Auto merge of #116152 - cjgillot:unchunck, r=nnethercote

Only use dense bitsets in dataflow analyses

When a dataflow state has the size close to the number of locals, we should prefer a dense bitset, like we already store locals in a dense vector.
Other occurrences of `ChunkedBitSet` need to be justified by the size of the dataflow state.

compiler/rustc_borrowck/src/type_check/liveness/trace.rs

@@ -1,6 +1,6 @@
 use rustc_data_structures::fx::{FxIndexMap, FxIndexSet};
 use rustc_data_structures::graph::WithSuccessors;
-use rustc_index::bit_set::HybridBitSet;
+use rustc_index::bit_set::BitSet;
 use rustc_index::interval::IntervalSet;
 use rustc_infer::infer::canonical::QueryRegionConstraints;
 use rustc_infer::infer::outlives::for_liveness;
@@ -135,7 +135,7 @@ struct LivenessResults<'me, 'typeck, 'flow, 'tcx> {
     cx: LivenessContext<'me, 'typeck, 'flow, 'tcx>,
 
     /// Set of points that define the current local.
-    defs: HybridBitSet<PointIndex>,
+    defs: BitSet<PointIndex>,
 
     /// Points where the current variable is "use live" -- meaning
     /// that there is a future "full use" that may use its value.
@@ -158,7 +158,7 @@ impl<'me, 'typeck, 'flow, 'tcx> LivenessResults<'me, 'typeck, 'flow, 'tcx> {
         let num_points = cx.elements.num_points();
         LivenessResults {
             cx,
-            defs: HybridBitSet::new_empty(num_points),
+            defs: BitSet::new_empty(num_points),
             use_live_at: IntervalSet::new(num_points),
             drop_live_at: IntervalSet::new(num_points),
             drop_locations: vec![],

compiler/rustc_index/src/bit_set.rs

@@ -284,7 +284,7 @@ impl<T: Idx> BitSet<T> {
         not_already
     }
 
-    fn last_set_in(&self, range: impl RangeBounds<T>) -> Option<T> {
+    pub fn last_set_in(&self, range: impl RangeBounds<T>) -> Option<T> {
         let (start, end) = inclusive_start_end(range, self.domain_size)?;
         let (start_word_index, _) = word_index_and_mask(start);
         let (end_word_index, end_mask) = word_index_and_mask(end);
@@ -1299,7 +1299,7 @@ impl<T: Idx> SparseBitSet<T> {
 }
 
 impl<T: Idx + Ord> SparseBitSet<T> {
-    fn last_set_in(&self, range: impl RangeBounds<T>) -> Option<T> {
+    pub fn last_set_in(&self, range: impl RangeBounds<T>) -> Option<T> {
         let mut last_leq = None;
         for e in self.iter() {
             if range.contains(e) {

compiler/rustc_mir_dataflow/src/impls/initialized.rs

@@ -305,7 +305,10 @@ impl<'a, 'tcx> DefinitelyInitializedPlaces<'a, 'tcx> {
 }
 
 impl<'tcx> AnalysisDomain<'tcx> for MaybeInitializedPlaces<'_, 'tcx> {
+    /// There can be many more `MovePathIndex` than there are locals in a MIR body.
+    /// We use a chunked bitset to avoid paying too high a memory footprint.
     type Domain = MaybeReachable<ChunkedBitSet<MovePathIndex>>;
+
     const NAME: &'static str = "maybe_init";
 
     fn bottom_value(&self, _: &mir::Body<'tcx>) -> Self::Domain {
@@ -437,6 +440,8 @@ impl<'tcx> GenKillAnalysis<'tcx> for MaybeInitializedPlaces<'_, 'tcx> {
 }
 
 impl<'tcx> AnalysisDomain<'tcx> for MaybeUninitializedPlaces<'_, 'tcx> {
+    /// There can be many more `MovePathIndex` than there are locals in a MIR body.
+    /// We use a chunked bitset to avoid paying too high a memory footprint.
     type Domain = ChunkedBitSet<MovePathIndex>;
 
     const NAME: &'static str = "maybe_uninit";
@@ -636,6 +641,8 @@ impl<'tcx> GenKillAnalysis<'tcx> for DefinitelyInitializedPlaces<'_, 'tcx> {
 }
 
 impl<'tcx> AnalysisDomain<'tcx> for EverInitializedPlaces<'_, 'tcx> {
+    /// There can be many more `InitIndex` than there are locals in a MIR body.
+    /// We use a chunked bitset to avoid paying too high a memory footprint.
     type Domain = ChunkedBitSet<InitIndex>;
 
     const NAME: &'static str = "ever_init";

compiler/rustc_mir_dataflow/src/impls/liveness.rs

@@ -1,4 +1,4 @@
-use rustc_index::bit_set::{BitSet, ChunkedBitSet};
+use rustc_index::bit_set::BitSet;
 use rustc_middle::mir::visit::{MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor};
 use rustc_middle::mir::{
     self, CallReturnPlaces, Local, Location, Place, StatementKind, TerminatorEdges,
@@ -26,14 +26,14 @@ use crate::{Analysis, AnalysisDomain, Backward, GenKill, GenKillAnalysis};
 pub struct MaybeLiveLocals;
 
 impl<'tcx> AnalysisDomain<'tcx> for MaybeLiveLocals {
-    type Domain = ChunkedBitSet<Local>;
+    type Domain = BitSet<Local>;
     type Direction = Backward;
 
     const NAME: &'static str = "liveness";
 
     fn bottom_value(&self, body: &mir::Body<'tcx>) -> Self::Domain {
         // bottom = not live
-        ChunkedBitSet::new_empty(body.local_decls.len())
+        BitSet::new_empty(body.local_decls.len())
     }
 
     fn initialize_start_block(&self, _: &mir::Body<'tcx>, _: &mut Self::Domain) {
@@ -233,14 +233,14 @@ impl<'a> MaybeTransitiveLiveLocals<'a> {
 }
 
 impl<'a, 'tcx> AnalysisDomain<'tcx> for MaybeTransitiveLiveLocals<'a> {
-    type Domain = ChunkedBitSet<Local>;
+    type Domain = BitSet<Local>;
     type Direction = Backward;
 
     const NAME: &'static str = "transitive liveness";
 
     fn bottom_value(&self, body: &mir::Body<'tcx>) -> Self::Domain {
         // bottom = not live
-        ChunkedBitSet::new_empty(body.local_decls.len())
+        BitSet::new_empty(body.local_decls.len())
     }
 
     fn initialize_start_block(&self, _: &mir::Body<'tcx>, _: &mut Self::Domain) {

compiler/rustc_mir_dataflow/src/rustc_peek.rs

@@ -12,7 +12,7 @@ use crate::MoveDataParamEnv;
 use crate::{Analysis, JoinSemiLattice, ResultsCursor};
 use rustc_ast::MetaItem;
 use rustc_hir::def_id::DefId;
-use rustc_index::bit_set::ChunkedBitSet;
+use rustc_index::bit_set::BitSet;
 use rustc_middle::mir::MirPass;
 use rustc_middle::mir::{self, Body, Local, Location};
 use rustc_middle::ty::{self, Ty, TyCtxt};
@@ -275,7 +275,7 @@ impl<'tcx> RustcPeekAt<'tcx> for MaybeLiveLocals {
         &self,
         tcx: TyCtxt<'tcx>,
         place: mir::Place<'tcx>,
-        flow_state: &ChunkedBitSet<Local>,
+        flow_state: &BitSet<Local>,
         call: PeekCall,
     ) {
         info!(?place, "peek_at");

compiler/rustc_mir_transform/src/nrvo.rs

@@ -1,7 +1,7 @@
 //! See the docs for [`RenameReturnPlace`].
 
 use rustc_hir::Mutability;
-use rustc_index::bit_set::HybridBitSet;
+use rustc_index::bit_set::BitSet;
 use rustc_middle::mir::visit::{MutVisitor, NonUseContext, PlaceContext, Visitor};
 use rustc_middle::mir::{self, BasicBlock, Local, Location};
 use rustc_middle::ty::TyCtxt;
@@ -123,7 +123,7 @@ fn find_local_assigned_to_return_place(
     body: &mut mir::Body<'_>,
 ) -> Option<Local> {
     let mut block = start;
-    let mut seen = HybridBitSet::new_empty(body.basic_blocks.len());
+    let mut seen = BitSet::new_empty(body.basic_blocks.len());
 
     // Iterate as long as `block` has exactly one predecessor that we have not yet visited.
     while seen.insert(block) {