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;
31 use rustc_index::vec::{Idx, IndexVec};
32 use rustc_middle::mir::visit::{MutVisitor, MutatingUseContext, PlaceContext, Visitor};
33 use rustc_middle::mir::*;
34 use rustc_middle::ty::TyCtxt;
35 use smallvec::SmallVec;
37 use std::convert::TryInto;
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>, body: &mut Body<'tcx>) {
63 debug!("SimplifyCfg({:?}) - simplifying {:?}", self.label, body.source);
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);
152 if start != START_BLOCK {
153 debug_assert!(self.pred_count[START_BLOCK] == 0);
154 self.basic_blocks.swap(START_BLOCK, start);
155 self.pred_count.swap(START_BLOCK, start);
157 // pred_count == 1 if the start block has no predecessor _blocks_.
158 if self.pred_count[START_BLOCK] > 1 {
159 for (bb, data) in self.basic_blocks.iter_enumerated_mut() {
160 if self.pred_count[bb] == 0 {
164 for target in data.terminator_mut().successors_mut() {
165 if *target == start {
166 *target = START_BLOCK;
174 /// This function will return `None` if
175 /// * the block has statements
176 /// * the block has a terminator other than `goto`
177 /// * the block has no terminator (meaning some other part of the current optimization stole it)
178 fn take_terminator_if_simple_goto(&mut self, bb: BasicBlock) -> Option<Terminator<'tcx>> {
179 match self.basic_blocks[bb] {
183 ref mut terminator @ Some(Terminator { kind: TerminatorKind::Goto { .. }, .. }),
185 } if statements.is_empty() => terminator.take(),
186 // if `terminator` is None, this means we are in a loop. In that
187 // case, let all the loop collapse to its entry.
192 /// Collapse a goto chain starting from `start`
193 fn collapse_goto_chain(&mut self, start: &mut BasicBlock, changed: &mut bool) {
194 // Using `SmallVec` here, because in some logs on libcore oli-obk saw many single-element
195 // goto chains. We should probably benchmark different sizes.
196 let mut terminators: SmallVec<[_; 1]> = Default::default();
197 let mut current = *start;
198 while let Some(terminator) = self.take_terminator_if_simple_goto(current) {
199 let target = match terminator {
200 Terminator { kind: TerminatorKind::Goto { target }, .. } => target,
203 terminators.push((current, terminator));
208 while let Some((current, mut terminator)) = terminators.pop() {
209 let target = match terminator {
210 Terminator { kind: TerminatorKind::Goto { ref mut target }, .. } => target,
213 *changed |= *target != last;
215 debug!("collapsing goto chain from {:?} to {:?}", current, target);
217 if self.pred_count[current] == 1 {
218 // This is the last reference to current, so the pred-count to
219 // to target is moved into the current block.
220 self.pred_count[current] = 0;
222 self.pred_count[*target] += 1;
223 self.pred_count[current] -= 1;
225 self.basic_blocks[current].terminator = Some(terminator);
229 // merge a block with 1 `goto` predecessor to its parent
232 merged_blocks: &mut Vec<BasicBlock>,
233 terminator: &mut Terminator<'tcx>,
235 let target = match terminator.kind {
236 TerminatorKind::Goto { target } if self.pred_count[target] == 1 => target,
240 debug!("merging block {:?} into {:?}", target, terminator);
241 *terminator = match self.basic_blocks[target].terminator.take() {
242 Some(terminator) => terminator,
244 // unreachable loop - this should not be possible, as we
245 // don't strand blocks, but handle it correctly.
250 merged_blocks.push(target);
251 self.pred_count[target] = 0;
256 // turn a branch with all successors identical to a goto
257 fn simplify_branch(&mut self, terminator: &mut Terminator<'tcx>) -> bool {
258 match terminator.kind {
259 TerminatorKind::SwitchInt { .. } => {}
264 if let Some(&first_succ) = terminator.successors().next() {
265 if terminator.successors().all(|s| *s == first_succ) {
266 let count = terminator.successors().count();
267 self.pred_count[first_succ] -= (count - 1) as u32;
277 debug!("simplifying branch {:?}", terminator);
278 terminator.kind = TerminatorKind::Goto { target: first_succ };
282 fn strip_nops(&mut self) {
283 for blk in self.basic_blocks.iter_mut() {
284 blk.statements.retain(|stmt| !matches!(stmt.kind, StatementKind::Nop))
289 pub fn remove_dead_blocks(body: &mut Body<'_>) {
290 let reachable = traversal::reachable_as_bitset(body);
291 let num_blocks = body.basic_blocks().len();
292 if num_blocks == reachable.count() {
296 let basic_blocks = body.basic_blocks_mut();
297 let mut replacements: Vec<_> = (0..num_blocks).map(BasicBlock::new).collect();
298 let mut used_blocks = 0;
299 for alive_index in reachable.iter() {
300 let alive_index = alive_index.index();
301 replacements[alive_index] = BasicBlock::new(used_blocks);
302 if alive_index != used_blocks {
303 // Swap the next alive block data with the current available slot. Since
304 // alive_index is non-decreasing this is a valid operation.
305 basic_blocks.raw.swap(alive_index, used_blocks);
309 basic_blocks.raw.truncate(used_blocks);
311 for block in basic_blocks {
312 for target in block.terminator_mut().successors_mut() {
313 *target = replacements[target.index()];
318 pub struct SimplifyLocals;
320 impl<'tcx> MirPass<'tcx> for SimplifyLocals {
321 fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
322 trace!("running SimplifyLocals on {:?}", body.source);
323 simplify_locals(body, tcx);
327 pub fn simplify_locals<'tcx>(body: &mut Body<'tcx>, tcx: TyCtxt<'tcx>) {
328 // First, we're going to get a count of *actual* uses for every `Local`.
329 let mut used_locals = UsedLocals::new(body);
331 // Next, we're going to remove any `Local` with zero actual uses. When we remove those
332 // `Locals`, we're also going to subtract any uses of other `Locals` from the `used_locals`
333 // count. For example, if we removed `_2 = discriminant(_1)`, then we'll subtract one from
334 // `use_counts[_1]`. That in turn might make `_1` unused, so we loop until we hit a
335 // fixedpoint where there are no more unused locals.
336 remove_unused_definitions(&mut used_locals, body);
338 // Finally, we'll actually do the work of shrinking `body.local_decls` and remapping the `Local`s.
339 let map = make_local_map(&mut body.local_decls, &used_locals);
341 // Only bother running the `LocalUpdater` if we actually found locals to remove.
342 if map.iter().any(Option::is_none) {
343 // Update references to all vars and tmps now
344 let mut updater = LocalUpdater { map, tcx };
345 updater.visit_body(body);
347 body.local_decls.shrink_to_fit();
351 /// Construct the mapping while swapping out unused stuff out from the `vec`.
352 fn make_local_map<V>(
353 local_decls: &mut IndexVec<Local, V>,
354 used_locals: &UsedLocals,
355 ) -> IndexVec<Local, Option<Local>> {
356 let mut map: IndexVec<Local, Option<Local>> = IndexVec::from_elem(None, &*local_decls);
357 let mut used = Local::new(0);
359 for alive_index in local_decls.indices() {
360 // `is_used` treats the `RETURN_PLACE` and arguments as used.
361 if !used_locals.is_used(alive_index) {
365 map[alive_index] = Some(used);
366 if alive_index != used {
367 local_decls.swap(alive_index, used);
369 used.increment_by(1);
371 local_decls.truncate(used.index());
375 /// Keeps track of used & unused locals.
379 use_count: IndexVec<Local, u32>,
383 /// Determines which locals are used & unused in the given body.
384 fn new(body: &Body<'_>) -> Self {
385 let mut this = Self {
387 arg_count: body.arg_count.try_into().unwrap(),
388 use_count: IndexVec::from_elem(0, &body.local_decls),
390 this.visit_body(body);
394 /// Checks if local is used.
396 /// Return place and arguments are always considered used.
397 fn is_used(&self, local: Local) -> bool {
398 trace!("is_used({:?}): use_count: {:?}", local, self.use_count[local]);
399 local.as_u32() <= self.arg_count || self.use_count[local] != 0
402 /// Updates the use counts to reflect the removal of given statement.
403 fn statement_removed(&mut self, statement: &Statement<'tcx>) {
404 self.increment = false;
406 // The location of the statement is irrelevant.
407 let location = Location { block: START_BLOCK, statement_index: 0 };
408 self.visit_statement(statement, location);
411 /// Visits a left-hand side of an assignment.
412 fn visit_lhs(&mut self, place: &Place<'tcx>, location: Location) {
413 if place.is_indirect() {
414 // A use, not a definition.
415 self.visit_place(place, PlaceContext::MutatingUse(MutatingUseContext::Store), location);
417 // A definition. Although, it still might use other locals for indexing.
418 self.super_projection(
420 PlaceContext::MutatingUse(MutatingUseContext::Projection),
427 impl Visitor<'_> for UsedLocals {
428 fn visit_statement(&mut self, statement: &Statement<'tcx>, location: Location) {
429 match statement.kind {
430 StatementKind::LlvmInlineAsm(..)
431 | StatementKind::CopyNonOverlapping(..)
432 | StatementKind::Retag(..)
433 | StatementKind::Coverage(..)
434 | StatementKind::FakeRead(..)
435 | StatementKind::AscribeUserType(..) => {
436 self.super_statement(statement, location);
439 StatementKind::Nop => {}
441 StatementKind::StorageLive(_local) | StatementKind::StorageDead(_local) => {}
443 StatementKind::Assign(box (ref place, ref rvalue)) => {
444 self.visit_lhs(place, location);
445 self.visit_rvalue(rvalue, location);
448 StatementKind::SetDiscriminant { ref place, variant_index: _ } => {
449 self.visit_lhs(place, location);
454 fn visit_local(&mut self, local: &Local, _ctx: PlaceContext, _location: Location) {
456 self.use_count[*local] += 1;
458 assert_ne!(self.use_count[*local], 0);
459 self.use_count[*local] -= 1;
464 /// Removes unused definitions. Updates the used locals to reflect the changes made.
465 fn remove_unused_definitions<'a, 'tcx>(used_locals: &'a mut UsedLocals, body: &mut Body<'tcx>) {
466 // The use counts are updated as we remove the statements. A local might become unused
467 // during the retain operation, leading to a temporary inconsistency (storage statements or
468 // definitions referencing the local might remain). For correctness it is crucial that this
469 // computation reaches a fixed point.
471 let mut modified = true;
475 for data in body.basic_blocks_mut() {
476 // Remove unnecessary StorageLive and StorageDead annotations.
477 data.statements.retain(|statement| {
478 let keep = match &statement.kind {
479 StatementKind::StorageLive(local) | StatementKind::StorageDead(local) => {
480 used_locals.is_used(*local)
482 StatementKind::Assign(box (place, _)) => used_locals.is_used(place.local),
484 StatementKind::SetDiscriminant { ref place, .. } => {
485 used_locals.is_used(place.local)
491 trace!("removing statement {:?}", statement);
493 used_locals.statement_removed(statement);
502 struct LocalUpdater<'tcx> {
503 map: IndexVec<Local, Option<Local>>,
507 impl<'tcx> MutVisitor<'tcx> for LocalUpdater<'tcx> {
508 fn tcx(&self) -> TyCtxt<'tcx> {
512 fn visit_local(&mut self, l: &mut Local, _: PlaceContext, _: Location) {
513 *l = self.map[*l].unwrap();