[NLL] Remove LiveVar

src/librustc_mir/borrow_check/nll/mod.rs

@@ -2,7 +2,6 @@ use crate::borrow_check::borrow_set::BorrowSet;
 use crate::borrow_check::location::{LocationIndex, LocationTable};
 use crate::borrow_check::nll::facts::AllFactsExt;
 use crate::borrow_check::nll::type_check::{MirTypeckResults, MirTypeckRegionConstraints};
-use crate::borrow_check::nll::type_check::liveness::liveness_map::NllLivenessMap;
 use crate::borrow_check::nll::region_infer::values::RegionValueElements;
 use crate::dataflow::indexes::BorrowIndex;
 use crate::dataflow::move_paths::MoveData;

src/librustc_mir/borrow_check/nll/type_check/liveness/local_use_map.rs

@@ -1,6 +1,5 @@
 use crate::borrow_check::nll::region_infer::values::{PointIndex, RegionValueElements};
-use crate::borrow_check::nll::type_check::liveness::liveness_map::{LiveVar, NllLivenessMap};
-use crate::util::liveness::{categorize, DefUse, LiveVariableMap};
+use crate::util::liveness::{categorize, DefUse};
 use rustc::mir::visit::{PlaceContext, Visitor};
 use rustc::mir::{Local, Location, Mir};
 use rustc_data_structures::indexed_vec::{Idx, IndexVec};
@@ -9,26 +8,33 @@ use rustc_data_structures::vec_linked_list as vll;
 /// A map that cross references each local with the locations where it
 /// is defined (assigned), used, or dropped. Used during liveness
 /// computation.
-crate struct LocalUseMap<'me> {
-    liveness_map: &'me NllLivenessMap,
-
+///
+/// We keep track only of `Local`s we'll do the liveness analysis later,
+/// this means that our internal `IndexVec`s will only be sparsely populated.
+/// In the time-memory trade-off between keeping compact vectors with new
+/// indexes (and needing to continuously map the `Local` index to its compact
+/// counterpart) and having `IndexVec`s that we only use a fraction of, time
+/// (and code simplicity) was favored. The rationale is that we only keep
+/// a small number of `IndexVec`s throughout the entire analysis while, in
+/// contrast, we're accessing each `Local` *many* times.
+crate struct LocalUseMap {
     /// Head of a linked list of **definitions** of each variable --
     /// definition in this context means assignment, e.g., `x` is
     /// defined in `x = y` but not `y`; that first def is the head of
     /// a linked list that lets you enumerate all places the variable
     /// is assigned.
-    first_def_at: IndexVec<LiveVar, Option<AppearanceIndex>>,
+    first_def_at: IndexVec<Local, Option<AppearanceIndex>>,
 
     /// Head of a linked list of **uses** of each variable -- use in
     /// this context means that the existing value of the variable is
     /// read or modified. e.g., `y` is used in `x = y` but not `x`.
     /// Note that `DROP(x)` terminators are excluded from this list.
-    first_use_at: IndexVec<LiveVar, Option<AppearanceIndex>>,
+    first_use_at: IndexVec<Local, Option<AppearanceIndex>>,
 
     /// Head of a linked list of **drops** of each variable -- these
     /// are a special category of uses corresponding to the drop that
     /// we add for each local variable.
-    first_drop_at: IndexVec<LiveVar, Option<AppearanceIndex>>,
+    first_drop_at: IndexVec<Local, Option<AppearanceIndex>>,
 
     appearances: IndexVec<AppearanceIndex, Appearance>,
 }
@@ -50,52 +56,68 @@ impl vll::LinkElem for Appearance {
     }
 }
 
-impl LocalUseMap<'me> {
+impl LocalUseMap {
     crate fn build(
-        liveness_map: &'me NllLivenessMap,
+        live_locals: &Vec<Local>,
         elements: &RegionValueElements,
         mir: &Mir<'_>,
     ) -> Self {
-        let nones = IndexVec::from_elem_n(None, liveness_map.num_variables());
+        let nones = IndexVec::from_elem_n(None, mir.local_decls.len());
         let mut local_use_map = LocalUseMap {
-            liveness_map,
             first_def_at: nones.clone(),
             first_use_at: nones.clone(),
             first_drop_at: nones,
             appearances: IndexVec::new(),
         };
 
+        let mut locals_with_use_data: IndexVec<Local, bool> =
+            IndexVec::from_elem_n(false, mir.local_decls.len());
+        live_locals
+            .iter()
+            .for_each(|&local| locals_with_use_data[local] = true);
+
         LocalUseMapBuild {
             local_use_map: &mut local_use_map,
             elements,
-        }.visit_mir(mir);
+            locals_with_use_data,
+        }
+        .visit_mir(mir);
 
         local_use_map
     }
 
-    crate fn defs(&self, local: LiveVar) -> impl Iterator<Item = PointIndex> + '_ {
+    crate fn defs(&self, local: Local) -> impl Iterator<Item = PointIndex> + '_ {
         vll::iter(self.first_def_at[local], &self.appearances)
             .map(move |aa| self.appearances[aa].point_index)
     }
 
-    crate fn uses(&self, local: LiveVar) -> impl Iterator<Item = PointIndex> + '_ {
+    crate fn uses(&self, local: Local) -> impl Iterator<Item = PointIndex> + '_ {
         vll::iter(self.first_use_at[local], &self.appearances)
             .map(move |aa| self.appearances[aa].point_index)
     }
 
-    crate fn drops(&self, local: LiveVar) -> impl Iterator<Item = PointIndex> + '_ {
+    crate fn drops(&self, local: Local) -> impl Iterator<Item = PointIndex> + '_ {
         vll::iter(self.first_drop_at[local], &self.appearances)
             .map(move |aa| self.appearances[aa].point_index)
     }
 }
 
-struct LocalUseMapBuild<'me, 'map: 'me> {
-    local_use_map: &'me mut LocalUseMap<'map>,
+struct LocalUseMapBuild<'me> {
+    local_use_map: &'me mut LocalUseMap,
     elements: &'me RegionValueElements,
+
+    // Vector used in `visit_local` to signal which `Local`s do we need
+    // def/use/drop information on, constructed from `live_locals` (that
+    // contains the variables we'll do the liveness analysis for).
+    // This vector serves optimization purposes only: we could have
+    // obtained the same information from `live_locals` but we want to
+    // avoid repeatedly calling `Vec::contains()` (see `LocalUseMap` for
+    // the rationale on the time-memory trade-off we're favoring here).
+    locals_with_use_data: IndexVec<Local, bool>,
 }
 
-impl LocalUseMapBuild<'_, '_> {
-    fn insert_def(&mut self, local: LiveVar, location: Location) {
+impl LocalUseMapBuild<'_> {
+    fn insert_def(&mut self, local: Local, location: Location) {
         Self::insert(
             self.elements,
             &mut self.local_use_map.first_def_at[local],
@@ -104,7 +126,7 @@ impl LocalUseMapBuild<'_, '_> {
         );
     }
 
-    fn insert_use(&mut self, local: LiveVar, location: Location) {
+    fn insert_use(&mut self, local: Local, location: Location) {
         Self::insert(
             self.elements,
             &mut self.local_use_map.first_use_at[local],
@@ -113,7 +135,7 @@ impl LocalUseMapBuild<'_, '_> {
         );
     }
 
-    fn insert_drop(&mut self, local: LiveVar, location: Location) {
+    fn insert_drop(&mut self, local: Local, location: Location) {
         Self::insert(
             self.elements,
             &mut self.local_use_map.first_drop_at[local],
@@ -137,13 +159,13 @@ impl LocalUseMapBuild<'_, '_> {
     }
 }
 
-impl Visitor<'tcx> for LocalUseMapBuild<'_, '_> {
+impl Visitor<'tcx> for LocalUseMapBuild<'_> {
     fn visit_local(&mut self, &local: &Local, context: PlaceContext<'tcx>, location: Location) {
-        if let Some(local_with_region) = self.local_use_map.liveness_map.from_local(local) {
+        if self.locals_with_use_data[local] {
             match categorize(context) {
-                Some(DefUse::Def) => self.insert_def(local_with_region, location),
-                Some(DefUse::Use) => self.insert_use(local_with_region, location),
-                Some(DefUse::Drop) => self.insert_drop(local_with_region, location),
+                Some(DefUse::Def) => self.insert_def(local, location),
+                Some(DefUse::Use) => self.insert_use(local, location),
+                Some(DefUse::Drop) => self.insert_drop(local, location),
                 _ => (),
             }
         }

src/librustc_mir/borrow_check/nll/type_check/liveness/mod.rs

@@ -1,19 +1,19 @@
 use crate::borrow_check::location::LocationTable;
-use crate::borrow_check::nll::region_infer::values::RegionValueElements;
 use crate::borrow_check::nll::constraints::ConstraintSet;
-use crate::borrow_check::nll::NllLivenessMap;
+use crate::borrow_check::nll::facts::{AllFacts, AllFactsExt};
+use crate::borrow_check::nll::region_infer::values::RegionValueElements;
 use crate::borrow_check::nll::universal_regions::UniversalRegions;
+use crate::borrow_check::nll::ToRegionVid;
 use crate::dataflow::move_paths::MoveData;
-use crate::dataflow::MaybeInitializedPlaces;
 use crate::dataflow::FlowAtLocation;
-use rustc::mir::Mir;
-use rustc::ty::RegionVid;
+use crate::dataflow::MaybeInitializedPlaces;
+use rustc::mir::{Local, Mir};
+use rustc::ty::{RegionVid, TyCtxt};
 use rustc_data_structures::fx::FxHashSet;
 use std::rc::Rc;
 
 use super::TypeChecker;
 
-crate mod liveness_map;
 mod local_use_map;
 mod trace;
 
@@ -34,16 +34,71 @@ pub(super) fn generate<'gcx, 'tcx>(
     location_table: &LocationTable,
 ) {
     debug!("liveness::generate");
-    let free_regions = {
-        let borrowck_context = typeck.borrowck_context.as_ref().unwrap();
-        regions_that_outlive_free_regions(
-            typeck.infcx.num_region_vars(),
-            &borrowck_context.universal_regions,
-            &borrowck_context.constraints.outlives_constraints,
-        )
+
+    let live_locals: Vec<Local> = if AllFacts::enabled(typeck.tcx()) {
+        // If "dump facts from NLL analysis" was requested perform
+        // the liveness analysis for all `Local`s. This case opens
+        // the possibility of the variables being analyzed in `trace`
+        // to be *any* `Local`, not just the "live" ones, so we can't
+        // make any assumptions past this point as to the characteristics
+        // of the `live_locals`.
+        // FIXME: Review "live" terminology past this point, we should
+        // not be naming the `Local`s as live.
+        mir.local_decls.indices().collect()
+    } else {
+        let free_regions = {
+            let borrowck_context = typeck.borrowck_context.as_ref().unwrap();
+            regions_that_outlive_free_regions(
+                typeck.infcx.num_region_vars(),
+                &borrowck_context.universal_regions,
+                &borrowck_context.constraints.outlives_constraints,
+            )
+        };
+        compute_live_locals(typeck.tcx(), &free_regions, mir)
     };
-    let liveness_map = NllLivenessMap::compute(typeck.tcx(), &free_regions, mir);
-    trace::trace(typeck, mir, elements, flow_inits, move_data, &liveness_map, location_table);
+
+    if !live_locals.is_empty() {
+        trace::trace(
+            typeck,
+            mir,
+            elements,
+            flow_inits,
+            move_data,
+            live_locals,
+            location_table,
+        );
+    }
+}
+
+// The purpose of `compute_live_locals` is to define the subset of `Local`
+// variables for which we need to do a liveness computation. We only need
+// to compute whether a variable `X` is live if that variable contains
+// some region `R` in its type where `R` is not known to outlive a free
+// region (i.e., where `R` may be valid for just a subset of the fn body).
+fn compute_live_locals(
+    tcx: TyCtxt<'_, '_, 'tcx>,
+    free_regions: &FxHashSet<RegionVid>,
+    mir: &Mir<'tcx>,
+) -> Vec<Local> {
+    let live_locals: Vec<Local> = mir
+        .local_decls
+        .iter_enumerated()
+        .filter_map(|(local, local_decl)| {
+            if tcx.all_free_regions_meet(&local_decl.ty, |r| {
+                free_regions.contains(&r.to_region_vid())
+            }) {
+                None
+            } else {
+                Some(local)
+            }
+        })
+        .collect();
+
+    debug!("{} total variables", mir.local_decls.len());
+    debug!("{} variables need liveness", live_locals.len());
+    debug!("{} regions outlive free regions", free_regions.len());
+
+    live_locals
 }
 
 /// Computes all regions that are (currently) known to outlive free