1 use crate::stable_hasher;
2 use rustc_serialize::{Decodable, Encodable};
3 use std::convert::TryInto;
4 use std::hash::{Hash, Hasher};
9 #[derive(Eq, PartialEq, Ord, PartialOrd, Debug, Clone, Copy)]
11 pub struct Fingerprint(u64, u64);
14 pub const ZERO: Fingerprint = Fingerprint(0, 0);
17 pub fn new(_0: u64, _1: u64) -> Fingerprint {
22 pub fn from_smaller_hash(hash: u64) -> Fingerprint {
23 Fingerprint(hash, hash)
27 pub fn to_smaller_hash(&self) -> u64 {
28 // Even though both halves of the fingerprint are expected to be good
29 // quality hash values, let's still combine the two values because the
30 // Fingerprints in DefPathHash have the StableCrateId portion which is
31 // the same for all DefPathHashes from the same crate. Combining the
32 // two halfs makes sure we get a good quality hash in such cases too.
33 self.0.wrapping_mul(3).wrapping_add(self.1)
37 pub fn as_value(&self) -> (u64, u64) {
42 pub fn combine(self, other: Fingerprint) -> Fingerprint {
43 // See https://stackoverflow.com/a/27952689 on why this function is
44 // implemented this way.
46 self.0.wrapping_mul(3).wrapping_add(other.0),
47 self.1.wrapping_mul(3).wrapping_add(other.1),
51 // Combines two hashes in an order independent way. Make sure this is what
54 pub fn combine_commutative(self, other: Fingerprint) -> Fingerprint {
55 let a = u128::from(self.1) << 64 | u128::from(self.0);
56 let b = u128::from(other.1) << 64 | u128::from(other.0);
58 let c = a.wrapping_add(b);
60 Fingerprint(c as u64, (c >> 64) as u64)
63 pub fn to_hex(&self) -> String {
64 format!("{:x}{:x}", self.0, self.1)
68 pub fn to_le_bytes(&self) -> [u8; 16] {
69 // This seems to optimize to the same machine code as
70 // `unsafe { mem::transmute(*k) }`. Well done, LLVM! :)
71 let mut result = [0u8; 16];
73 let first_half: &mut [u8; 8] = (&mut result[0..8]).try_into().unwrap();
74 *first_half = self.0.to_le_bytes();
76 let second_half: &mut [u8; 8] = (&mut result[8..16]).try_into().unwrap();
77 *second_half = self.1.to_le_bytes();
83 pub fn from_le_bytes(bytes: [u8; 16]) -> Fingerprint {
85 u64::from_le_bytes(bytes[0..8].try_into().unwrap()),
86 u64::from_le_bytes(bytes[8..16].try_into().unwrap()),
91 impl std::fmt::Display for Fingerprint {
92 fn fmt(&self, formatter: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
93 write!(formatter, "{:x}-{:x}", self.0, self.1)
97 impl Hash for Fingerprint {
99 fn hash<H: Hasher>(&self, state: &mut H) {
100 state.write_fingerprint(self);
104 trait FingerprintHasher {
105 fn write_fingerprint(&mut self, fingerprint: &Fingerprint);
108 impl<H: Hasher> FingerprintHasher for H {
110 default fn write_fingerprint(&mut self, fingerprint: &Fingerprint) {
111 self.write_u64(fingerprint.0);
112 self.write_u64(fingerprint.1);
116 impl FingerprintHasher for crate::unhash::Unhasher {
118 fn write_fingerprint(&mut self, fingerprint: &Fingerprint) {
119 // Even though both halves of the fingerprint are expected to be good
120 // quality hash values, let's still combine the two values because the
121 // Fingerprints in DefPathHash have the StableCrateId portion which is
122 // the same for all DefPathHashes from the same crate. Combining the
123 // two halfs makes sure we get a good quality hash in such cases too.
125 // Since `Unhasher` is used only in the context of HashMaps, it is OK
126 // to combine the two components in an order-independent way (which is
127 // cheaper than the more robust Fingerprint::to_smaller_hash()). For
128 // HashMaps we don't really care if Fingerprint(x,y) and
129 // Fingerprint(y, x) result in the same hash value. Collision
130 // probability will still be much better than with FxHash.
131 self.write_u64(fingerprint.0.wrapping_add(fingerprint.1));
135 impl stable_hasher::StableHasherResult for Fingerprint {
137 fn finish(hasher: stable_hasher::StableHasher) -> Self {
138 let (_0, _1) = hasher.finalize();
143 impl_stable_hash_via_hash!(Fingerprint);
145 impl<E: rustc_serialize::Encoder> Encodable<E> for Fingerprint {
147 fn encode(&self, s: &mut E) -> Result<(), E::Error> {
148 s.emit_raw_bytes(&self.to_le_bytes())?;
153 impl<D: rustc_serialize::Decoder> Decodable<D> for Fingerprint {
155 fn decode(d: &mut D) -> Self {
156 Fingerprint::from_le_bytes(d.read_raw_bytes(16).try_into().unwrap())
160 // `PackedFingerprint` wraps a `Fingerprint`. Its purpose is to, on certain
161 // architectures, behave like a `Fingerprint` without alignment requirements.
162 // This behavior is only enabled on x86 and x86_64, where the impact of
163 // unaligned accesses is tolerable in small doses.
165 // This may be preferable to use in large collections of structs containing
166 // fingerprints, as it can reduce memory consumption by preventing the padding
167 // that the more strictly-aligned `Fingerprint` can introduce. An application of
168 // this is in the query dependency graph, which contains a large collection of
169 // `DepNode`s. As of this writing, the size of a `DepNode` decreases by ~30%
170 // (from 24 bytes to 17) by using the packed representation here, which
171 // noticeably decreases total memory usage when compiling large crates.
173 // The wrapped `Fingerprint` is private to reduce the chance of a client
174 // invoking undefined behavior by taking a reference to the packed field.
175 #[cfg_attr(any(target_arch = "x86", target_arch = "x86_64"), repr(packed))]
176 #[derive(Eq, PartialEq, Ord, PartialOrd, Debug, Clone, Copy, Hash)]
177 pub struct PackedFingerprint(Fingerprint);
179 impl std::fmt::Display for PackedFingerprint {
181 fn fmt(&self, formatter: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
182 // Copy to avoid taking reference to packed field.
188 impl<E: rustc_serialize::Encoder> Encodable<E> for PackedFingerprint {
190 fn encode(&self, s: &mut E) -> Result<(), E::Error> {
191 // Copy to avoid taking reference to packed field.
197 impl<D: rustc_serialize::Decoder> Decodable<D> for PackedFingerprint {
199 fn decode(d: &mut D) -> Self {
200 Self(Fingerprint::decode(d))
204 impl From<Fingerprint> for PackedFingerprint {
206 fn from(f: Fingerprint) -> PackedFingerprint {
211 impl From<PackedFingerprint> for Fingerprint {
213 fn from(f: PackedFingerprint) -> Fingerprint {