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]
slice_copy(src: *const u8, dst: *mut u8, num_bytes: usize)21 pub 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]
wild_copy_from_src<const SIZE: usize>( mut source: *const u8, mut dst: *mut u8, num_bytes: usize, )64 fn 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]
short_copy(src: *const u8, dst: *mut u8, len: usize)93 fn 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]
double_copy_trick<const SIZE: usize>(src: *const u8, dst: *mut u8, len: usize)106 fn 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)]
117 mod 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]
test_fast_short_slice_copy_edge_cases()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]
test_fail2()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]
test_fail()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 }
166