocks[bb].statements = statements;
144 self.basic_blocks[bb].terminator = Some(terminator);
146 changed |= inner_changed;
154 if start != START_BLOCK {
155 debug_assert!(self.pred_count[START_BLOCK] == 0);
156 self.basic_blocks.swap(START_BLOCK, start);
157 self.pred_count.swap(START_BLOCK, start);
159 // pred_count == 1 if the start block has no predecessor _blocks_.
160 if self.pred_count[START_BLOCK] > 1 {
161 for (bb, data) in self.basic_blocks.iter_enumerated_mut() {
162 if self.pred_count[bb] == 0 {
166 for target in data.terminator_mut().successors_mut() {
167 if *target == start {
168 *target = START_BLOCK;
176 /// This function will return `None` if
177 /// * the block has statements
178 /// * the block has a terminator other than `goto`
179 /// * the block has no terminator (meaning some other part of the current optimization stole it)
180 fn take_terminator_if_simple_goto(&mut self, bb: BasicBlock) -> Option<Terminator<'tcx>> {
181 match self.basic_blocks[bb] {
185 ref mut terminator @ Some(Terminator { kind: TerminatorKind::Goto { .. }, .. }),
187 } if statements.is_empty() => terminator.take(),
188 // if `terminator` is None, this means we are in a loop. In that
189 // case, let all the loop collapse to its entry.
194 /// Collapse a goto chain starting from `start`
195 fn collapse_goto_chain(&mut self, start: &mut BasicBlock, changed: &mut bool) {
196 // Using `SmallVec` here, because in some logs on libcore oli-obk saw many single-element
197 // goto chains. We should probably benchmark different sizes.
198 let mut terminators: SmallVec<[_; 1]> = Default::default();
199 let mut current = *start;
200 while let Some(terminator) = self.take_terminator_if_simple_goto(current) {
201 let target = match terminator {
202 Terminator { kind: TerminatorKind::Goto { target }, .. } => target,
205 terminators.push((current, terminator));
210 while let Some((current, mut terminator)) = terminators.pop() {
211 let target = match terminator {
212 Terminator { kind: TerminatorKind::Goto { ref mut target }, .. } => target,
216 debug!("collapsing goto chain from {:?} to {:?}", current, target);
218 if self.pred_count[current] == 1 {
219 // This is the last reference to current, so the pred-count to
220 // to target is moved into the current block.
221 self.pred_count[current] = 0;
223 self.pred_count[*target] += 1;
224 self.pred_count[current] -= 1;
227 self.basic_blocks[current].terminator = Some(terminator);
231 // merge a block with 1 `goto` predecessor to its parent
234 merged_blocks: &mut Vec<BasicBlock>,
235 terminator: &mut Terminator<'tcx>,
237 let target = match terminator.kind {
238 TerminatorKind::Goto { target } if self.pred_count[target] == 1 => target,
242 debug!("merging block {:?} into {:?}", target, terminator);
243 *terminator = match self.basic_blocks[target].terminator.take() {
244 Some(terminator) => terminator,
246 // unreachable loop - this should not be possible, as we
247 // don't strand blocks, but handle it correctly.
252 merged_blocks.push(target);
253 self.pred_count[target] = 0;
258 // turn a branch with all successors identical to a goto
259 fn simplify_branch(&mut self, terminator: &mut Terminator<'tcx>) -> bool {
260 match terminator.kind {
261 TerminatorKind::SwitchInt { .. } => {}
266 if let Some(&first_succ) = terminator.successors().next() {
267 if terminator.successors().all(|s| *s == first_succ) {
268 let count = terminator.successors().count();
269 self.pred_count[first_succ] -= (count - 1) as u32;
279 debug!("simplifying branch {:?}", terminator);
280 terminator.kind = TerminatorKind::Goto { target: first_succ };
284 fn strip_nops(&mut self) {
285 for blk in self.basic_blocks.iter_mut() {
287 .retain(|stmt| if let StatementKind::Nop = stmt.kind { false } else { true })
292 pub fn remove_dead_blocks(body: &mut Body<'_>) {
293 let mut seen = BitSet::new_empty(body.basic_blocks().len());
294 for (bb, _) in traversal::preorder(body) {
295 seen.insert(bb.index());
298 let basic_blocks = body.basic_blocks_mut();
300 let num_blocks = basic_blocks.len();
301 let mut replacements: Vec<_> = (0..num_blocks).map(BasicBlock::new).collect();
302 let mut used_blocks = 0;
303 for alive_index in seen.iter() {
304 replacements[alive_index] = BasicBlock::new(used_blocks);
305 if alive_index != used_blocks {
306 // Swap the next alive block data with the current available slot. Since
307 // alive_index is non-decreasing this is a valid operation.
308 basic_blocks.raw.swap(alive_index, used_blocks);
312 basic_blocks.raw.truncate(used_blocks);
314 for block in basic_blocks {
315 for target in block.terminator_mut().successors_mut() {
316 *target = replacements[target.index()];
321 pub struct SimplifyLocals;
323 impl<'tcx> MirPass<'tcx> for SimplifyLocals {
324 fn run_pass(&self, tcx: TyCtxt<'tcx>, source: MirSource<'tcx>, body: &mut Body<'tcx>) {
325 trace!("running SimplifyLocals on {:?}", source);
327 // First, we're going to get a count of *actual* uses for every `Local`.
328 // Take a look at `DeclMarker::visit_local()` to see exactly what is ignored.
329 let mut used_locals = {
330 let mut marker = DeclMarker::new(body);
331 marker.visit_body(&body);
336 let arg_count = body.arg_count;
338 // Next, we're going to remove any `Local` with zero actual uses. When we remove those
339 // `Locals`, we're also going to subtract any uses of other `Locals` from the `used_locals`
340 // count. For example, if we removed `_2 = discriminant(_1)`, then we'll subtract one from
341 // `use_counts[_1]`. That in turn might make `_1` unused, so we loop until we hit a
342 // fixedpoint where there are no more unused locals.
344 let mut remove_statements = RemoveStatements::new(&mut used_locals, arg_count, tcx);
345 remove_statements.visit_body(body);
347 if !remove_statements.modified {
352 // Finally, we'll actually do the work of shrinking `body.local_decls` and remapping the `Local`s.
353 let map = make_local_map(&mut body.local_decls, used_locals, arg_count);
355 // Only bother running the `LocalUpdater` if we actually found locals to remove.
356 if map.iter().any(Option::is_none) {
357 // Update references to all vars and tmps now
358 let mut updater = LocalUpdater { map, tcx };
359 updater.visit_body(body);
361 body.local_decls.shrink_to_fit();
366 /// Construct the mapping while swapping out unused stuff out from the `vec`.
367 fn make_local_map<V>(
368 local_decls: &mut IndexVec<Local, V>,
369 used_locals: IndexVec<Local, usize>,
371 ) -> IndexVec<Local, Option<Local>> {
372 let mut map: IndexVec<Local, Option<Local>> = IndexVec::from_elem(None, &*local_decls);
373 let mut used = Local::new(0);
374 for (alive_index, count) in used_locals.iter_enumerated() {
375 // The `RETURN_PLACE` and arguments are always live.
376 if alive_index.as_usize() > arg_count && *count == 0 {
380 map[alive_index] = Some(used);
381 if alive_index != used {
382 local_decls.swap(alive_index, used);
384 used.increment_by(1);
386 local_decls.truncate(used.index());
390 struct DeclMarker<'a, 'tcx> {
391 pub local_counts: IndexVec<Local, usize>,
392 pub body: &'a Body<'tcx>,
395 impl<'a, 'tcx> DeclMarker<'a, 'tcx> {
396 pub fn new(body: &'a Body<'tcx>) -> Self {
397 Self { local_counts: IndexVec::from_elem(0, &body.local_decls), body }
401 impl<'a, 'tcx> Visitor<'tcx> for DeclMarker<'a, 'tcx> {
402 fn visit_local(&mut self, local: &Local, ctx: PlaceContext, location: Location) {
403 // Ignore storage markers altogether, they get removed along with their otherwise unused
405 // FIXME: Extend this to all non-uses.
406 if ctx.is_storage_marker() {
410 // Ignore stores of constants because `ConstProp` and `CopyProp` can remove uses of many
411 // of these locals. However, if the local is still needed, then it will be referenced in
412 // another place and we'll mark it as being used there.
413 if ctx == PlaceContext::MutatingUse(MutatingUseContext::Store)
414 || ctx == PlaceContext::MutatingUse(MutatingUseContext::Projection)
416 let block = &self.body.basic_blocks()[location.block];
417 if location.statement_index != block.statements.len() {
418 let stmt = &block.statements[location.statement_index];
420 if let StatementKind::Assign(box (dest, rvalue)) = &stmt.kind {
421 if !dest.is_indirect() && dest.local == *local {
422 let can_skip = match rvalue {
424 | Rvalue::Discriminant(_)
425 | Rvalue::BinaryOp(_, _, _)
426 | Rvalue::CheckedBinaryOp(_, _, _)
427 | Rvalue::Repeat(_, _)
428 | Rvalue::AddressOf(_, _)
430 | Rvalue::UnaryOp(_, _)
431 | Rvalue::Aggregate(_, _) => true,
437 trace!("skipping store of {:?} to {:?}", rvalue, dest);
445 self.local_counts[*local] += 1;
449 struct StatementDeclMarker<'a, 'tcx> {
450 used_locals: &'a mut IndexVec<Local, usize>,
451 statement: &'a Statement<'tcx>,
454 impl<'a, 'tcx> StatementDeclMarker<'a, 'tcx> {
456 used_locals: &'a mut IndexVec<Local, usize>,
457 statement: &'a Statement<'tcx>,
459 Self { used_locals, statement }
463 impl<'a, 'tcx> Visitor<'tcx> for StatementDeclMarker<'a, 'tcx> {
464 fn visit_local(&mut self, local: &Local, context: PlaceContext, _location: Location) {
465 // Skip the lvalue for assignments
466 if let StatementKind::Assign(box (p, _)) = self.statement.kind {
467 if p.local == *local && context.is_place_assignment() {
472 let use_count = &mut self.used_locals[*local];
473 // If this is the local we're removing...
480 struct RemoveStatements<'a, 'tcx> {
481 used_locals: &'a mut IndexVec<Local, usize>,
487 impl<'a, 'tcx> RemoveStatements<'a, 'tcx> {
489 used_locals: &'a mut IndexVec<Local, usize>,
493 Self { used_locals, arg_count, tcx, modified: false }
496 fn keep_local(&self, l: Local) -> bool {
497 trace!("keep_local({:?}): count: {:?}", l, self.used_locals[l]);
498 l.as_usize() <= self.arg_count || self.used_locals[l] != 0
502 impl<'a, 'tcx> MutVisitor<'tcx> for RemoveStatements<'a, 'tcx> {
503 fn tcx(&self) -> TyCtxt<'tcx> {
507 fn visit_basic_block_data(&mut self, block: BasicBlock, data: &mut BasicBlockData<'tcx>) {
508 // Remove unnecessary StorageLive and StorageDead annotations.
510 data.statements.retain(|stmt| {
511 let keep = match &stmt.kind {
512 StatementKind::StorageLive(l) | StatementKind::StorageDead(l) => {
515 StatementKind::Assign(box (place, _)) => self.keep_local(place.local),
520 trace!("removing statement {:?}", stmt);
521 self.modified = true;
523 let mut visitor = StatementDeclMarker::new(self.used_locals, stmt);
524 visitor.visit_statement(stmt, Location { block, statement_index: i });
532 self.super_basic_block_data(block, data);
536 struct LocalUpdater<'tcx> {
537 map: IndexVec<Local, Option<Local>>,
541 impl<'tcx> MutVisitor<'tcx> for LocalUpdater<'tcx> {
542 fn tcx(&self) -> TyCtxt<'tcx> {
546 fn visit_local(&mut self, l: &mut Local, _: PlaceContext, _: Location) {
547 *l = self.map[*l].unwrap();