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|
//! # FastCpy
//!
//! The Rust Compiler calls `memcpy` for slices of unknown length.
//! This crate provides a faster implementation of `memcpy` for slices up to 32bytes (64bytes with `avx`).
//! If you know most of you copy operations are not too big you can use `fastcpy` to speed up your program.
//!
//! `fastcpy` is designed to contain not too much assembly, so the overhead is low.
//!
//! As fall back the standard `memcpy` is called
//!
//! ## Double Copy Trick
//! `fastcpy` employs a double copy trick to copy slices of length 4-32bytes (64bytes with `avx`).
//! E.g. Slice of length 6 can be copied with two uncoditional copy operations.
//!
//! /// [1, 2, 3, 4, 5, 6]
//! /// [1, 2, 3, 4]
//! /// [3, 4, 5, 6]
//!
#[inline]
pub fn slice_copy(src: &[u8], dst: &mut [u8]) {
#[inline(never)]
#[cold]
#[track_caller]
fn len_mismatch_fail(dst_len: usize, src_len: usize) -> ! {
panic!(
"source slice length ({}) does not match destination slice length ({})",
src_len, dst_len,
);
}
if src.len() != dst.len() {
len_mismatch_fail(src.len(), dst.len());
}
let len = src.len();
if src.is_empty() {
return;
}
if len < 4 {
short_copy(src, dst);
return;
}
if len < 8 {
double_copy_trick::<4>(src, dst);
return;
}
if len <= 16 {
double_copy_trick::<8>(src, dst);
return;
}
if len <= 32 {
double_copy_trick::<16>(src, dst);
return;
}
/// The code will use the vmovdqu instruction to copy 32 bytes at a time.
#[cfg(target_feature = "avx")]
{
if len <= 64 {
double_copy_trick::<32>(src, dst);
return;
}
}
// For larger sizes we use the default, which calls memcpy
// memcpy does some virtual memory tricks to copy large chunks of memory.
//
// The theory should be that the checks above don't cost much relative to the copy call for
// larger copies.
// The bounds checks in `copy_from_slice` are elided.
dst.copy_from_slice(src);
}
#[inline(always)]
fn short_copy(src: &[u8], dst: &mut [u8]) {
let len = src.len();
// length 1-3
dst[0] = src[0];
if len >= 2 {
double_copy_trick::<2>(src, dst);
}
}
#[inline(always)]
/// [1, 2, 3, 4, 5, 6]
/// [1, 2, 3, 4]
/// [3, 4, 5, 6]
fn double_copy_trick<const SIZE: usize>(src: &[u8], dst: &mut [u8]) {
dst[0..SIZE].copy_from_slice(&src[0..SIZE]);
dst[src.len() - SIZE..].copy_from_slice(&src[src.len() - SIZE..]);
}
#[cfg(test)]
mod tests {
use super::slice_copy;
use alloc::vec::Vec;
use proptest::prelude::*;
proptest! {
#[test]
fn test_fast_short_slice_copy(left: Vec<u8>) {
let mut right = vec![0u8; left.len()];
slice_copy(&left, &mut right);
prop_assert_eq!(&left, &right);
}
}
#[test]
fn test_fast_short_slice_copy_edge_cases() {
for len in 0..(512 * 2) {
let left = (0..len).map(|i| i as u8).collect::<Vec<_>>();
let mut right = vec![0u8; len];
slice_copy(&left, &mut right);
assert_eq!(left, right);
}
}
#[test]
fn test_fail2() {
let left = vec![
0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23,
24, 25, 26, 27, 28, 29, 30, 31, 32,
];
let mut right = vec![0u8; left.len()];
slice_copy(&left, &mut right);
assert_eq!(left, right);
}
#[test]
fn test_fail() {
let left = vec![
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,
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,
0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
];
let mut right = vec![0u8; left.len()];
slice_copy(&left, &mut right);
assert_eq!(left, right);
}
}
|
