1 //! Liveness analysis which computes liveness of MIR local variables at the boundary of basic
4 //! This analysis considers references as being used only at the point of the
5 //! borrow. This means that this does not track uses because of references that
11 //! // `x` is live here ...
12 //! GLOBAL = &x: *const u32;
13 //! // ... but not here, even while it can be accessed through `GLOBAL`.
16 //! // `x` is live again here, because it is assigned to `OTHER_GLOBAL`.
17 //! OTHER_GLOBAL = &x: *const u32;
22 //! This means that users of this analysis still have to check whether
23 //! pre-existing references can be used to access the value (e.g., at movable
24 //! generator yield points, all pre-existing references are invalidated, so this
27 use rustc::mir::visit::{
28 PlaceContext, Visitor, MutatingUseContext, NonMutatingUseContext, NonUseContext,
30 use rustc::mir::Local;
32 use rustc::ty::{self, TyCtxt};
33 use rustc_index::bit_set::BitSet;
34 use rustc_index::vec::{Idx, IndexVec};
35 use rustc_data_structures::work_queue::WorkQueue;
37 use std::io::{self, Write};
38 use std::path::{Path, PathBuf};
39 use crate::transform::MirSource;
40 use crate::util::pretty::{dump_enabled, write_basic_block, write_mir_intro};
42 pub type LiveVarSet = BitSet<Local>;
44 /// This gives the result of the liveness analysis at the boundary of
47 /// The `V` type defines the set of variables that we computed
48 /// liveness for. This is often `Local`, in which case we computed
49 /// liveness for all variables -- but it can also be some other type,
50 /// which indicates a subset of the variables within the graph.
51 pub struct LivenessResult {
52 /// Live variables on exit to each basic block. This is equal to
53 /// the union of the `ins` for each successor.
54 pub outs: IndexVec<BasicBlock, LiveVarSet>,
57 /// Computes which local variables are live within the given function
58 /// `mir`, including drops.
59 pub fn liveness_of_locals(
60 body: ReadOnlyBodyAndCache<'_, '_>,
62 let num_live_vars = body.local_decls.len();
64 let def_use: IndexVec<_, DefsUses> = body
67 .map(|b| block(b, num_live_vars))
70 let mut outs: IndexVec<_, LiveVarSet> = body
73 .map(|_| LiveVarSet::new_empty(num_live_vars))
76 let mut bits = LiveVarSet::new_empty(num_live_vars);
78 // The dirty queue contains the set of basic blocks whose entry sets have changed since they
79 // were last processed. At the start of the analysis, we initialize the queue in post-order to
80 // make it more likely that the entry set for a given basic block will have the effects of all
81 // its successors in the CFG applied before it is processed.
83 // FIXME(ecstaticmorse): Reverse post-order on the reverse CFG may generate a better iteration
84 // order when cycles are present, but the overhead of computing the reverse CFG may outweigh
85 // any benefits. Benchmark this and find out.
86 let mut dirty_queue: WorkQueue<BasicBlock>
87 = WorkQueue::with_none(body.basic_blocks().len());
88 for (bb, _) in traversal::postorder(&body) {
89 dirty_queue.insert(bb);
92 // Add blocks which are not reachable from START_BLOCK to the work queue. These blocks will
93 // be processed after the ones added above.
94 for bb in body.basic_blocks().indices() {
95 dirty_queue.insert(bb);
98 let predecessors = body.predecessors();
100 while let Some(bb) = dirty_queue.pop() {
101 // bits = use ∪ (bits - def)
102 bits.overwrite(&outs[bb]);
103 def_use[bb].apply(&mut bits);
105 // `bits` now contains the live variables on entry. Therefore,
106 // add `bits` to the `out` set for each predecessor; if those
107 // bits were not already present, then enqueue the predecessor
110 // (note that `union` returns true if the `self` set changed)
111 for &pred_bb in &predecessors[bb] {
112 if outs[pred_bb].union(&bits) {
113 dirty_queue.insert(pred_bb);
118 LivenessResult { outs }
121 #[derive(Eq, PartialEq, Clone)]
128 pub fn categorize(context: PlaceContext) -> Option<DefUse> {
130 ///////////////////////////////////////////////////////////////////////////
133 PlaceContext::MutatingUse(MutatingUseContext::Store) |
135 // This is potentially both a def and a use...
136 PlaceContext::MutatingUse(MutatingUseContext::AsmOutput) |
138 // We let Call define the result in both the success and
139 // unwind cases. This is not really correct, however it
140 // does not seem to be observable due to the way that we
141 // generate MIR. To do things properly, we would apply
142 // the def in call only to the input from the success
143 // path and not the unwind path. -nmatsakis
144 PlaceContext::MutatingUse(MutatingUseContext::Call) |
146 // Storage live and storage dead aren't proper defines, but we can ignore
147 // values that come before them.
148 PlaceContext::NonUse(NonUseContext::StorageLive) |
149 PlaceContext::NonUse(NonUseContext::StorageDead) => Some(DefUse::Def),
151 ///////////////////////////////////////////////////////////////////////////
154 // These are uses that occur *outside* of a drop. For the
155 // purposes of NLL, these are special in that **all** the
156 // lifetimes appearing in the variable must be live for each regular use.
158 PlaceContext::NonMutatingUse(NonMutatingUseContext::Projection) |
159 PlaceContext::MutatingUse(MutatingUseContext::Projection) |
161 // Borrows only consider their local used at the point of the borrow.
162 // This won't affect the results since we use this analysis for generators
163 // and we only care about the result at suspension points. Borrows cannot
164 // cross suspension points so this behavior is unproblematic.
165 PlaceContext::MutatingUse(MutatingUseContext::Borrow) |
166 PlaceContext::NonMutatingUse(NonMutatingUseContext::SharedBorrow) |
167 PlaceContext::NonMutatingUse(NonMutatingUseContext::ShallowBorrow) |
168 PlaceContext::NonMutatingUse(NonMutatingUseContext::UniqueBorrow) |
170 PlaceContext::MutatingUse(MutatingUseContext::AddressOf) |
171 PlaceContext::NonMutatingUse(NonMutatingUseContext::AddressOf) |
172 PlaceContext::NonMutatingUse(NonMutatingUseContext::Inspect) |
173 PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy) |
174 PlaceContext::NonMutatingUse(NonMutatingUseContext::Move) |
175 PlaceContext::NonUse(NonUseContext::AscribeUserTy) |
176 PlaceContext::MutatingUse(MutatingUseContext::Retag) =>
179 ///////////////////////////////////////////////////////////////////////////
182 // These are uses that occur in a DROP (a MIR drop, not a
183 // call to `std::mem::drop()`). For the purposes of NLL,
184 // uses in drop are special because `#[may_dangle]`
185 // attributes can affect whether lifetimes must be live.
187 PlaceContext::MutatingUse(MutatingUseContext::Drop) =>
190 // Debug info is neither def nor use.
191 PlaceContext::NonUse(NonUseContext::VarDebugInfo) => None,
195 struct DefsUsesVisitor
200 #[derive(Eq, PartialEq, Clone)]
207 fn apply(&self, bits: &mut LiveVarSet) -> bool {
208 bits.subtract(&self.defs) | bits.union(&self.uses)
211 fn add_def(&mut self, index: Local) {
212 // If it was used already in the block, remove that use
213 // now that we found a definition.
217 // // Defs = {X}, Uses = {}
219 // // Defs = {}, Uses = {X}
221 self.uses.remove(index);
222 self.defs.insert(index);
225 fn add_use(&mut self, index: Local) {
230 // // Defs = {}, Uses = {X}
232 // // Defs = {X}, Uses = {}
234 // // Defs = {}, Uses = {X}
236 self.defs.remove(index);
237 self.uses.insert(index);
241 impl<'tcx> Visitor<'tcx> for DefsUsesVisitor
243 fn visit_local(&mut self, &local: &Local, context: PlaceContext, _: Location) {
244 match categorize(context) {
245 Some(DefUse::Def) => self.defs_uses.add_def(local),
246 Some(DefUse::Use) | Some(DefUse::Drop) => self.defs_uses.add_use(local),
253 b: &BasicBlockData<'_>,
256 let mut visitor = DefsUsesVisitor {
257 defs_uses: DefsUses {
258 defs: LiveVarSet::new_empty(locals),
259 uses: LiveVarSet::new_empty(locals),
263 let dummy_location = Location {
264 block: BasicBlock::new(0),
268 // Visit the various parts of the basic block in reverse. If we go
269 // forward, the logic in `add_def` and `add_use` would be wrong.
270 visitor.visit_terminator(b.terminator(), dummy_location);
271 for statement in b.statements.iter().rev() {
272 visitor.visit_statement(statement, dummy_location);
278 pub fn dump_mir<'tcx>(
281 source: MirSource<'tcx>,
283 result: &LivenessResult,
285 if !dump_enabled(tcx, pass_name, source) {
288 let node_path = ty::print::with_forced_impl_filename_line(|| {
289 // see notes on #41697 below
290 tcx.def_path_str(source.def_id())
292 dump_matched_mir_node(tcx, pass_name, &node_path, source, body, result);
295 fn dump_matched_mir_node<'tcx>(
299 source: MirSource<'tcx>,
301 result: &LivenessResult,
303 let mut file_path = PathBuf::new();
304 file_path.push(Path::new(&tcx.sess.opts.debugging_opts.dump_mir_dir));
305 let item_id = tcx.hir().as_local_hir_id(source.def_id()).unwrap();
306 let file_name = format!("rustc.node{}{}-liveness.mir", item_id, pass_name);
307 file_path.push(&file_name);
308 let _ = fs::File::create(&file_path).and_then(|mut file| {
309 writeln!(file, "// MIR local liveness analysis for `{}`", node_path)?;
310 writeln!(file, "// source = {:?}", source)?;
311 writeln!(file, "// pass_name = {}", pass_name)?;
313 write_mir_fn(tcx, source, body, &mut file, result)?;
318 pub fn write_mir_fn<'tcx>(
320 src: MirSource<'tcx>,
323 result: &LivenessResult,
324 ) -> io::Result<()> {
325 write_mir_intro(tcx, src, body, w)?;
326 for block in body.basic_blocks().indices() {
327 let print = |w: &mut dyn Write, prefix, result: &IndexVec<BasicBlock, LiveVarSet>| {
328 let live: Vec<String> = result[block]
330 .map(|local| format!("{:?}", local))
332 writeln!(w, "{} {{{}}}", prefix, live.join(", "))
334 write_basic_block(tcx, block, body, &mut |_, _| Ok(()), w)?;
335 print(w, " ", &result.outs)?;
336 if block.index() + 1 != body.basic_blocks().len() {