Limit number of tracked places, and some other perf improvements

Author: jachris

compiler/rustc_mir_dataflow/src/value_analysis.rs

@@ -47,7 +47,7 @@
 
 use std::fmt::{Debug, Formatter};
 
-use rustc_data_structures::fx::FxHashMap;
+use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_index::vec::IndexVec;
 use rustc_middle::mir::tcx::PlaceTy;
 use rustc_middle::mir::visit::{PlaceContext, Visitor};
@@ -405,12 +405,31 @@ rustc_index::newtype_index!(
 );
 
 /// See [`State`].
-#[derive(PartialEq, Eq, Clone, Debug)]
+#[derive(PartialEq, Eq, Debug)]
 enum StateData<V> {
     Reachable(IndexVec<ValueIndex, V>),
     Unreachable,
 }
 
+impl<V: Clone> Clone for StateData<V> {
+    fn clone(&self) -> Self {
+        match self {
+            Self::Reachable(x) => Self::Reachable(x.clone()),
+            Self::Unreachable => Self::Unreachable,
+        }
+    }
+
+    fn clone_from(&mut self, source: &Self) {
+        match (&mut *self, source) {
+            (Self::Reachable(x), Self::Reachable(y)) => {
+                // We go through `raw` here, because `IndexVec` currently has a naive `clone_from`.
+                x.raw.clone_from(&y.raw);
+            }
+            _ => *self = source.clone(),
+        }
+    }
+}
+
 /// The dataflow state for an instance of [`ValueAnalysis`].
 ///
 /// Every instance specifies a lattice that represents the possible values of a single tracked
@@ -421,9 +440,19 @@ enum StateData<V> {
 /// reachable state). All operations on unreachable states are ignored.
 ///
 /// Flooding means assigning a value (by default `⊤`) to all tracked projections of a given place.
-#[derive(PartialEq, Eq, Clone, Debug)]
+#[derive(PartialEq, Eq, Debug)]
 pub struct State<V>(StateData<V>);
 
+impl<V: Clone> Clone for State<V> {
+    fn clone(&self) -> Self {
+        Self(self.0.clone())
+    }
+
+    fn clone_from(&mut self, source: &Self) {
+        self.0.clone_from(&source.0);
+    }
+}
+
 impl<V: Clone + HasTop + HasBottom> State<V> {
     pub fn is_reachable(&self) -> bool {
         matches!(&self.0, StateData::Reachable(_))
@@ -590,6 +619,7 @@ impl Map {
     ///
     /// This is currently the only way to create a [`Map`]. The way in which the tracked places are
     /// chosen is an implementation detail and may not be relied upon.
+    #[instrument(skip_all, level = "debug")]
     pub fn from_filter<'tcx>(
         tcx: TyCtxt<'tcx>,
         body: &Body<'tcx>,
@@ -604,11 +634,12 @@ impl Map {
         if tcx.sess.opts.unstable_opts.unsound_mir_opts {
             // We might want to add additional limitations. If a struct has 10 boxed fields of
             // itself, there will currently be `10.pow(max_derefs)` tracked places.
-            map.register_with_filter(tcx, body, 2, filter, &[]);
+            map.register_with_filter(tcx, body, 2, filter, &FxHashSet::default());
         } else {
             map.register_with_filter(tcx, body, 0, filter, &escaped_places(body));
         }
 
+        debug!("registered {} places ({} nodes in total)", map.value_count, map.places.len());
         map
     }
 
@@ -619,7 +650,7 @@ impl Map {
         body: &Body<'tcx>,
         max_derefs: u32,
         mut filter: impl FnMut(Ty<'tcx>) -> bool,
-        exclude: &[Place<'tcx>],
+        exclude: &FxHashSet<Place<'tcx>>,
     ) {
         // This is used to tell whether a type is `!Freeze`.
         let param_env = tcx.param_env_reveal_all_normalized(body.source.def_id());
@@ -648,10 +679,10 @@ impl Map {
         ty: Ty<'tcx>,
         filter: &mut impl FnMut(Ty<'tcx>) -> bool,
         param_env: ty::ParamEnv<'tcx>,
-        exclude: &[Place<'tcx>],
+        exclude: &FxHashSet<Place<'tcx>>,
     ) {
-        // This currently does a linear scan, could be improved.
         if exclude.contains(&Place { local, projection: tcx.intern_place_elems(projection) }) {
+            // This will also exclude all projections of the excluded place.
             return;
         }
 
@@ -764,6 +795,10 @@ impl Map {
         Ok(())
     }
 
+    pub fn tracked_places(&self) -> usize {
+        self.value_count
+    }
+
     pub fn apply(&self, place: PlaceIndex, elem: TrackElem) -> Option<PlaceIndex> {
         self.projections.get(&(place, elem)).copied()
     }
@@ -929,20 +964,20 @@ fn iter_fields<'tcx>(
 /// Returns all places, that have their reference or address taken.
 ///
 /// This includes shared references.
-fn escaped_places<'tcx>(body: &Body<'tcx>) -> Vec<Place<'tcx>> {
+fn escaped_places<'tcx>(body: &Body<'tcx>) -> FxHashSet<Place<'tcx>> {
     struct Collector<'tcx> {
-        result: Vec<Place<'tcx>>,
+        result: FxHashSet<Place<'tcx>>,
     }
 
     impl<'tcx> Visitor<'tcx> for Collector<'tcx> {
         fn visit_place(&mut self, place: &Place<'tcx>, context: PlaceContext, _location: Location) {
             if context.is_borrow() || context.is_address_of() {
-                self.result.push(*place);
+                self.result.insert(*place);
             }
         }
     }
 
-    let mut collector = Collector { result: Vec::new() };
+    let mut collector = Collector { result: FxHashSet::default() };
     collector.visit_body(body);
     collector.result
 }

compiler/rustc_mir_transform/src/dataflow_const_prop.rs

@@ -15,25 +15,42 @@ use rustc_span::DUMMY_SP;
 
 use crate::MirPass;
 
+const TRACKING_LIMIT: usize = 1000;
+
 pub struct DataflowConstProp;
 
 impl<'tcx> MirPass<'tcx> for DataflowConstProp {
     fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
         sess.mir_opt_level() >= 1
     }
 
+    #[instrument(skip_all level = "debug")]
     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
         // Decide which places to track during the analysis.
         let map = Map::from_filter(tcx, body, Ty::is_scalar);
 
+        // We want to have a somewhat linear runtime w.r.t. the number of statements/terminators.
+        // Let's call this number `n`. Dataflow analysis has `O(h*n)` transfer function
+        // applications, where `h` is the height of the lattice. Because the height of our lattice
+        // is linear w.r.t. the number of tracked places, this is `O(tracked_places * n)`. However,
+        // because every transfer function application could traverse the whole map, this becomes
+        // `O(num_nodes * tracked_places * n)` in terms of time complexity. Since the number of
+        // map nodes is strongly correlated to the number of tracked places, this becomes more or
+        // less `O(n)` if we place a constant limit on the number of tracked places.
+        if map.tracked_places() > TRACKING_LIMIT {
+            debug!("aborted dataflow const prop due to too many tracked places");
+            return;
+        }
+
         // Perform the actual dataflow analysis.
         let analysis = ConstAnalysis::new(tcx, body, map);
-        let results = analysis.wrap().into_engine(tcx, body).iterate_to_fixpoint();
+        let results = debug_span!("analyze")
+            .in_scope(|| analysis.wrap().into_engine(tcx, body).iterate_to_fixpoint());
 
         // Collect results and patch the body afterwards.
         let mut visitor = CollectAndPatch::new(tcx, &results.analysis.0.map);
-        results.visit_reachable_with(body, &mut visitor);
-        visitor.visit_body(body);
+        debug_span!("collect").in_scope(|| results.visit_reachable_with(body, &mut visitor));
+        debug_span!("patch").in_scope(|| visitor.visit_body(body));
     }
 }