Adding upstream version 1:115.7.0.upstream/1%115.7.0upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 422 insertions, 0 deletions

diff --git a/servo/components/selectors/bloom.rs b/servo/components/selectors/bloom.rs
new file mode 100644
index 0000000000..98461d1ba2
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+++ b/servo/components/selectors/bloom.rs
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+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+//! Counting and non-counting Bloom filters tuned for use as ancestor filters
+//! for selector matching.
+
+use std::fmt::{self, Debug};
+
+// The top 8 bits of the 32-bit hash value are not used by the bloom filter.
+// Consumers may rely on this to pack hashes more efficiently.
+pub const BLOOM_HASH_MASK: u32 = 0x00ffffff;
+const KEY_SIZE: usize = 12;
+
+const ARRAY_SIZE: usize = 1 << KEY_SIZE;
+const KEY_MASK: u32 = (1 << KEY_SIZE) - 1;
+
+/// A counting Bloom filter with 8-bit counters.
+pub type BloomFilter = CountingBloomFilter<BloomStorageU8>;
+
+/// A counting Bloom filter with parameterized storage to handle
+/// counters of different sizes.  For now we assume that having two hash
+/// functions is enough, but we may revisit that decision later.
+///
+/// The filter uses an array with 2**KeySize entries.
+///
+/// Assuming a well-distributed hash function, a Bloom filter with
+/// array size M containing N elements and
+/// using k hash function has expected false positive rate exactly
+///
+/// $  (1 - (1 - 1/M)^{kN})^k  $
+///
+/// because each array slot has a
+///
+/// $  (1 - 1/M)^{kN}  $
+///
+/// chance of being 0, and the expected false positive rate is the
+/// probability that all of the k hash functions will hit a nonzero
+/// slot.
+///
+/// For reasonable assumptions (M large, kN large, which should both
+/// hold if we're worried about false positives) about M and kN this
+/// becomes approximately
+///
+/// $$  (1 - \exp(-kN/M))^k   $$
+///
+/// For our special case of k == 2, that's $(1 - \exp(-2N/M))^2$,
+/// or in other words
+///
+/// $$    N/M = -0.5 * \ln(1 - \sqrt(r))   $$
+///
+/// where r is the false positive rate.  This can be used to compute
+/// the desired KeySize for a given load N and false positive rate r.
+///
+/// If N/M is assumed small, then the false positive rate can
+/// further be approximated as 4*N^2/M^2.  So increasing KeySize by
+/// 1, which doubles M, reduces the false positive rate by about a
+/// factor of 4, and a false positive rate of 1% corresponds to
+/// about M/N == 20.
+///
+/// What this means in practice is that for a few hundred keys using a
+/// KeySize of 12 gives false positive rates on the order of 0.25-4%.
+///
+/// Similarly, using a KeySize of 10 would lead to a 4% false
+/// positive rate for N == 100 and to quite bad false positive
+/// rates for larger N.
+#[derive(Clone, Default)]
+pub struct CountingBloomFilter<S>
+where
+    S: BloomStorage,
+{
+    storage: S,
+}
+
+impl<S> CountingBloomFilter<S>
+where
+    S: BloomStorage,
+{
+    /// Creates a new bloom filter.
+    #[inline]
+    pub fn new() -> Self {
+        Default::default()
+    }
+
+    #[inline]
+    pub fn clear(&mut self) {
+        self.storage = Default::default();
+    }
+
+    // Slow linear accessor to make sure the bloom filter is zeroed. This should
+    // never be used in release builds.
+    #[cfg(debug_assertions)]
+    pub fn is_zeroed(&self) -> bool {
+        self.storage.is_zeroed()
+    }
+
+    #[cfg(not(debug_assertions))]
+    pub fn is_zeroed(&self) -> bool {
+        unreachable!()
+    }
+
+    /// Inserts an item with a particular hash into the bloom filter.
+    #[inline]
+    pub fn insert_hash(&mut self, hash: u32) {
+        self.storage.adjust_first_slot(hash, true);
+        self.storage.adjust_second_slot(hash, true);
+    }
+
+    /// Removes an item with a particular hash from the bloom filter.
+    #[inline]
+    pub fn remove_hash(&mut self, hash: u32) {
+        self.storage.adjust_first_slot(hash, false);
+        self.storage.adjust_second_slot(hash, false);
+    }
+
+    /// Check whether the filter might contain an item with the given hash.
+    /// This can sometimes return true even if the item is not in the filter,
+    /// but will never return false for items that are actually in the filter.
+    #[inline]
+    pub fn might_contain_hash(&self, hash: u32) -> bool {
+        !self.storage.first_slot_is_empty(hash) && !self.storage.second_slot_is_empty(hash)
+    }
+}
+
+impl<S> Debug for CountingBloomFilter<S>
+where
+    S: BloomStorage,
+{
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        let mut slots_used = 0;
+        for i in 0..ARRAY_SIZE {
+            if !self.storage.slot_is_empty(i) {
+                slots_used += 1;
+            }
+        }
+        write!(f, "BloomFilter({}/{})", slots_used, ARRAY_SIZE)
+    }
+}
+
+pub trait BloomStorage: Clone + Default {
+    fn slot_is_empty(&self, index: usize) -> bool;
+    fn adjust_slot(&mut self, index: usize, increment: bool);
+    fn is_zeroed(&self) -> bool;
+
+    #[inline]
+    fn first_slot_is_empty(&self, hash: u32) -> bool {
+        self.slot_is_empty(Self::first_slot_index(hash))
+    }
+
+    #[inline]
+    fn second_slot_is_empty(&self, hash: u32) -> bool {
+        self.slot_is_empty(Self::second_slot_index(hash))
+    }
+
+    #[inline]
+    fn adjust_first_slot(&mut self, hash: u32, increment: bool) {
+        self.adjust_slot(Self::first_slot_index(hash), increment)
+    }
+
+    #[inline]
+    fn adjust_second_slot(&mut self, hash: u32, increment: bool) {
+        self.adjust_slot(Self::second_slot_index(hash), increment)
+    }
+
+    #[inline]
+    fn first_slot_index(hash: u32) -> usize {
+        hash1(hash) as usize
+    }
+
+    #[inline]
+    fn second_slot_index(hash: u32) -> usize {
+        hash2(hash) as usize
+    }
+}
+
+/// Storage class for a CountingBloomFilter that has 8-bit counters.
+pub struct BloomStorageU8 {
+    counters: [u8; ARRAY_SIZE],
+}
+
+impl BloomStorage for BloomStorageU8 {
+    #[inline]
+    fn adjust_slot(&mut self, index: usize, increment: bool) {
+        let slot = &mut self.counters[index];
+        if *slot != 0xff {
+            // full
+            if increment {
+                *slot += 1;
+            } else {
+                *slot -= 1;
+            }
+        }
+    }
+
+    #[inline]
+    fn slot_is_empty(&self, index: usize) -> bool {
+        self.counters[index] == 0
+    }
+
+    #[inline]
+    fn is_zeroed(&self) -> bool {
+        self.counters.iter().all(|x| *x == 0)
+    }
+}
+
+impl Default for BloomStorageU8 {
+    fn default() -> Self {
+        BloomStorageU8 {
+            counters: [0; ARRAY_SIZE],
+        }
+    }
+}
+
+impl Clone for BloomStorageU8 {
+    fn clone(&self) -> Self {
+        BloomStorageU8 {
+            counters: self.counters,
+        }
+    }
+}
+
+/// Storage class for a CountingBloomFilter that has 1-bit counters.
+pub struct BloomStorageBool {
+    counters: [u8; ARRAY_SIZE / 8],
+}
+
+impl BloomStorage for BloomStorageBool {
+    #[inline]
+    fn adjust_slot(&mut self, index: usize, increment: bool) {
+        let bit = 1 << (index % 8);
+        let byte = &mut self.counters[index / 8];
+
+        // Since we have only one bit for storage, decrementing it
+        // should never do anything.  Assert against an accidental
+        // decrementing of a bit that was never set.
+        assert!(
+            increment || (*byte & bit) != 0,
+            "should not decrement if slot is already false"
+        );
+
+        if increment {
+            *byte |= bit;
+        }
+    }
+
+    #[inline]
+    fn slot_is_empty(&self, index: usize) -> bool {
+        let bit = 1 << (index % 8);
+        (self.counters[index / 8] & bit) == 0
+    }
+
+    #[inline]
+    fn is_zeroed(&self) -> bool {
+        self.counters.iter().all(|x| *x == 0)
+    }
+}
+
+impl Default for BloomStorageBool {
+    fn default() -> Self {
+        BloomStorageBool {
+            counters: [0; ARRAY_SIZE / 8],
+        }
+    }
+}
+
+impl Clone for BloomStorageBool {
+    fn clone(&self) -> Self {
+        BloomStorageBool {
+            counters: self.counters,
+        }
+    }
+}
+
+#[inline]
+fn hash1(hash: u32) -> u32 {
+    hash & KEY_MASK
+}
+
+#[inline]
+fn hash2(hash: u32) -> u32 {
+    (hash >> KEY_SIZE) & KEY_MASK
+}
+
+#[test]
+fn create_and_insert_some_stuff() {
+    use fxhash::FxHasher;
+    use std::hash::{Hash, Hasher};
+    use std::mem::transmute;
+
+    fn hash_as_str(i: usize) -> u32 {
+        let mut hasher = FxHasher::default();
+        let s = i.to_string();
+        s.hash(&mut hasher);
+        let hash: u64 = hasher.finish();
+        (hash >> 32) as u32 ^ (hash as u32)
+    }
+
+    let mut bf = BloomFilter::new();
+
+    // Statically assert that ARRAY_SIZE is a multiple of 8, which
+    // BloomStorageBool relies on.
+    unsafe {
+        transmute::<[u8; ARRAY_SIZE % 8], [u8; 0]>([]);
+    }
+
+    for i in 0_usize..1000 {
+        bf.insert_hash(hash_as_str(i));
+    }
+
+    for i in 0_usize..1000 {
+        assert!(bf.might_contain_hash(hash_as_str(i)));
+    }
+
+    let false_positives = (1001_usize..2000)
+        .filter(|i| bf.might_contain_hash(hash_as_str(*i)))
+        .count();
+
+    assert!(false_positives < 190, "{} is not < 190", false_positives); // 19%.
+
+    for i in 0_usize..100 {
+        bf.remove_hash(hash_as_str(i));
+    }
+
+    for i in 100_usize..1000 {
+        assert!(bf.might_contain_hash(hash_as_str(i)));
+    }
+
+    let false_positives = (0_usize..100)
+        .filter(|i| bf.might_contain_hash(hash_as_str(*i)))
+        .count();
+
+    assert!(false_positives < 20, "{} is not < 20", false_positives); // 20%.
+
+    bf.clear();
+
+    for i in 0_usize..2000 {
+        assert!(!bf.might_contain_hash(hash_as_str(i)));
+    }
+}
+
+#[cfg(feature = "bench")]
+#[cfg(test)]
+mod bench {
+    extern crate test;
+    use super::BloomFilter;
+
+    #[derive(Default)]
+    struct HashGenerator(u32);
+
+    impl HashGenerator {
+        fn next(&mut self) -> u32 {
+            // Each hash is split into two twelve-bit segments, which are used
+            // as an index into an array. We increment each by 64 so that we
+            // hit the next cache line, and then another 1 so that our wrapping
+            // behavior leads us to different entries each time.
+            //
+            // Trying to simulate cold caches is rather difficult with the cargo
+            // benchmarking setup, so it may all be moot depending on the number
+            // of iterations that end up being run. But we might as well.
+            self.0 += (65) + (65 << super::KEY_SIZE);
+            self.0
+        }
+    }
+
+    #[bench]
+    fn create_insert_1000_remove_100_lookup_100(b: &mut test::Bencher) {
+        b.iter(|| {
+            let mut gen1 = HashGenerator::default();
+            let mut gen2 = HashGenerator::default();
+            let mut bf = BloomFilter::new();
+            for _ in 0_usize..1000 {
+                bf.insert_hash(gen1.next());
+            }
+            for _ in 0_usize..100 {
+                bf.remove_hash(gen2.next());
+            }
+            for _ in 100_usize..200 {
+                test::black_box(bf.might_contain_hash(gen2.next()));
+            }
+        });
+    }
+
+    #[bench]
+    fn might_contain_10(b: &mut test::Bencher) {
+        let bf = BloomFilter::new();
+        let mut gen = HashGenerator::default();
+        b.iter(|| {
+            for _ in 0..10 {
+                test::black_box(bf.might_contain_hash(gen.next()));
+            }
+        });
+    }
+
+    #[bench]
+    fn clear(b: &mut test::Bencher) {
+        let mut bf = Box::new(BloomFilter::new());
+        b.iter(|| test::black_box(&mut bf).clear());
+    }
+
+    #[bench]
+    fn insert_10(b: &mut test::Bencher) {
+        let mut bf = BloomFilter::new();
+        let mut gen = HashGenerator::default();
+        b.iter(|| {
+            for _ in 0..10 {
+                test::black_box(bf.insert_hash(gen.next()));
+            }
+        });
+    }
+
+    #[bench]
+    fn remove_10(b: &mut test::Bencher) {
+        let mut bf = BloomFilter::new();
+        let mut gen = HashGenerator::default();
+        // Note: this will underflow, and that's ok.
+        b.iter(|| {
+            for _ in 0..10 {
+                bf.remove_hash(gen.next())
+            }
+        });
+    }
+}