src/librustc_mir/util/liveness.rs

   1 //! Liveness analysis which computes liveness of MIR local variables at the boundary of basic
   2 //! blocks.
   3 //!
   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
   6 //! already exist:
   7 //!
   8 //! ```rust
   9 //! fn foo() {
  10 //!     x = 0;
  11 //!     // `x` is live here ...
  12 //!     GLOBAL = &x: *const u32;
  13 //!     // ... but not here, even while it can be accessed through `GLOBAL`.
  14 //!     foo();
  15 //!     x = 1;
  16 //!     // `x` is live again here, because it is assigned to `OTHER_GLOBAL`.
  17 //!     OTHER_GLOBAL = &x: *const u32;
  18 //!     // ...
  19 //! }
  20 //! ```
  21 //!
  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
  25 //! doesn't matter).
  26
  27 use rustc::mir::visit::{
  28     PlaceContext, Visitor, MutatingUseContext, NonMutatingUseContext, NonUseContext,
  29 };
  30 use rustc::mir::Local;
  31 use rustc::mir::*;
  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;
  36 use std::fs;
  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};
  41
  42 pub type LiveVarSet = BitSet<Local>;
  43
  44 /// This gives the result of the liveness analysis at the boundary of
  45 /// basic blocks.
  46 ///
  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>,
  55 }
  56
  57 /// Computes which local variables are live within the given function
  58 /// `mir`, including drops.
  59 pub fn liveness_of_locals<'tcx>(
  60     body: &Body<'tcx>,
  61 ) -> LivenessResult {
  62     let num_live_vars = body.local_decls.len();
  63
  64     let def_use: IndexVec<_, DefsUses> = body
  65         .basic_blocks()
  66         .iter()
  67         .map(|b| block(b, num_live_vars))
  68         .collect();
  69
  70     let mut outs: IndexVec<_, LiveVarSet> = body
  71         .basic_blocks()
  72         .indices()
  73         .map(|_| LiveVarSet::new_empty(num_live_vars))
  74         .collect();
  75
  76     let mut bits = LiveVarSet::new_empty(num_live_vars);
  77
  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.
  82     //
  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> = WorkQueue::with_none(body.basic_blocks().len());
  87     for (bb, _) in traversal::postorder(body) {
  88         dirty_queue.insert(bb);
  89     }
  90
  91     // Add blocks which are not reachable from START_BLOCK to the work queue. These blocks will
  92     // be processed after the ones added above.
  93     for bb in body.basic_blocks().indices() {
  94         dirty_queue.insert(bb);
  95     }
  96
  97     let predecessors = body.predecessors();
  98
  99     while let Some(bb) = dirty_queue.pop() {
 100         // bits = use ∪ (bits - def)
 101         bits.overwrite(&outs[bb]);
 102         def_use[bb].apply(&mut bits);
 103
 104         // `bits` now contains the live variables on entry. Therefore,
 105         // add `bits` to the `out` set for each predecessor; if those
 106         // bits were not already present, then enqueue the predecessor
 107         // as dirty.
 108         //
 109         // (note that `union` returns true if the `self` set changed)
 110         for &pred_bb in &predecessors[bb] {
 111             if outs[pred_bb].union(&bits) {
 112                 dirty_queue.insert(pred_bb);
 113             }
 114         }
 115     }
 116
 117     LivenessResult { outs }
 118 }
 119
 120 #[derive(Eq, PartialEq, Clone)]
 121 pub enum DefUse {
 122     Def,
 123     Use,
 124     Drop,
 125 }
 126
 127 pub fn categorize(context: PlaceContext) -> Option<DefUse> {
 128     match context {
 129         ///////////////////////////////////////////////////////////////////////////
 130         // DEFS
 131
 132         PlaceContext::MutatingUse(MutatingUseContext::Store) |
 133
 134         // This is potentially both a def and a use...
 135         PlaceContext::MutatingUse(MutatingUseContext::AsmOutput) |
 136
 137         // We let Call define the result in both the success and
 138         // unwind cases. This is not really correct, however it
 139         // does not seem to be observable due to the way that we
 140         // generate MIR. To do things properly, we would apply
 141         // the def in call only to the input from the success
 142         // path and not the unwind path. -nmatsakis
 143         PlaceContext::MutatingUse(MutatingUseContext::Call) |
 144
 145         // Storage live and storage dead aren't proper defines, but we can ignore
 146         // values that come before them.
 147         PlaceContext::NonUse(NonUseContext::StorageLive) |
 148         PlaceContext::NonUse(NonUseContext::StorageDead) => Some(DefUse::Def),
 149
 150         ///////////////////////////////////////////////////////////////////////////
 151         // REGULAR USES
 152         //
 153         // These are uses that occur *outside* of a drop. For the
 154         // purposes of NLL, these are special in that **all** the
 155         // lifetimes appearing in the variable must be live for each regular use.
 156
 157         PlaceContext::NonMutatingUse(NonMutatingUseContext::Projection) |
 158         PlaceContext::MutatingUse(MutatingUseContext::Projection) |
 159
 160         // Borrows only consider their local used at the point of the borrow.
 161         // This won't affect the results since we use this analysis for generators
 162         // and we only care about the result at suspension points. Borrows cannot
 163         // cross suspension points so this behavior is unproblematic.
 164         PlaceContext::MutatingUse(MutatingUseContext::Borrow) |
 165         PlaceContext::NonMutatingUse(NonMutatingUseContext::SharedBorrow) |
 166         PlaceContext::NonMutatingUse(NonMutatingUseContext::ShallowBorrow) |
 167         PlaceContext::NonMutatingUse(NonMutatingUseContext::UniqueBorrow) |
 168
 169         PlaceContext::NonMutatingUse(NonMutatingUseContext::Inspect) |
 170         PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy) |
 171         PlaceContext::NonMutatingUse(NonMutatingUseContext::Move) |
 172         PlaceContext::NonUse(NonUseContext::AscribeUserTy) |
 173         PlaceContext::MutatingUse(MutatingUseContext::Retag) =>
 174             Some(DefUse::Use),
 175
 176         ///////////////////////////////////////////////////////////////////////////
 177         // DROP USES
 178         //
 179         // These are uses that occur in a DROP (a MIR drop, not a
 180         // call to `std::mem::drop()`). For the purposes of NLL,
 181         // uses in drop are special because `#[may_dangle]`
 182         // attributes can affect whether lifetimes must be live.
 183
 184         PlaceContext::MutatingUse(MutatingUseContext::Drop) =>
 185             Some(DefUse::Drop),
 186     }
 187 }
 188
 189 struct DefsUsesVisitor
 190 {
 191     defs_uses: DefsUses,
 192 }
 193
 194 #[derive(Eq, PartialEq, Clone)]
 195 struct DefsUses {
 196     defs: LiveVarSet,
 197     uses: LiveVarSet,
 198 }
 199
 200 impl DefsUses {
 201     fn apply(&self, bits: &mut LiveVarSet) -> bool {
 202         bits.subtract(&self.defs) | bits.union(&self.uses)
 203     }
 204
 205     fn add_def(&mut self, index: Local) {
 206         // If it was used already in the block, remove that use
 207         // now that we found a definition.
 208         //
 209         // Example:
 210         //
 211         //     // Defs = {X}, Uses = {}
 212         //     X = 5
 213         //     // Defs = {}, Uses = {X}
 214         //     use(X)
 215         self.uses.remove(index);
 216         self.defs.insert(index);
 217     }
 218
 219     fn add_use(&mut self, index: Local) {
 220         // Inverse of above.
 221         //
 222         // Example:
 223         //
 224         //     // Defs = {}, Uses = {X}
 225         //     use(X)
 226         //     // Defs = {X}, Uses = {}
 227         //     X = 5
 228         //     // Defs = {}, Uses = {X}
 229         //     use(X)
 230         self.defs.remove(index);
 231         self.uses.insert(index);
 232     }
 233 }
 234
 235 impl<'tcx> Visitor<'tcx> for DefsUsesVisitor
 236 {
 237     fn visit_local(&mut self, &local: &Local, context: PlaceContext, _: Location) {
 238         match categorize(context) {
 239             Some(DefUse::Def) => self.defs_uses.add_def(local),
 240             Some(DefUse::Use) | Some(DefUse::Drop) => self.defs_uses.add_use(local),
 241             _ => (),
 242         }
 243     }
 244 }
 245
 246 fn block<'tcx>(
 247     b: &BasicBlockData<'tcx>,
 248     locals: usize,
 249 ) -> DefsUses {
 250     let mut visitor = DefsUsesVisitor {
 251         defs_uses: DefsUses {
 252             defs: LiveVarSet::new_empty(locals),
 253             uses: LiveVarSet::new_empty(locals),
 254         },
 255     };
 256
 257     let dummy_location = Location {
 258         block: BasicBlock::new(0),
 259         statement_index: 0,
 260     };
 261
 262     // Visit the various parts of the basic block in reverse. If we go
 263     // forward, the logic in `add_def` and `add_use` would be wrong.
 264     visitor.visit_terminator(b.terminator(), dummy_location);
 265     for statement in b.statements.iter().rev() {
 266         visitor.visit_statement(statement, dummy_location);
 267     }
 268
 269     visitor.defs_uses
 270 }
 271
 272 pub fn dump_mir<'tcx>(
 273     tcx: TyCtxt<'tcx>,
 274     pass_name: &str,
 275     source: MirSource<'tcx>,
 276     body: &Body<'tcx>,
 277     result: &LivenessResult,
 278 ) {
 279     if !dump_enabled(tcx, pass_name, source) {
 280         return;
 281     }
 282     let node_path = ty::print::with_forced_impl_filename_line(|| {
 283         // see notes on #41697 below
 284         tcx.def_path_str(source.def_id())
 285     });
 286     dump_matched_mir_node(tcx, pass_name, &node_path, source, body, result);
 287 }
 288
 289 fn dump_matched_mir_node<'tcx>(
 290     tcx: TyCtxt<'tcx>,
 291     pass_name: &str,
 292     node_path: &str,
 293     source: MirSource<'tcx>,
 294     body: &Body<'tcx>,
 295     result: &LivenessResult,
 296 ) {
 297     let mut file_path = PathBuf::new();
 298     file_path.push(Path::new(&tcx.sess.opts.debugging_opts.dump_mir_dir));
 299     let item_id = tcx.hir().as_local_hir_id(source.def_id()).unwrap();
 300     let file_name = format!("rustc.node{}{}-liveness.mir", item_id, pass_name);
 301     file_path.push(&file_name);
 302     let _ = fs::File::create(&file_path).and_then(|mut file| {
 303         writeln!(file, "// MIR local liveness analysis for `{}`", node_path)?;
 304         writeln!(file, "// source = {:?}", source)?;
 305         writeln!(file, "// pass_name = {}", pass_name)?;
 306         writeln!(file, "")?;
 307         write_mir_fn(tcx, source, body, &mut file, result)?;
 308         Ok(())
 309     });
 310 }
 311
 312 pub fn write_mir_fn<'tcx>(
 313     tcx: TyCtxt<'tcx>,
 314     src: MirSource<'tcx>,
 315     body: &Body<'tcx>,
 316     w: &mut dyn Write,
 317     result: &LivenessResult,
 318 ) -> io::Result<()> {
 319     write_mir_intro(tcx, src, body, w)?;
 320     for block in body.basic_blocks().indices() {
 321         let print = |w: &mut dyn Write, prefix, result: &IndexVec<BasicBlock, LiveVarSet>| {
 322             let live: Vec<String> = result[block]
 323                 .iter()
 324                 .map(|local| format!("{:?}", local))
 325                 .collect();
 326             writeln!(w, "{} {{{}}}", prefix, live.join(", "))
 327         };
 328         write_basic_block(tcx, block, body, &mut |_, _| Ok(()), w)?;
 329         print(w, "   ", &result.outs)?;
 330         if block.index() + 1 != body.basic_blocks().len() {
 331             writeln!(w, "")?;
 332         }
 333     }
 334
 335     writeln!(w, "}}")?;
 336     Ok(())
 337 }