proc_macro/bridge/
fxhash.rs

1//! This is a copy of the `rustc_hash` crate, adapted to work as a module.
2//!
3//! If in the future it becomes more reasonable to add dependencies to
4//! `proc_macro`, this module should be removed and replaced with a dependency
5//! on the `rustc_hash` crate.
6
7use std::collections::HashMap;
8use std::hash::{BuildHasherDefault, Hasher};
9use std::ops::BitXor;
10
11/// Type alias for a hashmap using the `fx` hash algorithm.
12pub(super) type FxHashMap<K, V> = HashMap<K, V, BuildHasherDefault<FxHasher>>;
13
14/// A speedy hash algorithm for use within rustc. The hashmap in alloc by
15/// default uses SipHash which isn't quite as speedy as we want. In the compiler
16/// we're not really worried about DOS attempts, so we use a fast
17/// non-cryptographic hash.
18///
19/// This is the same as the algorithm used by Firefox -- which is a homespun
20/// one not based on any widely-known algorithm -- though modified to produce
21/// 64-bit hash values instead of 32-bit hash values. It consistently
22/// out-performs an FNV-based hash within rustc itself -- the collision rate is
23/// similar or slightly worse than FNV, but the speed of the hash function
24/// itself is much higher because it works on up to 8 bytes at a time.
25#[derive(Default)]
26pub(super) struct FxHasher {
27    hash: usize,
28}
29
30#[cfg(target_pointer_width = "32")]
31const K: usize = 0x9e3779b9;
32#[cfg(target_pointer_width = "64")]
33const K: usize = 0x517cc1b727220a95;
34
35impl FxHasher {
36    #[inline]
37    fn add_to_hash(&mut self, i: usize) {
38        self.hash = self.hash.rotate_left(5).bitxor(i).wrapping_mul(K);
39    }
40}
41
42impl Hasher for FxHasher {
43    #[inline]
44    fn write(&mut self, mut bytes: &[u8]) {
45        #[cfg(target_pointer_width = "32")]
46        let read_usize = |bytes: &[u8]| u32::from_ne_bytes(bytes[..4].try_into().unwrap());
47        #[cfg(target_pointer_width = "64")]
48        let read_usize = |bytes: &[u8]| u64::from_ne_bytes(bytes[..8].try_into().unwrap());
49
50        let mut hash = FxHasher { hash: self.hash };
51        assert!(size_of::<usize>() <= 8);
52        while bytes.len() >= size_of::<usize>() {
53            hash.add_to_hash(read_usize(bytes) as usize);
54            bytes = &bytes[size_of::<usize>()..];
55        }
56        if (size_of::<usize>() > 4) && (bytes.len() >= 4) {
57            hash.add_to_hash(u32::from_ne_bytes(bytes[..4].try_into().unwrap()) as usize);
58            bytes = &bytes[4..];
59        }
60        if (size_of::<usize>() > 2) && bytes.len() >= 2 {
61            hash.add_to_hash(u16::from_ne_bytes(bytes[..2].try_into().unwrap()) as usize);
62            bytes = &bytes[2..];
63        }
64        if (size_of::<usize>() > 1) && !bytes.is_empty() {
65            hash.add_to_hash(bytes[0] as usize);
66        }
67        self.hash = hash.hash;
68    }
69
70    #[inline]
71    fn write_u8(&mut self, i: u8) {
72        self.add_to_hash(i as usize);
73    }
74
75    #[inline]
76    fn write_u16(&mut self, i: u16) {
77        self.add_to_hash(i as usize);
78    }
79
80    #[inline]
81    fn write_u32(&mut self, i: u32) {
82        self.add_to_hash(i as usize);
83    }
84
85    #[cfg(target_pointer_width = "32")]
86    #[inline]
87    fn write_u64(&mut self, i: u64) {
88        self.add_to_hash(i as usize);
89        self.add_to_hash((i >> 32) as usize);
90    }
91
92    #[cfg(target_pointer_width = "64")]
93    #[inline]
94    fn write_u64(&mut self, i: u64) {
95        self.add_to_hash(i as usize);
96    }
97
98    #[inline]
99    fn write_usize(&mut self, i: usize) {
100        self.add_to_hash(i);
101    }
102
103    #[inline]
104    fn finish(&self) -> u64 {
105        self.hash as u64
106    }
107}
proc_macro/bridge/fxhash.rs

proc_macro/bridge/
fxhash.rs
