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_data_structures::bit_set::BitSet;
34 use rustc_data_structures::indexed_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<'tcx>(
62 let num_live_vars = mir.local_decls.len();
64 let def_use: IndexVec<_, DefsUses> = mir
67 .map(|b| block(b, num_live_vars))
70 let mut outs: IndexVec<_, LiveVarSet> = mir
73 .map(|_| LiveVarSet::new_empty(num_live_vars))
76 let mut bits = LiveVarSet::new_empty(num_live_vars);
78 // queue of things that need to be re-processed, and a set containing
79 // the things currently in the queue
80 let mut dirty_queue: WorkQueue<BasicBlock> = WorkQueue::with_all(mir.basic_blocks().len());
82 let predecessors = mir.predecessors();
84 while let Some(bb) = dirty_queue.pop() {
85 // bits = use ∪ (bits - def)
86 bits.overwrite(&outs[bb]);
87 def_use[bb].apply(&mut bits);
89 // `bits` now contains the live variables on entry. Therefore,
90 // add `bits` to the `out` set for each predecessor; if those
91 // bits were not already present, then enqueue the predecessor
94 // (note that `union` returns true if the `self` set changed)
95 for &pred_bb in &predecessors[bb] {
96 if outs[pred_bb].union(&bits) {
97 dirty_queue.insert(pred_bb);
102 LivenessResult { outs }
105 #[derive(Eq, PartialEq, Clone)]
112 pub fn categorize<'tcx>(context: PlaceContext) -> Option<DefUse> {
114 ///////////////////////////////////////////////////////////////////////////
117 PlaceContext::MutatingUse(MutatingUseContext::Store) |
119 // This is potentially both a def and a use...
120 PlaceContext::MutatingUse(MutatingUseContext::AsmOutput) |
122 // We let Call define the result in both the success and
123 // unwind cases. This is not really correct, however it
124 // does not seem to be observable due to the way that we
125 // generate MIR. To do things properly, we would apply
126 // the def in call only to the input from the success
127 // path and not the unwind path. -nmatsakis
128 PlaceContext::MutatingUse(MutatingUseContext::Call) |
130 // Storage live and storage dead aren't proper defines, but we can ignore
131 // values that come before them.
132 PlaceContext::NonUse(NonUseContext::StorageLive) |
133 PlaceContext::NonUse(NonUseContext::StorageDead) => Some(DefUse::Def),
135 ///////////////////////////////////////////////////////////////////////////
138 // These are uses that occur *outside* of a drop. For the
139 // purposes of NLL, these are special in that **all** the
140 // lifetimes appearing in the variable must be live for each regular use.
142 PlaceContext::NonMutatingUse(NonMutatingUseContext::Projection) |
143 PlaceContext::MutatingUse(MutatingUseContext::Projection) |
145 // Borrows only consider their local used at the point of the borrow.
146 // This won't affect the results since we use this analysis for generators
147 // and we only care about the result at suspension points. Borrows cannot
148 // cross suspension points so this behavior is unproblematic.
149 PlaceContext::MutatingUse(MutatingUseContext::Borrow) |
150 PlaceContext::NonMutatingUse(NonMutatingUseContext::SharedBorrow) |
151 PlaceContext::NonMutatingUse(NonMutatingUseContext::ShallowBorrow) |
152 PlaceContext::NonMutatingUse(NonMutatingUseContext::UniqueBorrow) |
154 PlaceContext::NonMutatingUse(NonMutatingUseContext::Inspect) |
155 PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy) |
156 PlaceContext::NonMutatingUse(NonMutatingUseContext::Move) |
157 PlaceContext::NonUse(NonUseContext::AscribeUserTy) |
158 PlaceContext::MutatingUse(MutatingUseContext::Retag) =>
161 ///////////////////////////////////////////////////////////////////////////
164 // These are uses that occur in a DROP (a MIR drop, not a
165 // call to `std::mem::drop()`). For the purposes of NLL,
166 // uses in drop are special because `#[may_dangle]`
167 // attributes can affect whether lifetimes must be live.
169 PlaceContext::MutatingUse(MutatingUseContext::Drop) =>
174 struct DefsUsesVisitor
179 #[derive(Eq, PartialEq, Clone)]
186 fn apply(&self, bits: &mut LiveVarSet) -> bool {
187 bits.subtract(&self.defs) | bits.union(&self.uses)
190 fn add_def(&mut self, index: Local) {
191 // If it was used already in the block, remove that use
192 // now that we found a definition.
196 // // Defs = {X}, Uses = {}
198 // // Defs = {}, Uses = {X}
200 self.uses.remove(index);
201 self.defs.insert(index);
204 fn add_use(&mut self, index: Local) {
209 // // Defs = {}, Uses = {X}
211 // // Defs = {X}, Uses = {}
213 // // Defs = {}, Uses = {X}
215 self.defs.remove(index);
216 self.uses.insert(index);
220 impl<'tcx> Visitor<'tcx> for DefsUsesVisitor
222 fn visit_local(&mut self, &local: &Local, context: PlaceContext, _: Location) {
223 match categorize(context) {
224 Some(DefUse::Def) => self.defs_uses.add_def(local),
225 Some(DefUse::Use) | Some(DefUse::Drop) => self.defs_uses.add_use(local),
232 b: &BasicBlockData<'tcx>,
235 let mut visitor = DefsUsesVisitor {
236 defs_uses: DefsUses {
237 defs: LiveVarSet::new_empty(locals),
238 uses: LiveVarSet::new_empty(locals),
242 let dummy_location = Location {
243 block: BasicBlock::new(0),
247 // Visit the various parts of the basic block in reverse. If we go
248 // forward, the logic in `add_def` and `add_use` would be wrong.
249 visitor.visit_terminator(b.terminator(), dummy_location);
250 for statement in b.statements.iter().rev() {
251 visitor.visit_statement(statement, dummy_location);
257 pub fn dump_mir<'a, 'tcx>(
258 tcx: TyCtxt<'a, 'tcx, 'tcx>,
260 source: MirSource<'tcx>,
262 result: &LivenessResult,
264 if !dump_enabled(tcx, pass_name, source) {
267 let node_path = ty::print::with_forced_impl_filename_line(|| {
268 // see notes on #41697 below
269 tcx.def_path_str(source.def_id())
271 dump_matched_mir_node(tcx, pass_name, &node_path, source, mir, result);
274 fn dump_matched_mir_node<'a, 'tcx>(
275 tcx: TyCtxt<'a, 'tcx, 'tcx>,
278 source: MirSource<'tcx>,
280 result: &LivenessResult,
282 let mut file_path = PathBuf::new();
283 file_path.push(Path::new(&tcx.sess.opts.debugging_opts.dump_mir_dir));
284 let item_id = tcx.hir().as_local_hir_id(source.def_id()).unwrap();
285 let file_name = format!("rustc.node{}{}-liveness.mir", item_id, pass_name);
286 file_path.push(&file_name);
287 let _ = fs::File::create(&file_path).and_then(|mut file| {
288 writeln!(file, "// MIR local liveness analysis for `{}`", node_path)?;
289 writeln!(file, "// source = {:?}", source)?;
290 writeln!(file, "// pass_name = {}", pass_name)?;
292 write_mir_fn(tcx, source, mir, &mut file, result)?;
297 pub fn write_mir_fn<'a, 'tcx>(
298 tcx: TyCtxt<'a, 'tcx, 'tcx>,
299 src: MirSource<'tcx>,
302 result: &LivenessResult,
303 ) -> io::Result<()> {
304 write_mir_intro(tcx, src, mir, w)?;
305 for block in mir.basic_blocks().indices() {
306 let print = |w: &mut dyn Write, prefix, result: &IndexVec<BasicBlock, LiveVarSet>| {
307 let live: Vec<String> = result[block]
309 .map(|local| format!("{:?}", local))
311 writeln!(w, "{} {{{}}}", prefix, live.join(", "))
313 write_basic_block(tcx, block, mir, &mut |_, _| Ok(()), w)?;
314 print(w, " ", &result.outs)?;
315 if block.index() + 1 != mir.basic_blocks().len() {