bjoernager/rust
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Clarify ChunkSize invariants.

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`ChunkedBitSet::is_empty` currently does an unnecessary check. This
commit removes that check and adds clarifying comments and an assertion
that demonstrate why it's unnecessary.
This commit is contained in:
Nicholas Nethercote 2024-11-23 13:28:30 +11:00
parent 6b6a867ae9
commit 15b24c46b4
1 changed files with 8 additions and 4 deletions
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12
compiler/rustc_index/src/bit_set.rs
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@ -369,9 +369,11 @@ pub struct ChunkedBitSet<T> {
#[derive(Clone, Debug, PartialEq, Eq)] #[derive(Clone, Debug, PartialEq, Eq)]
enum Chunk { enum Chunk {
/// A chunk that is all zeros; we don't represent the zeros explicitly. /// A chunk that is all zeros; we don't represent the zeros explicitly.
/// The `ChunkSize` is always non-zero.
Zeros(ChunkSize), Zeros(ChunkSize),
/// A chunk that is all ones; we don't represent the ones explicitly. /// A chunk that is all ones; we don't represent the ones explicitly.
/// `ChunkSize` is always non-zero.
Ones(ChunkSize), Ones(ChunkSize),
/// A chunk that has a mix of zeros and ones, which are represented /// A chunk that has a mix of zeros and ones, which are represented
@ -384,8 +386,10 @@ enum Chunk {
/// words are always be zero, as are any excess bits in the final in-use /// words are always be zero, as are any excess bits in the final in-use
/// word. /// word.
/// ///
/// The second field is the count of 1s set in the chunk, and must satisfy /// The first `ChunkSize` field is always non-zero.
/// `0 < count < chunk_domain_size`. ///
/// The second `ChunkSize` field is the count of 1s set in the chunk, and
/// must satisfy `0 < count < chunk_domain_size`.
/// ///
/// The words are within an `Rc` because it's surprisingly common to /// The words are within an `Rc` because it's surprisingly common to
/// duplicate an entire chunk, e.g. in `ChunkedBitSet::clone_from()`, or /// duplicate an entire chunk, e.g. in `ChunkedBitSet::clone_from()`, or
@ -461,7 +465,7 @@ impl<T: Idx> ChunkedBitSet<T> {
} }
pub fn is_empty(&self) -> bool { pub fn is_empty(&self) -> bool {
self.chunks.iter().all(|chunk| matches!(chunk, Chunk::Zeros(..) | Chunk::Ones(0))) self.chunks.iter().all(|chunk| matches!(chunk, Chunk::Zeros(..)))
} }
/// Returns `true` if `self` contains `elem`. /// Returns `true` if `self` contains `elem`.
@ -1005,7 +1009,7 @@ impl Chunk {
} }
fn new(chunk_domain_size: usize, is_empty: bool) -> Self { fn new(chunk_domain_size: usize, is_empty: bool) -> Self {
debug_assert!(chunk_domain_size <= CHUNK_BITS); debug_assert!(0 < chunk_domain_size && chunk_domain_size <= CHUNK_BITS);
let chunk_domain_size = chunk_domain_size as ChunkSize; let chunk_domain_size = chunk_domain_size as ChunkSize;
if is_empty { Zeros(chunk_domain_size) } else { Ones(chunk_domain_size) } if is_empty { Zeros(chunk_domain_size) } else { Ones(chunk_domain_size) }
} }