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c30ab7b3 | 1 | //! A number of passes which remove various redundancies in the CFG. |
3157f602 | 2 | //! |
c30ab7b3 SL |
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. | |
3157f602 | 5 | //! |
c30ab7b3 SL |
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 | |
94b46f34 | 8 | //! either, so running the pass once, right before codegen, should suffice. |
c30ab7b3 SL |
9 | //! |
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 | |
13 | //! ill-typed. | |
14 | //! | |
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: | |
3157f602 | 19 | //! |
3157f602 XL |
20 | //! ```rust |
21 | //! fn example() { | |
22 | //! let _a: char = { return; }; | |
23 | //! } | |
24 | //! ``` | |
25 | //! | |
c30ab7b3 SL |
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 | |
28 | //! return. | |
3157f602 | 29 | |
29967ef6 | 30 | use crate::transform::MirPass; |
e74abb32 | 31 | use rustc_index::vec::{Idx, IndexVec}; |
17df50a5 | 32 | use rustc_middle::mir::coverage::*; |
ba9703b0 XL |
33 | use rustc_middle::mir::visit::{MutVisitor, MutatingUseContext, PlaceContext, Visitor}; |
34 | use rustc_middle::mir::*; | |
f9f354fc | 35 | use rustc_middle::ty::TyCtxt; |
3dfed10e | 36 | use smallvec::SmallVec; |
cdc7bbd5 XL |
37 | use std::borrow::Cow; |
38 | use std::convert::TryInto; | |
92a42be0 | 39 | |
dfeec247 XL |
40 | pub struct SimplifyCfg { |
41 | label: String, | |
42 | } | |
a7813a04 | 43 | |
7cac9316 XL |
44 | impl SimplifyCfg { |
45 | pub fn new(label: &str) -> Self { | |
46 | SimplifyCfg { label: format!("SimplifyCfg-{}", label) } | |
92a42be0 | 47 | } |
a7813a04 XL |
48 | } |
49 | ||
17df50a5 | 50 | pub fn simplify_cfg(tcx: TyCtxt<'tcx>, body: &mut Body<'_>) { |
dc9dc135 | 51 | CfgSimplifier::new(body).simplify(); |
17df50a5 | 52 | remove_dead_blocks(tcx, body); |
cc61c64b XL |
53 | |
54 | // FIXME: Should probably be moved into some kind of pass manager | |
dc9dc135 | 55 | body.basic_blocks_mut().raw.shrink_to_fit(); |
cc61c64b XL |
56 | } |
57 | ||
e1599b0c | 58 | impl<'tcx> MirPass<'tcx> for SimplifyCfg { |
416331ca | 59 | fn name(&self) -> Cow<'_, str> { |
7cac9316 | 60 | Cow::Borrowed(&self.label) |
3157f602 | 61 | } |
3157f602 | 62 | |
17df50a5 | 63 | fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) { |
5869c6ff | 64 | debug!("SimplifyCfg({:?}) - simplifying {:?}", self.label, body.source); |
17df50a5 | 65 | simplify_cfg(tcx, body); |
a7813a04 | 66 | } |
3157f602 XL |
67 | } |
68 | ||
dc9dc135 | 69 | pub struct CfgSimplifier<'a, 'tcx> { |
3157f602 | 70 | basic_blocks: &'a mut IndexVec<BasicBlock, BasicBlockData<'tcx>>, |
dfeec247 | 71 | pred_count: IndexVec<BasicBlock, u32>, |
a7813a04 XL |
72 | } |
73 | ||
dc9dc135 | 74 | impl<'a, 'tcx> CfgSimplifier<'a, 'tcx> { |
f9f354fc | 75 | pub fn new(body: &'a mut Body<'tcx>) -> Self { |
dc9dc135 | 76 | let mut pred_count = IndexVec::from_elem(0u32, body.basic_blocks()); |
a7813a04 | 77 | |
3157f602 XL |
78 | // we can't use mir.predecessors() here because that counts |
79 | // dead blocks, which we don't want to. | |
c30ab7b3 SL |
80 | pred_count[START_BLOCK] = 1; |
81 | ||
dc9dc135 | 82 | for (_, data) in traversal::preorder(body) { |
3157f602 | 83 | if let Some(ref term) = data.terminator { |
83c7162d | 84 | for &tgt in term.successors() { |
3157f602 XL |
85 | pred_count[tgt] += 1; |
86 | } | |
a7813a04 XL |
87 | } |
88 | } | |
3157f602 | 89 | |
dc9dc135 | 90 | let basic_blocks = body.basic_blocks_mut(); |
3157f602 | 91 | |
dfeec247 | 92 | CfgSimplifier { basic_blocks, pred_count } |
a7813a04 XL |
93 | } |
94 | ||
8bb4bdeb | 95 | pub fn simplify(mut self) { |
3b2f2976 XL |
96 | self.strip_nops(); |
97 | ||
0731742a XL |
98 | let mut start = START_BLOCK; |
99 | ||
ba9703b0 XL |
100 | // Vec of the blocks that should be merged. We store the indices here, instead of the |
101 | // statements itself to avoid moving the (relatively) large statements twice. | |
102 | // We do not push the statements directly into the target block (`bb`) as that is slower | |
103 | // due to additional reallocations | |
104 | let mut merged_blocks = Vec::new(); | |
3157f602 XL |
105 | loop { |
106 | let mut changed = false; | |
107 | ||
0731742a XL |
108 | self.collapse_goto_chain(&mut start, &mut changed); |
109 | ||
110 | for bb in self.basic_blocks.indices() { | |
3157f602 | 111 | if self.pred_count[bb] == 0 { |
dfeec247 | 112 | continue; |
a7813a04 XL |
113 | } |
114 | ||
3157f602 XL |
115 | debug!("simplifying {:?}", bb); |
116 | ||
dfeec247 XL |
117 | let mut terminator = |
118 | self.basic_blocks[bb].terminator.take().expect("invalid terminator state"); | |
3157f602 XL |
119 | |
120 | for successor in terminator.successors_mut() { | |
121 | self.collapse_goto_chain(successor, &mut changed); | |
a7813a04 XL |
122 | } |
123 | ||
3157f602 | 124 | let mut inner_changed = true; |
ba9703b0 | 125 | merged_blocks.clear(); |
3157f602 XL |
126 | while inner_changed { |
127 | inner_changed = false; | |
128 | inner_changed |= self.simplify_branch(&mut terminator); | |
ba9703b0 | 129 | inner_changed |= self.merge_successor(&mut merged_blocks, &mut terminator); |
3157f602 | 130 | changed |= inner_changed; |
92a42be0 | 131 | } |
92a42be0 | 132 | |
ba9703b0 XL |
133 | let statements_to_merge = |
134 | merged_blocks.iter().map(|&i| self.basic_blocks[i].statements.len()).sum(); | |
135 | ||
136 | if statements_to_merge > 0 { | |
137 | let mut statements = std::mem::take(&mut self.basic_blocks[bb].statements); | |
138 | statements.reserve(statements_to_merge); | |
139 | for &from in &merged_blocks { | |
140 | statements.append(&mut self.basic_blocks[from].statements); | |
141 | } | |
142 | self.basic_blocks[bb].statements = statements; | |
143 | } | |
144 | ||
145 | self.basic_blocks[bb].terminator = Some(terminator); | |
a7813a04 | 146 | } |
92a42be0 | 147 | |
dfeec247 XL |
148 | if !changed { |
149 | break; | |
150 | } | |
a7813a04 | 151 | } |
0731742a XL |
152 | |
153 | if start != START_BLOCK { | |
154 | debug_assert!(self.pred_count[START_BLOCK] == 0); | |
155 | self.basic_blocks.swap(START_BLOCK, start); | |
156 | self.pred_count.swap(START_BLOCK, start); | |
157 | ||
158 | // pred_count == 1 if the start block has no predecessor _blocks_. | |
159 | if self.pred_count[START_BLOCK] > 1 { | |
160 | for (bb, data) in self.basic_blocks.iter_enumerated_mut() { | |
161 | if self.pred_count[bb] == 0 { | |
162 | continue; | |
163 | } | |
164 | ||
165 | for target in data.terminator_mut().successors_mut() { | |
166 | if *target == start { | |
167 | *target = START_BLOCK; | |
168 | } | |
169 | } | |
170 | } | |
171 | } | |
172 | } | |
a7813a04 | 173 | } |
a7813a04 | 174 | |
3dfed10e XL |
175 | /// This function will return `None` if |
176 | /// * the block has statements | |
177 | /// * the block has a terminator other than `goto` | |
178 | /// * the block has no terminator (meaning some other part of the current optimization stole it) | |
179 | fn take_terminator_if_simple_goto(&mut self, bb: BasicBlock) -> Option<Terminator<'tcx>> { | |
180 | match self.basic_blocks[bb] { | |
3157f602 XL |
181 | BasicBlockData { |
182 | ref statements, | |
dfeec247 XL |
183 | terminator: |
184 | ref mut terminator @ Some(Terminator { kind: TerminatorKind::Goto { .. }, .. }), | |
185 | .. | |
3157f602 XL |
186 | } if statements.is_empty() => terminator.take(), |
187 | // if `terminator` is None, this means we are in a loop. In that | |
188 | // case, let all the loop collapse to its entry. | |
3dfed10e XL |
189 | _ => None, |
190 | } | |
191 | } | |
3157f602 | 192 | |
3dfed10e XL |
193 | /// Collapse a goto chain starting from `start` |
194 | fn collapse_goto_chain(&mut self, start: &mut BasicBlock, changed: &mut bool) { | |
195 | // Using `SmallVec` here, because in some logs on libcore oli-obk saw many single-element | |
196 | // goto chains. We should probably benchmark different sizes. | |
197 | let mut terminators: SmallVec<[_; 1]> = Default::default(); | |
198 | let mut current = *start; | |
199 | while let Some(terminator) = self.take_terminator_if_simple_goto(current) { | |
200 | let target = match terminator { | |
201 | Terminator { kind: TerminatorKind::Goto { target }, .. } => target, | |
202 | _ => unreachable!(), | |
203 | }; | |
204 | terminators.push((current, terminator)); | |
205 | current = target; | |
206 | } | |
207 | let last = current; | |
208 | *start = last; | |
209 | while let Some((current, mut terminator)) = terminators.pop() { | |
210 | let target = match terminator { | |
211 | Terminator { kind: TerminatorKind::Goto { ref mut target }, .. } => target, | |
212 | _ => unreachable!(), | |
213 | }; | |
214 | *changed |= *target != last; | |
215 | *target = last; | |
216 | debug!("collapsing goto chain from {:?} to {:?}", current, target); | |
217 | ||
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; | |
222 | } else { | |
223 | self.pred_count[*target] += 1; | |
224 | self.pred_count[current] -= 1; | |
92a42be0 | 225 | } |
3dfed10e | 226 | self.basic_blocks[current].terminator = Some(terminator); |
c30ab7b3 | 227 | } |
3157f602 | 228 | } |
7453a54e | 229 | |
3157f602 | 230 | // merge a block with 1 `goto` predecessor to its parent |
dfeec247 XL |
231 | fn merge_successor( |
232 | &mut self, | |
ba9703b0 | 233 | merged_blocks: &mut Vec<BasicBlock>, |
dfeec247 XL |
234 | terminator: &mut Terminator<'tcx>, |
235 | ) -> bool { | |
3157f602 | 236 | let target = match terminator.kind { |
dfeec247 XL |
237 | TerminatorKind::Goto { target } if self.pred_count[target] == 1 => target, |
238 | _ => return false, | |
3157f602 XL |
239 | }; |
240 | ||
241 | debug!("merging block {:?} into {:?}", target, terminator); | |
242 | *terminator = match self.basic_blocks[target].terminator.take() { | |
243 | Some(terminator) => terminator, | |
244 | None => { | |
245 | // unreachable loop - this should not be possible, as we | |
246 | // don't strand blocks, but handle it correctly. | |
dfeec247 | 247 | return false; |
3157f602 XL |
248 | } |
249 | }; | |
ba9703b0 XL |
250 | |
251 | merged_blocks.push(target); | |
3157f602 | 252 | self.pred_count[target] = 0; |
7453a54e | 253 | |
3157f602 XL |
254 | true |
255 | } | |
256 | ||
257 | // turn a branch with all successors identical to a goto | |
258 | fn simplify_branch(&mut self, terminator: &mut Terminator<'tcx>) -> bool { | |
259 | match terminator.kind { | |
dfeec247 XL |
260 | TerminatorKind::SwitchInt { .. } => {} |
261 | _ => return false, | |
3157f602 XL |
262 | }; |
263 | ||
264 | let first_succ = { | |
74b04a01 | 265 | if let Some(&first_succ) = terminator.successors().next() { |
83c7162d XL |
266 | if terminator.successors().all(|s| *s == first_succ) { |
267 | let count = terminator.successors().count(); | |
268 | self.pred_count[first_succ] -= (count - 1) as u32; | |
3157f602 XL |
269 | first_succ |
270 | } else { | |
dfeec247 | 271 | return false; |
92a42be0 | 272 | } |
3157f602 | 273 | } else { |
dfeec247 | 274 | return false; |
92a42be0 | 275 | } |
3157f602 XL |
276 | }; |
277 | ||
278 | debug!("simplifying branch {:?}", terminator); | |
279 | terminator.kind = TerminatorKind::Goto { target: first_succ }; | |
280 | true | |
281 | } | |
8bb4bdeb XL |
282 | |
283 | fn strip_nops(&mut self) { | |
284 | for blk in self.basic_blocks.iter_mut() { | |
1b1a35ee | 285 | blk.statements.retain(|stmt| !matches!(stmt.kind, StatementKind::Nop)) |
8bb4bdeb XL |
286 | } |
287 | } | |
3157f602 XL |
288 | } |
289 | ||
17df50a5 | 290 | pub fn remove_dead_blocks(tcx: TyCtxt<'tcx>, body: &mut Body<'_>) { |
5869c6ff XL |
291 | let reachable = traversal::reachable_as_bitset(body); |
292 | let num_blocks = body.basic_blocks().len(); | |
293 | if num_blocks == reachable.count() { | |
294 | return; | |
3157f602 XL |
295 | } |
296 | ||
dc9dc135 | 297 | let basic_blocks = body.basic_blocks_mut(); |
dfeec247 | 298 | let mut replacements: Vec<_> = (0..num_blocks).map(BasicBlock::new).collect(); |
3157f602 | 299 | let mut used_blocks = 0; |
5869c6ff XL |
300 | for alive_index in reachable.iter() { |
301 | let alive_index = alive_index.index(); | |
3157f602 XL |
302 | replacements[alive_index] = BasicBlock::new(used_blocks); |
303 | if alive_index != used_blocks { | |
60c5eb7d XL |
304 | // Swap the next alive block data with the current available slot. Since |
305 | // alive_index is non-decreasing this is a valid operation. | |
3157f602 | 306 | basic_blocks.raw.swap(alive_index, used_blocks); |
92a42be0 | 307 | } |
3157f602 XL |
308 | used_blocks += 1; |
309 | } | |
17df50a5 XL |
310 | |
311 | if tcx.sess.instrument_coverage() { | |
312 | save_unreachable_coverage(basic_blocks, used_blocks); | |
313 | } | |
314 | ||
3157f602 | 315 | basic_blocks.raw.truncate(used_blocks); |
92a42be0 | 316 | |
3157f602 XL |
317 | for block in basic_blocks { |
318 | for target in block.terminator_mut().successors_mut() { | |
319 | *target = replacements[target.index()]; | |
92a42be0 | 320 | } |
92a42be0 SL |
321 | } |
322 | } | |
c30ab7b3 | 323 | |
17df50a5 XL |
324 | /// Some MIR transforms can determine at compile time that a sequences of |
325 | /// statements will never be executed, so they can be dropped from the MIR. | |
326 | /// For example, an `if` or `else` block that is guaranteed to never be executed | |
327 | /// because its condition can be evaluated at compile time, such as by const | |
328 | /// evaluation: `if false { ... }`. | |
329 | /// | |
330 | /// Those statements are bypassed by redirecting paths in the CFG around the | |
331 | /// `dead blocks`; but with `-Z instrument-coverage`, the dead blocks usually | |
332 | /// include `Coverage` statements representing the Rust source code regions to | |
333 | /// be counted at runtime. Without these `Coverage` statements, the regions are | |
334 | /// lost, and the Rust source code will show no coverage information. | |
335 | /// | |
336 | /// What we want to show in a coverage report is the dead code with coverage | |
337 | /// counts of `0`. To do this, we need to save the code regions, by injecting | |
338 | /// `Unreachable` coverage statements. These are non-executable statements whose | |
339 | /// code regions are still recorded in the coverage map, representing regions | |
340 | /// with `0` executions. | |
341 | fn save_unreachable_coverage( | |
342 | basic_blocks: &mut IndexVec<BasicBlock, BasicBlockData<'_>>, | |
343 | first_dead_block: usize, | |
344 | ) { | |
345 | let has_live_counters = basic_blocks.raw[0..first_dead_block].iter().any(|live_block| { | |
346 | live_block.statements.iter().any(|statement| { | |
347 | if let StatementKind::Coverage(coverage) = &statement.kind { | |
348 | matches!(coverage.kind, CoverageKind::Counter { .. }) | |
349 | } else { | |
350 | false | |
351 | } | |
352 | }) | |
353 | }); | |
354 | if !has_live_counters { | |
355 | // If there are no live `Counter` `Coverage` statements anymore, don't | |
356 | // move dead coverage to the `START_BLOCK`. Just allow the dead | |
357 | // `Coverage` statements to be dropped with the dead blocks. | |
358 | // | |
359 | // The `generator::StateTransform` MIR pass can create atypical | |
360 | // conditions, where all live `Counter`s are dropped from the MIR. | |
361 | // | |
362 | // At least one Counter per function is required by LLVM (and necessary, | |
363 | // to add the `function_hash` to the counter's call to the LLVM | |
364 | // intrinsic `instrprof.increment()`). | |
365 | return; | |
366 | } | |
367 | ||
368 | // Retain coverage info for dead blocks, so coverage reports will still | |
369 | // report `0` executions for the uncovered code regions. | |
370 | let mut dropped_coverage = Vec::new(); | |
371 | for dead_block in basic_blocks.raw[first_dead_block..].iter() { | |
372 | for statement in dead_block.statements.iter() { | |
373 | if let StatementKind::Coverage(coverage) = &statement.kind { | |
374 | if let Some(code_region) = &coverage.code_region { | |
375 | dropped_coverage.push((statement.source_info, code_region.clone())); | |
376 | } | |
377 | } | |
378 | } | |
379 | } | |
380 | ||
381 | let start_block = &mut basic_blocks[START_BLOCK]; | |
382 | for (source_info, code_region) in dropped_coverage { | |
383 | start_block.statements.push(Statement { | |
384 | source_info, | |
385 | kind: StatementKind::Coverage(box Coverage { | |
386 | kind: CoverageKind::Unreachable, | |
387 | code_region: Some(code_region), | |
388 | }), | |
389 | }) | |
390 | } | |
391 | } | |
392 | ||
c30ab7b3 SL |
393 | pub struct SimplifyLocals; |
394 | ||
e1599b0c | 395 | impl<'tcx> MirPass<'tcx> for SimplifyLocals { |
29967ef6 XL |
396 | fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) { |
397 | trace!("running SimplifyLocals on {:?}", body.source); | |
6a06907d XL |
398 | simplify_locals(body, tcx); |
399 | } | |
400 | } | |
ba9703b0 | 401 | |
6a06907d XL |
402 | pub fn simplify_locals<'tcx>(body: &mut Body<'tcx>, tcx: TyCtxt<'tcx>) { |
403 | // First, we're going to get a count of *actual* uses for every `Local`. | |
cdc7bbd5 | 404 | let mut used_locals = UsedLocals::new(body); |
ba9703b0 | 405 | |
6a06907d XL |
406 | // Next, we're going to remove any `Local` with zero actual uses. When we remove those |
407 | // `Locals`, we're also going to subtract any uses of other `Locals` from the `used_locals` | |
408 | // count. For example, if we removed `_2 = discriminant(_1)`, then we'll subtract one from | |
409 | // `use_counts[_1]`. That in turn might make `_1` unused, so we loop until we hit a | |
410 | // fixedpoint where there are no more unused locals. | |
411 | remove_unused_definitions(&mut used_locals, body); | |
ba9703b0 | 412 | |
6a06907d | 413 | // Finally, we'll actually do the work of shrinking `body.local_decls` and remapping the `Local`s. |
cdc7bbd5 | 414 | let map = make_local_map(&mut body.local_decls, &used_locals); |
ba9703b0 | 415 | |
6a06907d XL |
416 | // Only bother running the `LocalUpdater` if we actually found locals to remove. |
417 | if map.iter().any(Option::is_none) { | |
418 | // Update references to all vars and tmps now | |
419 | let mut updater = LocalUpdater { map, tcx }; | |
420 | updater.visit_body(body); | |
ba9703b0 | 421 | |
6a06907d | 422 | body.local_decls.shrink_to_fit(); |
c30ab7b3 SL |
423 | } |
424 | } | |
425 | ||
426 | /// Construct the mapping while swapping out unused stuff out from the `vec`. | |
cdc7bbd5 | 427 | fn make_local_map<V>( |
ba9703b0 | 428 | local_decls: &mut IndexVec<Local, V>, |
cdc7bbd5 | 429 | used_locals: &UsedLocals, |
8faf50e0 | 430 | ) -> IndexVec<Local, Option<Local>> { |
cdc7bbd5 | 431 | let mut map: IndexVec<Local, Option<Local>> = IndexVec::from_elem(None, &*local_decls); |
8faf50e0 | 432 | let mut used = Local::new(0); |
29967ef6 XL |
433 | |
434 | for alive_index in local_decls.indices() { | |
cdc7bbd5 XL |
435 | // `is_used` treats the `RETURN_PLACE` and arguments as used. |
436 | if !used_locals.is_used(alive_index) { | |
437 | continue; | |
438 | } | |
439 | ||
440 | map[alive_index] = Some(used); | |
441 | if alive_index != used { | |
442 | local_decls.swap(alive_index, used); | |
c30ab7b3 | 443 | } |
cdc7bbd5 | 444 | used.increment_by(1); |
c30ab7b3 | 445 | } |
ba9703b0 | 446 | local_decls.truncate(used.index()); |
c30ab7b3 SL |
447 | map |
448 | } | |
449 | ||
29967ef6 | 450 | /// Keeps track of used & unused locals. |
cdc7bbd5 | 451 | struct UsedLocals { |
29967ef6 | 452 | increment: bool, |
cdc7bbd5 | 453 | arg_count: u32, |
29967ef6 | 454 | use_count: IndexVec<Local, u32>, |
c30ab7b3 SL |
455 | } |
456 | ||
cdc7bbd5 | 457 | impl UsedLocals { |
29967ef6 | 458 | /// Determines which locals are used & unused in the given body. |
cdc7bbd5 | 459 | fn new(body: &Body<'_>) -> Self { |
29967ef6 XL |
460 | let mut this = Self { |
461 | increment: true, | |
cdc7bbd5 | 462 | arg_count: body.arg_count.try_into().unwrap(), |
29967ef6 XL |
463 | use_count: IndexVec::from_elem(0, &body.local_decls), |
464 | }; | |
465 | this.visit_body(body); | |
466 | this | |
ba9703b0 | 467 | } |
ba9703b0 | 468 | |
29967ef6 | 469 | /// Checks if local is used. |
cdc7bbd5 XL |
470 | /// |
471 | /// Return place and arguments are always considered used. | |
29967ef6 XL |
472 | fn is_used(&self, local: Local) -> bool { |
473 | trace!("is_used({:?}): use_count: {:?}", local, self.use_count[local]); | |
cdc7bbd5 | 474 | local.as_u32() <= self.arg_count || self.use_count[local] != 0 |
29967ef6 | 475 | } |
e74abb32 | 476 | |
29967ef6 XL |
477 | /// Updates the use counts to reflect the removal of given statement. |
478 | fn statement_removed(&mut self, statement: &Statement<'tcx>) { | |
479 | self.increment = false; | |
e74abb32 | 480 | |
29967ef6 XL |
481 | // The location of the statement is irrelevant. |
482 | let location = Location { block: START_BLOCK, statement_index: 0 }; | |
483 | self.visit_statement(statement, location); | |
c30ab7b3 | 484 | } |
ba9703b0 | 485 | |
29967ef6 XL |
486 | /// Visits a left-hand side of an assignment. |
487 | fn visit_lhs(&mut self, place: &Place<'tcx>, location: Location) { | |
488 | if place.is_indirect() { | |
489 | // A use, not a definition. | |
490 | self.visit_place(place, PlaceContext::MutatingUse(MutatingUseContext::Store), location); | |
491 | } else { | |
cdc7bbd5 XL |
492 | // A definition. The base local itself is not visited, so this occurrence is not counted |
493 | // toward its use count. There might be other locals still, used in an indexing | |
494 | // projection. | |
29967ef6 | 495 | self.super_projection( |
6a06907d | 496 | place.as_ref(), |
29967ef6 XL |
497 | PlaceContext::MutatingUse(MutatingUseContext::Projection), |
498 | location, | |
499 | ); | |
500 | } | |
ba9703b0 XL |
501 | } |
502 | } | |
503 | ||
cdc7bbd5 | 504 | impl Visitor<'_> for UsedLocals { |
29967ef6 XL |
505 | fn visit_statement(&mut self, statement: &Statement<'tcx>, location: Location) { |
506 | match statement.kind { | |
507 | StatementKind::LlvmInlineAsm(..) | |
6a06907d | 508 | | StatementKind::CopyNonOverlapping(..) |
29967ef6 XL |
509 | | StatementKind::Retag(..) |
510 | | StatementKind::Coverage(..) | |
511 | | StatementKind::FakeRead(..) | |
512 | | StatementKind::AscribeUserType(..) => { | |
513 | self.super_statement(statement, location); | |
ba9703b0 | 514 | } |
ba9703b0 | 515 | |
29967ef6 | 516 | StatementKind::Nop => {} |
ba9703b0 | 517 | |
29967ef6 | 518 | StatementKind::StorageLive(_local) | StatementKind::StorageDead(_local) => {} |
c30ab7b3 | 519 | |
29967ef6 XL |
520 | StatementKind::Assign(box (ref place, ref rvalue)) => { |
521 | self.visit_lhs(place, location); | |
522 | self.visit_rvalue(rvalue, location); | |
523 | } | |
ba9703b0 | 524 | |
29967ef6 XL |
525 | StatementKind::SetDiscriminant { ref place, variant_index: _ } => { |
526 | self.visit_lhs(place, location); | |
527 | } | |
528 | } | |
ba9703b0 | 529 | } |
ba9703b0 | 530 | |
cdc7bbd5 | 531 | fn visit_local(&mut self, local: &Local, _ctx: PlaceContext, _location: Location) { |
29967ef6 XL |
532 | if self.increment { |
533 | self.use_count[*local] += 1; | |
534 | } else { | |
535 | assert_ne!(self.use_count[*local], 0); | |
536 | self.use_count[*local] -= 1; | |
537 | } | |
e74abb32 | 538 | } |
29967ef6 | 539 | } |
e74abb32 | 540 | |
29967ef6 | 541 | /// Removes unused definitions. Updates the used locals to reflect the changes made. |
cdc7bbd5 | 542 | fn remove_unused_definitions<'a, 'tcx>(used_locals: &'a mut UsedLocals, body: &mut Body<'tcx>) { |
29967ef6 XL |
543 | // The use counts are updated as we remove the statements. A local might become unused |
544 | // during the retain operation, leading to a temporary inconsistency (storage statements or | |
545 | // definitions referencing the local might remain). For correctness it is crucial that this | |
546 | // computation reaches a fixed point. | |
547 | ||
548 | let mut modified = true; | |
549 | while modified { | |
550 | modified = false; | |
551 | ||
552 | for data in body.basic_blocks_mut() { | |
553 | // Remove unnecessary StorageLive and StorageDead annotations. | |
554 | data.statements.retain(|statement| { | |
555 | let keep = match &statement.kind { | |
556 | StatementKind::StorageLive(local) | StatementKind::StorageDead(local) => { | |
557 | used_locals.is_used(*local) | |
558 | } | |
559 | StatementKind::Assign(box (place, _)) => used_locals.is_used(place.local), | |
ba9703b0 | 560 | |
29967ef6 XL |
561 | StatementKind::SetDiscriminant { ref place, .. } => { |
562 | used_locals.is_used(place.local) | |
563 | } | |
564 | _ => true, | |
565 | }; | |
ba9703b0 | 566 | |
29967ef6 XL |
567 | if !keep { |
568 | trace!("removing statement {:?}", statement); | |
569 | modified = true; | |
570 | used_locals.statement_removed(statement); | |
571 | } | |
ba9703b0 | 572 | |
29967ef6 XL |
573 | keep |
574 | }); | |
575 | } | |
c30ab7b3 | 576 | } |
ba9703b0 | 577 | } |
e74abb32 | 578 | |
ba9703b0 XL |
579 | struct LocalUpdater<'tcx> { |
580 | map: IndexVec<Local, Option<Local>>, | |
581 | tcx: TyCtxt<'tcx>, | |
582 | } | |
583 | ||
584 | impl<'tcx> MutVisitor<'tcx> for LocalUpdater<'tcx> { | |
585 | fn tcx(&self) -> TyCtxt<'tcx> { | |
586 | self.tcx | |
c30ab7b3 | 587 | } |
e74abb32 | 588 | |
ba9703b0 XL |
589 | fn visit_local(&mut self, l: &mut Local, _: PlaceContext, _: Location) { |
590 | *l = self.map[*l].unwrap(); | |
e74abb32 | 591 | } |
c30ab7b3 | 592 | } |