Introduce MixedBitSet.

It just uses `BitSet` for small/medium sizes (<= 2048 bits) and
`ChunkedBitSet` for larger sizes. This is good because `ChunkedBitSet`
is slow and memory-hungry at smaller sizes.

compiler/rustc_mir_dataflow/src/framework/fmt.rs

@@ -4,7 +4,7 @@
 use std::fmt;
 
 use rustc_index::Idx;
-use rustc_index::bit_set::{BitSet, ChunkedBitSet};
+use rustc_index::bit_set::{BitSet, ChunkedBitSet, MixedBitSet};
 
 use super::lattice::MaybeReachable;
 
@@ -127,6 +127,26 @@ where
     }
 }
 
+impl<T, C> DebugWithContext<C> for MixedBitSet<T>
+where
+    T: Idx + DebugWithContext<C>,
+{
+    fn fmt_with(&self, ctxt: &C, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            MixedBitSet::Small(set) => set.fmt_with(ctxt, f),
+            MixedBitSet::Large(set) => set.fmt_with(ctxt, f),
+        }
+    }
+
+    fn fmt_diff_with(&self, old: &Self, ctxt: &C, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match (self, old) {
+            (MixedBitSet::Small(set), MixedBitSet::Small(old)) => set.fmt_diff_with(old, ctxt, f),
+            (MixedBitSet::Large(set), MixedBitSet::Large(old)) => set.fmt_diff_with(old, ctxt, f),
+            _ => panic!("MixedBitSet size mismatch"),
+        }
+    }
+}
+
 impl<S, C> DebugWithContext<C> for MaybeReachable<S>
 where
     S: DebugWithContext<C>,

compiler/rustc_mir_dataflow/src/framework/lattice.rs

@@ -40,7 +40,7 @@
 
 use std::iter;
 
-use rustc_index::bit_set::{BitSet, ChunkedBitSet};
+use rustc_index::bit_set::{BitSet, ChunkedBitSet, MixedBitSet};
 use rustc_index::{Idx, IndexVec};
 
 use crate::framework::BitSetExt;
@@ -132,6 +132,12 @@ impl<T: Idx> JoinSemiLattice for ChunkedBitSet<T> {
     }
 }
 
+impl<T: Idx> JoinSemiLattice for MixedBitSet<T> {
+    fn join(&mut self, other: &Self) -> bool {
+        self.union(other)
+    }
+}
+
 /// Extends a type `T` with top and bottom elements to make it a partially ordered set in which no
 /// value of `T` is comparable with any other.
 ///

compiler/rustc_mir_dataflow/src/framework/mod.rs

@@ -35,7 +35,7 @@
 use std::cmp::Ordering;
 
 use rustc_data_structures::work_queue::WorkQueue;
-use rustc_index::bit_set::{BitSet, ChunkedBitSet};
+use rustc_index::bit_set::{BitSet, ChunkedBitSet, MixedBitSet};
 use rustc_index::{Idx, IndexVec};
 use rustc_middle::bug;
 use rustc_middle::mir::{self, BasicBlock, CallReturnPlaces, Location, TerminatorEdges, traversal};
@@ -77,6 +77,12 @@ impl<T: Idx> BitSetExt<T> for ChunkedBitSet<T> {
     }
 }
 
+impl<T: Idx> BitSetExt<T> for MixedBitSet<T> {
+    fn contains(&self, elem: T) -> bool {
+        self.contains(elem)
+    }
+}
+
 /// A dataflow problem with an arbitrarily complex transfer function.
 ///
 /// This trait specifies the lattice on which this analysis operates (the domain), its
@@ -337,6 +343,16 @@ impl<T: Idx> GenKill<T> for ChunkedBitSet<T> {
     }
 }
 
+impl<T: Idx> GenKill<T> for MixedBitSet<T> {
+    fn gen_(&mut self, elem: T) {
+        self.insert(elem);
+    }
+
+    fn kill(&mut self, elem: T) {
+        self.remove(elem);
+    }
+}
+
 impl<T, S: GenKill<T>> GenKill<T> for MaybeReachable<S> {
     fn gen_(&mut self, elem: T) {
         match self {