1 use crate::stable_hasher;
2 use rustc_serialize::{Decodable, Encodable};
3 use std::convert::TryInto;
4 use std::hash::{Hash, Hasher};
6 #[derive(Eq, PartialEq, Ord, PartialOrd, Debug, Clone, Copy)]
8 pub struct Fingerprint(u64, u64);
11 pub const ZERO: Fingerprint = Fingerprint(0, 0);
14 pub fn new(_0: u64, _1: u64) -> Fingerprint {
19 pub fn from_smaller_hash(hash: u64) -> Fingerprint {
20 Fingerprint(hash, hash)
24 pub fn to_smaller_hash(&self) -> u64 {
25 // Even though both halves of the fingerprint are expected to be good
26 // quality hash values, let's still combine the two values because the
27 // Fingerprints in DefPathHash have the StableCrateId portion which is
28 // the same for all DefPathHashes from the same crate. Combining the
29 // two halfs makes sure we get a good quality hash in such cases too.
30 self.0.wrapping_mul(3).wrapping_add(self.1)
34 pub fn as_value(&self) -> (u64, u64) {
39 pub fn combine(self, other: Fingerprint) -> Fingerprint {
40 // See https://stackoverflow.com/a/27952689 on why this function is
41 // implemented this way.
43 self.0.wrapping_mul(3).wrapping_add(other.0),
44 self.1.wrapping_mul(3).wrapping_add(other.1),
48 // Combines two hashes in an order independent way. Make sure this is what
51 pub fn combine_commutative(self, other: Fingerprint) -> Fingerprint {
52 let a = u128::from(self.1) << 64 | u128::from(self.0);
53 let b = u128::from(other.1) << 64 | u128::from(other.0);
55 let c = a.wrapping_add(b);
57 Fingerprint((c >> 64) as u64, c as u64)
60 pub fn to_hex(&self) -> String {
61 format!("{:x}{:x}", self.0, self.1)
65 pub fn to_le_bytes(&self) -> [u8; 16] {
66 // This seems to optimize to the same machine code as
67 // `unsafe { mem::transmute(*k) }`. Well done, LLVM! :)
68 let mut result = [0u8; 16];
70 let first_half: &mut [u8; 8] = (&mut result[0..8]).try_into().unwrap();
71 *first_half = self.0.to_le_bytes();
73 let second_half: &mut [u8; 8] = (&mut result[8..16]).try_into().unwrap();
74 *second_half = self.1.to_le_bytes();
80 pub fn from_le_bytes(bytes: [u8; 16]) -> Fingerprint {
82 u64::from_le_bytes(bytes[0..8].try_into().unwrap()),
83 u64::from_le_bytes(bytes[8..16].try_into().unwrap()),
88 impl std::fmt::Display for Fingerprint {
89 fn fmt(&self, formatter: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
90 write!(formatter, "{:x}-{:x}", self.0, self.1)
94 impl Hash for Fingerprint {
96 fn hash<H: Hasher>(&self, state: &mut H) {
97 state.write_fingerprint(self);
101 trait FingerprintHasher {
102 fn write_fingerprint(&mut self, fingerprint: &Fingerprint);
105 impl<H: Hasher> FingerprintHasher for H {
107 default fn write_fingerprint(&mut self, fingerprint: &Fingerprint) {
108 self.write_u64(fingerprint.0);
109 self.write_u64(fingerprint.1);
113 impl FingerprintHasher for crate::unhash::Unhasher {
115 fn write_fingerprint(&mut self, fingerprint: &Fingerprint) {
116 // Even though both halves of the fingerprint are expected to be good
117 // quality hash values, let's still combine the two values because the
118 // Fingerprints in DefPathHash have the StableCrateId portion which is
119 // the same for all DefPathHashes from the same crate. Combining the
120 // two halfs makes sure we get a good quality hash in such cases too.
122 // Since `Unhasher` is used only in the context of HashMaps, it is OK
123 // to combine the two components in an order-independent way (which is
124 // cheaper than the more robust Fingerprint::to_smaller_hash()). For
125 // HashMaps we don't really care if Fingerprint(x,y) and
126 // Fingerprint(y, x) result in the same hash value. Collision
127 // probability will still be much better than with FxHash.
128 self.write_u64(fingerprint.0.wrapping_add(fingerprint.1));
132 impl stable_hasher::StableHasherResult for Fingerprint {
134 fn finish(hasher: stable_hasher::StableHasher) -> Self {
135 let (_0, _1) = hasher.finalize();
140 impl_stable_hash_via_hash!(Fingerprint);
142 impl<E: rustc_serialize::Encoder> Encodable<E> for Fingerprint {
144 fn encode(&self, s: &mut E) -> Result<(), E::Error> {
145 s.emit_raw_bytes(&self.to_le_bytes()[..])?;
150 impl<D: rustc_serialize::Decoder> Decodable<D> for Fingerprint {
152 fn decode(d: &mut D) -> Result<Self, D::Error> {
153 let mut bytes = [0u8; 16];
154 d.read_raw_bytes_into(&mut bytes[..])?;
155 Ok(Fingerprint::from_le_bytes(bytes))
159 // `PackedFingerprint` wraps a `Fingerprint`. Its purpose is to, on certain
160 // architectures, behave like a `Fingerprint` without alignment requirements.
161 // This behavior is only enabled on x86 and x86_64, where the impact of
162 // unaligned accesses is tolerable in small doses.
164 // This may be preferable to use in large collections of structs containing
165 // fingerprints, as it can reduce memory consumption by preventing the padding
166 // that the more strictly-aligned `Fingerprint` can introduce. An application of
167 // this is in the query dependency graph, which contains a large collection of
168 // `DepNode`s. As of this writing, the size of a `DepNode` decreases by ~30%
169 // (from 24 bytes to 17) by using the packed representation here, which
170 // noticeably decreases total memory usage when compiling large crates.
172 // The wrapped `Fingerprint` is private to reduce the chance of a client
173 // invoking undefined behavior by taking a reference to the packed field.
174 #[cfg_attr(any(target_arch = "x86", target_arch = "x86_64"), repr(packed))]
175 #[derive(Eq, PartialEq, Ord, PartialOrd, Debug, Clone, Copy, Hash)]
176 pub struct PackedFingerprint(Fingerprint);
178 impl std::fmt::Display for PackedFingerprint {
180 fn fmt(&self, formatter: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
181 // Copy to avoid taking reference to packed field.
187 impl<E: rustc_serialize::Encoder> Encodable<E> for PackedFingerprint {
189 fn encode(&self, s: &mut E) -> Result<(), E::Error> {
190 // Copy to avoid taking reference to packed field.
196 impl<D: rustc_serialize::Decoder> Decodable<D> for PackedFingerprint {
198 fn decode(d: &mut D) -> Result<Self, D::Error> {
199 Fingerprint::decode(d).map(PackedFingerprint)
203 impl From<Fingerprint> for PackedFingerprint {
205 fn from(f: Fingerprint) -> PackedFingerprint {
210 impl From<PackedFingerprint> for Fingerprint {
212 fn from(f: PackedFingerprint) -> Fingerprint {