From dd28c40c295778fc3c3cb945c443efc6ec86ee2e Mon Sep 17 00:00:00 2001
From: Nicholas Nethercote <n.nethercote@gmail.com>
Date: Thu, 5 Dec 2024 12:49:04 +1100
Subject: [PATCH] Use `BitSet` in `SparseBitMatrix`.

A `ChunkedBitSet` has to be at least 2048 bits for it to outperform a
`BitSet`, because that's the chunk size. The largest `SparseBitMatrix`
encountered when compiling the compiler and the entire rustc-perf
benchmark suite is less than 600 bits.

This change is a tiny perf win, but the motivation is more about
avoiding uses of `ChunkedBitSet` outside of `MixedBitSet`.

The test change is necessary to avoid hitting the `<BitSet<T> as
BitRelations<ChunkedBitSet<T>>>::subtract` method that has
`unimplemented!` in its body and isn't otherwise used.
---
 compiler/rustc_index/src/bit_set.rs       | 20 ++++++++++----------
 compiler/rustc_index/src/bit_set/tests.rs |  6 +++---
 2 files changed, 13 insertions(+), 13 deletions(-)

diff --git a/compiler/rustc_index/src/bit_set.rs b/compiler/rustc_index/src/bit_set.rs
index 41bd47ea6d06..aba1e9382969 100644
--- a/compiler/rustc_index/src/bit_set.rs
+++ b/compiler/rustc_index/src/bit_set.rs
@@ -1529,7 +1529,7 @@ fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
 /// sparse representation.
 ///
 /// Initially, every row has no explicit representation. If any bit within a
-/// row is set, the entire row is instantiated as `Some(<ChunkedBitSet>)`.
+/// row is set, the entire row is instantiated as `Some(<BitSet>)`.
 /// Furthermore, any previously uninstantiated rows prior to it will be
 /// instantiated as `None`. Those prior rows may themselves become fully
 /// instantiated later on if any of their bits are set.
@@ -1543,7 +1543,7 @@ pub struct SparseBitMatrix<R, C>
     C: Idx,
 {
     num_columns: usize,
-    rows: IndexVec<R, Option<ChunkedBitSet<C>>>,
+    rows: IndexVec<R, Option<BitSet<C>>>,
 }
 
 impl<R: Idx, C: Idx> SparseBitMatrix<R, C> {
@@ -1552,10 +1552,10 @@ pub fn new(num_columns: usize) -> Self {
         Self { num_columns, rows: IndexVec::new() }
     }
 
-    fn ensure_row(&mut self, row: R) -> &mut ChunkedBitSet<C> {
-        // Instantiate any missing rows up to and including row `row` with an empty ChunkedBitSet.
-        // Then replace row `row` with a full ChunkedBitSet if necessary.
-        self.rows.get_or_insert_with(row, || ChunkedBitSet::new_empty(self.num_columns))
+    fn ensure_row(&mut self, row: R) -> &mut BitSet<C> {
+        // Instantiate any missing rows up to and including row `row` with an empty `BitSet`.
+        // Then replace row `row` with a full `BitSet` if necessary.
+        self.rows.get_or_insert_with(row, || BitSet::new_empty(self.num_columns))
     }
 
     /// Sets the cell at `(row, column)` to true. Put another way, insert
@@ -1629,7 +1629,7 @@ pub fn iter(&self, row: R) -> impl Iterator<Item = C> + '_ {
         self.row(row).into_iter().flat_map(|r| r.iter())
     }
 
-    pub fn row(&self, row: R) -> Option<&ChunkedBitSet<C>> {
+    pub fn row(&self, row: R) -> Option<&BitSet<C>> {
         self.rows.get(row)?.as_ref()
     }
 
@@ -1639,7 +1639,7 @@ pub fn row(&self, row: R) -> Option<&ChunkedBitSet<C>> {
     /// Returns true if the row was changed.
     pub fn intersect_row<Set>(&mut self, row: R, set: &Set) -> bool
     where
-        ChunkedBitSet<C>: BitRelations<Set>,
+        BitSet<C>: BitRelations<Set>,
     {
         match self.rows.get_mut(row) {
             Some(Some(row)) => row.intersect(set),
@@ -1653,7 +1653,7 @@ pub fn intersect_row<Set>(&mut self, row: R, set: &Set) -> bool
     /// Returns true if the row was changed.
     pub fn subtract_row<Set>(&mut self, row: R, set: &Set) -> bool
     where
-        ChunkedBitSet<C>: BitRelations<Set>,
+        BitSet<C>: BitRelations<Set>,
     {
         match self.rows.get_mut(row) {
             Some(Some(row)) => row.subtract(set),
@@ -1667,7 +1667,7 @@ pub fn subtract_row<Set>(&mut self, row: R, set: &Set) -> bool
     /// Returns true if the row was changed.
     pub fn union_row<Set>(&mut self, row: R, set: &Set) -> bool
     where
-        ChunkedBitSet<C>: BitRelations<Set>,
+        BitSet<C>: BitRelations<Set>,
     {
         self.ensure_row(row).union(set)
     }
diff --git a/compiler/rustc_index/src/bit_set/tests.rs b/compiler/rustc_index/src/bit_set/tests.rs
index 3f9198ce37f1..f61423239793 100644
--- a/compiler/rustc_index/src/bit_set/tests.rs
+++ b/compiler/rustc_index/src/bit_set/tests.rs
@@ -503,15 +503,15 @@ fn sparse_matrix_operations() {
     matrix.insert(2, 99);
     matrix.insert(4, 0);
 
-    let mut disjoint: ChunkedBitSet<usize> = ChunkedBitSet::new_empty(100);
+    let mut disjoint: BitSet<usize> = BitSet::new_empty(100);
     disjoint.insert(33);
 
-    let mut superset = ChunkedBitSet::new_empty(100);
+    let mut superset = BitSet::new_empty(100);
     superset.insert(22);
     superset.insert(75);
     superset.insert(33);
 
-    let mut subset = ChunkedBitSet::new_empty(100);
+    let mut subset = BitSet::new_empty(100);
     subset.insert(22);
 
     // SparseBitMatrix::remove