1use super::super::{BitMask, Tag};
2use core::{mem, ptr};
3
4// Use the native word size as the group size. Using a 64-bit group size on
5// a 32-bit architecture will just end up being more expensive because
6// shifts and multiplies will need to be emulated.
7
8cfg_if! {
9 if #[cfg(any(
10 target_pointer_width = "64",
11 target_arch = "aarch64",
12 target_arch = "x86_64",
13 target_arch = "wasm32",
14 ))] {
15 type GroupWord = u64;
16 type NonZeroGroupWord = core::num::NonZeroU64;
17 } else {
18 type GroupWord = u32;
19 type NonZeroGroupWord = core::num::NonZeroU32;
20 }
21}
22
23pub(crate) type BitMaskWord = GroupWord;
24pub(crate) type NonZeroBitMaskWord = NonZeroGroupWord;
25pub(crate) const BITMASK_STRIDE: usize = 8;
26// We only care about the highest bit of each tag for the mask.
27#[allow(clippy::cast_possible_truncation, clippy::unnecessary_cast)]
28pub(crate) const BITMASK_MASK: BitMaskWord = u64::from_ne_bytes([Tag::DELETED.0; 8]) as GroupWord;
29pub(crate) const BITMASK_ITER_MASK: BitMaskWord = !0;
30
31/// Helper function to replicate a tag across a `GroupWord`.
32#[inline]
33fn repeat(tag: Tag) -> GroupWord {
34 GroupWord::from_ne_bytes([tag.0; Group::WIDTH])
35}
36
37/// Abstraction over a group of control tags which can be scanned in
38/// parallel.
39///
40/// This implementation uses a word-sized integer.
41#[derive(Copy, Clone)]
42pub(crate) struct Group(GroupWord);
43
44// We perform all operations in the native endianness, and convert to
45// little-endian just before creating a BitMask. The can potentially
46// enable the compiler to eliminate unnecessary byte swaps if we are
47// only checking whether a BitMask is empty.
48#[allow(clippy::use_self)]
49impl Group {
50 /// Number of bytes in the group.
51 pub(crate) const WIDTH: usize = mem::size_of::<Self>();
52
53 /// Returns a full group of empty tags, suitable for use as the initial
54 /// value for an empty hash table.
55 ///
56 /// This is guaranteed to be aligned to the group size.
57 #[inline]
58 pub(crate) const fn static_empty() -> &'static [Tag; Group::WIDTH] {
59 #[repr(C)]
60 struct AlignedTags {
61 _align: [Group; 0],
62 tags: [Tag; Group::WIDTH],
63 }
64 const ALIGNED_TAGS: AlignedTags = AlignedTags {
65 _align: [],
66 tags: [Tag::EMPTY; Group::WIDTH],
67 };
68 &ALIGNED_TAGS.tags
69 }
70
71 /// Loads a group of tags starting at the given address.
72 #[inline]
73 #[allow(clippy::cast_ptr_alignment)] // unaligned load
74 pub(crate) unsafe fn load(ptr: *const Tag) -> Self {
75 Group(ptr::read_unaligned(ptr.cast()))
76 }
77
78 /// Loads a group of tags starting at the given address, which must be
79 /// aligned to `mem::align_of::<Group>()`.
80 #[inline]
81 #[allow(clippy::cast_ptr_alignment)]
82 pub(crate) unsafe fn load_aligned(ptr: *const Tag) -> Self {
83 debug_assert_eq!(ptr.align_offset(mem::align_of::<Self>()), 0);
84 Group(ptr::read(ptr.cast()))
85 }
86
87 /// Stores the group of tags to the given address, which must be
88 /// aligned to `mem::align_of::<Group>()`.
89 #[inline]
90 #[allow(clippy::cast_ptr_alignment)]
91 pub(crate) unsafe fn store_aligned(self, ptr: *mut Tag) {
92 debug_assert_eq!(ptr.align_offset(mem::align_of::<Self>()), 0);
93 ptr::write(ptr.cast(), self.0);
94 }
95
96 /// Returns a `BitMask` indicating all tags in the group which *may*
97 /// have the given value.
98 ///
99 /// This function may return a false positive in certain cases where
100 /// the tag in the group differs from the searched value only in its
101 /// lowest bit. This is fine because:
102 /// - This never happens for `EMPTY` and `DELETED`, only full entries.
103 /// - The check for key equality will catch these.
104 /// - This only happens if there is at least 1 true match.
105 /// - The chance of this happening is very low (< 1% chance per byte).
106 #[inline]
107 pub(crate) fn match_tag(self, tag: Tag) -> BitMask {
108 // This algorithm is derived from
109 // https://graphics.stanford.edu/~seander/bithacks.html##ValueInWord
110 let cmp = self.0 ^ repeat(tag);
111 BitMask((cmp.wrapping_sub(repeat(Tag(0x01))) & !cmp & repeat(Tag::DELETED)).to_le())
112 }
113
114 /// Returns a `BitMask` indicating all tags in the group which are
115 /// `EMPTY`.
116 #[inline]
117 pub(crate) fn match_empty(self) -> BitMask {
118 // If the high bit is set, then the tag must be either:
119 // 1111_1111 (EMPTY) or 1000_0000 (DELETED).
120 // So we can just check if the top two bits are 1 by ANDing them.
121 BitMask((self.0 & (self.0 << 1) & repeat(Tag::DELETED)).to_le())
122 }
123
124 /// Returns a `BitMask` indicating all tags in the group which are
125 /// `EMPTY` or `DELETED`.
126 #[inline]
127 pub(crate) fn match_empty_or_deleted(self) -> BitMask {
128 // A tag is EMPTY or DELETED iff the high bit is set
129 BitMask((self.0 & repeat(Tag::DELETED)).to_le())
130 }
131
132 /// Returns a `BitMask` indicating all tags in the group which are full.
133 #[inline]
134 pub(crate) fn match_full(self) -> BitMask {
135 self.match_empty_or_deleted().invert()
136 }
137
138 /// Performs the following transformation on all tags in the group:
139 /// - `EMPTY => EMPTY`
140 /// - `DELETED => EMPTY`
141 /// - `FULL => DELETED`
142 #[inline]
143 pub(crate) fn convert_special_to_empty_and_full_to_deleted(self) -> Self {
144 // Map high_bit = 1 (EMPTY or DELETED) to 1111_1111
145 // and high_bit = 0 (FULL) to 1000_0000
146 //
147 // Here's this logic expanded to concrete values:
148 // let full = 1000_0000 (true) or 0000_0000 (false)
149 // !1000_0000 + 1 = 0111_1111 + 1 = 1000_0000 (no carry)
150 // !0000_0000 + 0 = 1111_1111 + 0 = 1111_1111 (no carry)
151 let full = !self.0 & repeat(Tag::DELETED);
152 Group(!full + (full >> 7))
153 }
154}
155