lz4_flex/
fastcpy_unsafe.rs

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 alloc::vec::Vec;
120    use proptest::prelude::*;
121    proptest! {
122        #[test]
123        fn test_fast_short_slice_copy(left: Vec<u8>) {
124            if left.is_empty() {
125                return Ok(());
126            }
127            let mut right = vec![0u8; left.len()];
128            slice_copy(left.as_ptr(), right.as_mut_ptr(), left.len());
129            prop_assert_eq!(&left, &right);
130        }
131    }
132
133    #[test]
134    fn test_fast_short_slice_copy_edge_cases() {
135        for len in 1..(512 * 2) {
136            let left = (0..len).map(|i| i as u8).collect::<Vec<_>>();
137            let mut right = vec![0u8; len];
138            slice_copy(left.as_ptr(), right.as_mut_ptr(), left.len());
139            assert_eq!(left, right);
140        }
141    }
142
143    #[test]
144    fn test_fail2() {
145        let left = vec![
146            0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23,
147            24, 25, 26, 27, 28, 29, 30, 31, 32,
148        ];
149        let mut right = vec![0u8; left.len()];
150        slice_copy(left.as_ptr(), right.as_mut_ptr(), left.len());
151        assert_eq!(left, right);
152    }
153
154    #[test]
155    fn test_fail() {
156        let left = vec![
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, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
159            0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
160        ];
161        let mut right = vec![0u8; left.len()];
162        slice_copy(left.as_ptr(), right.as_mut_ptr(), left.len());
163        assert_eq!(left, right);
164    }
165}
lz4_flex/fastcpy_unsafe.rs

lz4_flex/
fastcpy_unsafe.rs
