lz4_flex/
fastcpy_unsafe.rs

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1//! # FastCpy
2//!
3//! The Rust Compiler calls `memcpy` for slices of unknown length.
4//! This crate provides a faster implementation of `memcpy` for slices up to 32bytes (64bytes with `avx`).
5//! If you know most of you copy operations are not too big you can use `fastcpy` to speed up your program.
6//!
7//! `fastcpy` is designed to contain not too much assembly, so the overhead is low.
8//!
9//! As fall back the standard `memcpy` is called
10//!
11//! ## Double Copy Trick
12//! `fastcpy` employs a double copy trick to copy slices of length 4-32bytes (64bytes with `avx`).
13//! E.g. Slice of length 6 can be copied with two uncoditional copy operations.
14//!
15//! /// [1, 2, 3, 4, 5, 6]
16//! /// [1, 2, 3, 4]
17//! ///       [3, 4, 5, 6]
18//!
19
20#[inline]
21pub fn slice_copy(src: *const u8, dst: *mut u8, num_bytes: usize) {
22    if num_bytes < 4 {
23        short_copy(src, dst, num_bytes);
24        return;
25    }
26
27    if num_bytes < 8 {
28        double_copy_trick::<4>(src, dst, num_bytes);
29        return;
30    }
31
32    if num_bytes <= 16 {
33        double_copy_trick::<8>(src, dst, num_bytes);
34        return;
35    }
36
37    //if num_bytes <= 32 {
38    //double_copy_trick::<16>(src, dst, num_bytes);
39    //return;
40    //}
41
42    // /// The code will use the vmovdqu instruction to copy 32 bytes at a time.
43    //#[cfg(target_feature = "avx")]
44    //{
45    //if num_bytes <= 64 {
46    //double_copy_trick::<32>(src, dst, num_bytes);
47    //return;
48    //}
49    //}
50
51    // For larger sizes we use the default, which calls memcpy
52    // memcpy does some virtual memory tricks to copy large chunks of memory.
53    //
54    // The theory should be that the checks above don't cost much relative to the copy call for
55    // larger copies.
56    // The bounds checks in `copy_from_slice` are elided.
57
58    //unsafe { core::ptr::copy_nonoverlapping(src, dst, num_bytes) }
59    wild_copy_from_src::<16>(src, dst, num_bytes)
60}
61
62// Inline never because otherwise we get a call to memcpy -.-
63#[inline]
64fn wild_copy_from_src<const SIZE: usize>(
65    mut source: *const u8,
66    mut dst: *mut u8,
67    num_bytes: usize,
68) {
69    // Note: if the compiler auto-vectorizes this it'll hurt performance!
70    // It's not the case for 16 bytes stepsize, but for 8 bytes.
71    let l_last = unsafe { source.add(num_bytes - SIZE) };
72    let r_last = unsafe { dst.add(num_bytes - SIZE) };
73    let num_bytes = (num_bytes / SIZE) * SIZE;
74
75    unsafe {
76        let dst_ptr_end = dst.add(num_bytes);
77        loop {
78            core::ptr::copy_nonoverlapping(source, dst, SIZE);
79            source = source.add(SIZE);
80            dst = dst.add(SIZE);
81            if dst >= dst_ptr_end {
82                break;
83            }
84        }
85    }
86
87    unsafe {
88        core::ptr::copy_nonoverlapping(l_last, r_last, SIZE);
89    }
90}
91
92#[inline]
93fn short_copy(src: *const u8, dst: *mut u8, len: usize) {
94    unsafe {
95        *dst = *src;
96    }
97    if len >= 2 {
98        double_copy_trick::<2>(src, dst, len);
99    }
100}
101
102#[inline(always)]
103/// [1, 2, 3, 4, 5, 6]
104/// [1, 2, 3, 4]
105///       [3, 4, 5, 6]
106fn double_copy_trick<const SIZE: usize>(src: *const u8, dst: *mut u8, len: usize) {
107    let l_end = unsafe { src.add(len - SIZE) };
108    let r_end = unsafe { dst.add(len - SIZE) };
109
110    unsafe {
111        core::ptr::copy_nonoverlapping(src, dst, SIZE);
112        core::ptr::copy_nonoverlapping(l_end, r_end, SIZE);
113    }
114}
115
116#[cfg(test)]
117mod tests {
118    use super::slice_copy;
119    use proptest::prelude::*;
120    proptest! {
121        #[test]
122        fn test_fast_short_slice_copy(left: Vec<u8>) {
123            if left.is_empty() {
124                return Ok(());
125            }
126            let mut right = vec![0u8; left.len()];
127            slice_copy(left.as_ptr(), right.as_mut_ptr(), left.len());
128            prop_assert_eq!(&left, &right);
129        }
130    }
131
132    #[test]
133    fn test_fast_short_slice_copy_edge_cases() {
134        for len in 1..(512 * 2) {
135            let left = (0..len).map(|i| i as u8).collect::<Vec<_>>();
136            let mut right = vec![0u8; len];
137            slice_copy(left.as_ptr(), right.as_mut_ptr(), left.len());
138            assert_eq!(left, right);
139        }
140    }
141
142    #[test]
143    fn test_fail2() {
144        let left = vec![
145            0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23,
146            24, 25, 26, 27, 28, 29, 30, 31, 32,
147        ];
148        let mut right = vec![0u8; left.len()];
149        slice_copy(left.as_ptr(), right.as_mut_ptr(), left.len());
150        assert_eq!(left, right);
151    }
152
153    #[test]
154    fn test_fail() {
155        let left = vec![
156            0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
157            0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
158            0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
159        ];
160        let mut right = vec![0u8; left.len()];
161        slice_copy(left.as_ptr(), right.as_mut_ptr(), left.len());
162        assert_eq!(left, right);
163    }
164}
lz4_flex/fastcpy_unsafe.rs

lz4_flex/
fastcpy_unsafe.rs
