Optimize `BitSet` iteration.
This commit removes an `Option` check in `BitIter::next()`, avoids
calling `trailing_zeros()` when it's not necessary, and avoids the need
for `enumerate()`. This gives a tiny (0.2%) instruction count win on a
couple of benchmarks.
The commit also adds some comments, which is good because this iteration
code is moderately complex.
diff --git a/src/librustc_index/bit_set.rs b/src/librustc_index/bit_set.rs
index bfaeb84..d8c6e4c 100644
--- a/src/librustc_index/bit_set.rs
+++ b/src/librustc_index/bit_set.rs
@@ -287,17 +287,32 @@
}
pub struct BitIter<'a, T: Idx> {
- cur: Option<(Word, usize)>,
- iter: iter::Enumerate<slice::Iter<'a, Word>>,
+ /// A copy of the current word, but with any already-visited bits cleared.
+ /// (This lets us use `trailing_zeros()` to find the next set bit.) When it
+ /// is reduced to 0, we move onto the next word.
+ word: Word,
+
+ /// The offset (measured in bits) of the current word.
+ offset: usize,
+
+ /// Underlying iterator over the words.
+ iter: slice::Iter<'a, Word>,
+
marker: PhantomData<T>
}
impl<'a, T: Idx> BitIter<'a, T> {
#[inline]
fn new(words: &'a [Word]) -> BitIter<'a, T> {
+ // We initialize `word` and `offset` to degenerate values. On the first
+ // call to `next()` we will fall through to getting the first word from
+ // `iter`, which sets `word` to the first word (if there is one) and
+ // `offset` to 0. Doing it this way saves us from having to maintain
+ // additional state about whether we have started.
BitIter {
- cur: None,
- iter: words.iter().enumerate(),
+ word: 0,
+ offset: std::usize::MAX - (WORD_BITS - 1),
+ iter: words.iter(),
marker: PhantomData,
}
}
@@ -307,17 +322,20 @@
type Item = T;
fn next(&mut self) -> Option<T> {
loop {
- if let Some((ref mut word, offset)) = self.cur {
- let bit_pos = word.trailing_zeros() as usize;
- if bit_pos != WORD_BITS {
- let bit = 1 << bit_pos;
- *word ^= bit;
- return Some(T::new(bit_pos + offset))
- }
+ if self.word != 0 {
+ // Get the position of the next set bit in the current word,
+ // then clear the bit.
+ let bit_pos = self.word.trailing_zeros() as usize;
+ let bit = 1 << bit_pos;
+ self.word ^= bit;
+ return Some(T::new(bit_pos + self.offset))
}
- let (i, word) = self.iter.next()?;
- self.cur = Some((*word, WORD_BITS * i));
+ // Move onto the next word. `wrapping_add()` is needed to handle
+ // the degenerate initial value given to `offset` in `new()`.
+ let word = self.iter.next()?;
+ self.word = *word;
+ self.offset = self.offset.wrapping_add(WORD_BITS);
}
}
}
