snap/
crc32.rs

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1use crate::bytes;
2use crate::crc32_table::{TABLE, TABLE16};
3
4/// Provides a simple API to generate "masked" CRC32C checksums specifically
5/// for use in Snappy. When available, this will make use of SSE 4.2 to compute
6/// checksums. Otherwise, it falls back to only-marginally-slower "slicing by
7/// 16" technique.
8///
9/// The main purpose of this type is to cache the CPU feature check and expose
10/// a safe API.
11#[derive(Clone, Copy, Debug)]
12pub struct CheckSummer {
13    sse42: bool,
14}
15
16impl CheckSummer {
17    /// Create a new checksummer that can compute CRC32C checksums on arbitrary
18    /// bytes.
19    #[cfg(not(target_arch = "x86_64"))]
20    pub fn new() -> CheckSummer {
21        CheckSummer { sse42: false }
22    }
23
24    /// Create a new checksummer that can compute CRC32C checksums on arbitrary
25    /// bytes.
26    #[cfg(target_arch = "x86_64")]
27    pub fn new() -> CheckSummer {
28        CheckSummer { sse42: is_x86_feature_detected!("sse4.2") }
29    }
30
31    /// Returns the "masked" CRC32 checksum of `buf` using the Castagnoli
32    /// polynomial. This "masked" checksum is defined by the Snappy frame
33    /// format. Masking is supposed to make the checksum robust with respect to
34    /// the data that contains the checksum itself.
35    pub fn crc32c_masked(&self, buf: &[u8]) -> u32 {
36        let sum = self.crc32c(buf);
37        (sum.wrapping_shr(15) | sum.wrapping_shl(17)).wrapping_add(0xA282EAD8)
38    }
39
40    /// Returns the CRC32 checksum of `buf` using the Castagnoli polynomial.
41    #[cfg(not(target_arch = "x86_64"))]
42    fn crc32c(&self, buf: &[u8]) -> u32 {
43        crc32c_slice16(buf)
44    }
45
46    /// Returns the CRC32 checksum of `buf` using the Castagnoli polynomial.
47    #[cfg(target_arch = "x86_64")]
48    fn crc32c(&self, buf: &[u8]) -> u32 {
49        if self.sse42 {
50            // SAFETY: When sse42 is true, we are guaranteed to be running on
51            // a CPU that supports SSE 4.2.
52            unsafe { crc32c_sse(buf) }
53        } else {
54            crc32c_slice16(buf)
55        }
56    }
57}
58
59#[cfg(target_arch = "x86_64")]
60#[target_feature(enable = "sse4.2")]
61unsafe fn crc32c_sse(buf: &[u8]) -> u32 {
62    use std::arch::x86_64::*;
63
64    let mut crc = !0u32;
65    // SAFETY: This is safe since alignment is handled by align_to (oh how I
66    // love you) and since 8 adjacent u8's are guaranteed to have the same
67    // in-memory representation as u64 for all possible values.
68    let (prefix, u64s, suffix) = buf.align_to::<u64>();
69    for &b in prefix {
70        // SAFETY: Safe since we have sse4.2 enabled.
71        crc = _mm_crc32_u8(crc, b);
72    }
73    for &n in u64s {
74        // SAFETY: Safe since we have sse4.2 enabled.
75        crc = _mm_crc32_u64(crc as u64, n) as u32;
76    }
77    for &b in suffix {
78        // SAFETY: Safe since we have sse4.2 enabled.
79        crc = _mm_crc32_u8(crc, b);
80    }
81    !crc
82}
83
84/// Returns the CRC32 checksum of `buf` using the Castagnoli polynomial.
85fn crc32c_slice16(mut buf: &[u8]) -> u32 {
86    let mut crc: u32 = !0;
87    while buf.len() >= 16 {
88        crc ^= bytes::read_u32_le(buf);
89        crc = TABLE16[0][buf[15] as usize]
90            ^ TABLE16[1][buf[14] as usize]
91            ^ TABLE16[2][buf[13] as usize]
92            ^ TABLE16[3][buf[12] as usize]
93            ^ TABLE16[4][buf[11] as usize]
94            ^ TABLE16[5][buf[10] as usize]
95            ^ TABLE16[6][buf[9] as usize]
96            ^ TABLE16[7][buf[8] as usize]
97            ^ TABLE16[8][buf[7] as usize]
98            ^ TABLE16[9][buf[6] as usize]
99            ^ TABLE16[10][buf[5] as usize]
100            ^ TABLE16[11][buf[4] as usize]
101            ^ TABLE16[12][(crc >> 24) as u8 as usize]
102            ^ TABLE16[13][(crc >> 16) as u8 as usize]
103            ^ TABLE16[14][(crc >> 8) as u8 as usize]
104            ^ TABLE16[15][(crc) as u8 as usize];
105        buf = &buf[16..];
106    }
107    for &b in buf {
108        crc = TABLE[((crc as u8) ^ b) as usize] ^ (crc >> 8);
109    }
110    !crc
111}
snap/crc32.rs

snap/
crc32.rs
