1 use crate::stable_hasher;
2 use rustc_serialize::{Decodable, Decoder, Encodable, Encoder};
3 use std::hash::{Hash, Hasher};
8 #[derive(Eq, PartialEq, Ord, PartialOrd, Debug, Clone, Copy)]
10 pub struct Fingerprint(u64, u64);
13 pub const ZERO: Fingerprint = Fingerprint(0, 0);
16 pub fn new(_0: u64, _1: u64) -> Fingerprint {
21 pub fn from_smaller_hash(hash: u64) -> Fingerprint {
22 Fingerprint(hash, hash)
26 pub fn to_smaller_hash(&self) -> u64 {
27 // Even though both halves of the fingerprint are expected to be good
28 // quality hash values, let's still combine the two values because the
29 // Fingerprints in DefPathHash have the StableCrateId portion which is
30 // the same for all DefPathHashes from the same crate. Combining the
31 // two halves makes sure we get a good quality hash in such cases too.
32 self.0.wrapping_mul(3).wrapping_add(self.1)
36 pub fn as_value(&self) -> (u64, u64) {
41 pub fn combine(self, other: Fingerprint) -> Fingerprint {
42 // See https://stackoverflow.com/a/27952689 on why this function is
43 // implemented this way.
45 self.0.wrapping_mul(3).wrapping_add(other.0),
46 self.1.wrapping_mul(3).wrapping_add(other.1),
50 // Combines two hashes in an order independent way. Make sure this is what
53 pub fn combine_commutative(self, other: Fingerprint) -> Fingerprint {
54 let a = u128::from(self.1) << 64 | u128::from(self.0);
55 let b = u128::from(other.1) << 64 | u128::from(other.0);
57 let c = a.wrapping_add(b);
59 Fingerprint(c as u64, (c >> 64) as u64)
62 pub fn to_hex(&self) -> String {
63 format!("{:x}{:x}", self.0, self.1)
67 pub fn to_le_bytes(&self) -> [u8; 16] {
68 // This seems to optimize to the same machine code as
69 // `unsafe { mem::transmute(*k) }`. Well done, LLVM! :)
70 let mut result = [0u8; 16];
72 let first_half: &mut [u8; 8] = (&mut result[0..8]).try_into().unwrap();
73 *first_half = self.0.to_le_bytes();
75 let second_half: &mut [u8; 8] = (&mut result[8..16]).try_into().unwrap();
76 *second_half = self.1.to_le_bytes();
82 pub fn from_le_bytes(bytes: [u8; 16]) -> Fingerprint {
84 u64::from_le_bytes(bytes[0..8].try_into().unwrap()),
85 u64::from_le_bytes(bytes[8..16].try_into().unwrap()),
90 impl std::fmt::Display for Fingerprint {
91 fn fmt(&self, formatter: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
92 write!(formatter, "{:x}-{:x}", self.0, self.1)
96 impl Hash for Fingerprint {
98 fn hash<H: Hasher>(&self, state: &mut H) {
99 state.write_fingerprint(self);
103 trait FingerprintHasher {
104 fn write_fingerprint(&mut self, fingerprint: &Fingerprint);
107 impl<H: Hasher> FingerprintHasher for H {
109 default fn write_fingerprint(&mut self, fingerprint: &Fingerprint) {
110 self.write_u64(fingerprint.0);
111 self.write_u64(fingerprint.1);
115 impl FingerprintHasher for crate::unhash::Unhasher {
117 fn write_fingerprint(&mut self, fingerprint: &Fingerprint) {
118 // Even though both halves of the fingerprint are expected to be good
119 // quality hash values, let's still combine the two values because the
120 // Fingerprints in DefPathHash have the StableCrateId portion which is
121 // the same for all DefPathHashes from the same crate. Combining the
122 // two halves makes sure we get a good quality hash in such cases too.
124 // Since `Unhasher` is used only in the context of HashMaps, it is OK
125 // to combine the two components in an order-independent way (which is
126 // cheaper than the more robust Fingerprint::to_smaller_hash()). For
127 // HashMaps we don't really care if Fingerprint(x,y) and
128 // Fingerprint(y, x) result in the same hash value. Collision
129 // probability will still be much better than with FxHash.
130 self.write_u64(fingerprint.0.wrapping_add(fingerprint.1));
134 impl stable_hasher::StableHasherResult for Fingerprint {
136 fn finish(hasher: stable_hasher::StableHasher) -> Self {
137 let (_0, _1) = hasher.finalize();
142 impl_stable_traits_for_trivial_type!(Fingerprint);
144 impl<E: Encoder> Encodable<E> for Fingerprint {
146 fn encode(&self, s: &mut E) {
147 s.emit_raw_bytes(&self.to_le_bytes());
151 impl<D: Decoder> Decodable<D> for Fingerprint {
153 fn decode(d: &mut D) -> Self {
154 Fingerprint::from_le_bytes(d.read_raw_bytes(16).try_into().unwrap())
158 // `PackedFingerprint` wraps a `Fingerprint`. Its purpose is to, on certain
159 // architectures, behave like a `Fingerprint` without alignment requirements.
160 // This behavior is only enabled on x86 and x86_64, where the impact of
161 // unaligned accesses is tolerable in small doses.
163 // This may be preferable to use in large collections of structs containing
164 // fingerprints, as it can reduce memory consumption by preventing the padding
165 // that the more strictly-aligned `Fingerprint` can introduce. An application of
166 // this is in the query dependency graph, which contains a large collection of
167 // `DepNode`s. As of this writing, the size of a `DepNode` decreases by ~30%
168 // (from 24 bytes to 17) by using the packed representation here, which
169 // noticeably decreases total memory usage when compiling large crates.
171 // The wrapped `Fingerprint` is private to reduce the chance of a client
172 // invoking undefined behavior by taking a reference to the packed field.
173 #[cfg_attr(any(target_arch = "x86", target_arch = "x86_64"), repr(packed))]
174 #[derive(Eq, PartialEq, Ord, PartialOrd, Debug, Clone, Copy, Hash)]
175 pub struct PackedFingerprint(Fingerprint);
177 impl std::fmt::Display for PackedFingerprint {
179 fn fmt(&self, formatter: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
180 // Copy to avoid taking reference to packed field.
186 impl<E: Encoder> Encodable<E> for PackedFingerprint {
188 fn encode(&self, s: &mut E) {
189 // Copy to avoid taking reference to packed field.
195 impl<D: Decoder> Decodable<D> for PackedFingerprint {
197 fn decode(d: &mut D) -> Self {
198 Self(Fingerprint::decode(d))
202 impl From<Fingerprint> for PackedFingerprint {
204 fn from(f: Fingerprint) -> PackedFingerprint {
209 impl From<PackedFingerprint> for Fingerprint {
211 fn from(f: PackedFingerprint) -> Fingerprint {