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Auto merge of #46713 - Manishearth:memchr, r=bluss
Use memchr to speed up [u8]::contains 3x None
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// Copyright 2015 The Rust Project Developers. See the COPYRIGHT | ||
// file at the top-level directory of this distribution and at | ||
// http://rust-lang.org/COPYRIGHT. | ||
// | ||
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or | ||
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license | ||
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your | ||
// option. This file may not be copied, modified, or distributed | ||
// except according to those terms. | ||
// | ||
// Original implementation taken from rust-memchr | ||
// Copyright 2015 Andrew Gallant, bluss and Nicolas Koch | ||
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use cmp; | ||
use mem; | ||
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const LO_U64: u64 = 0x0101010101010101; | ||
const HI_U64: u64 = 0x8080808080808080; | ||
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// use truncation | ||
const LO_USIZE: usize = LO_U64 as usize; | ||
const HI_USIZE: usize = HI_U64 as usize; | ||
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/// Return `true` if `x` contains any zero byte. | ||
/// | ||
/// From *Matters Computational*, J. Arndt | ||
/// | ||
/// "The idea is to subtract one from each of the bytes and then look for | ||
/// bytes where the borrow propagated all the way to the most significant | ||
/// bit." | ||
#[inline] | ||
fn contains_zero_byte(x: usize) -> bool { | ||
x.wrapping_sub(LO_USIZE) & !x & HI_USIZE != 0 | ||
} | ||
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#[cfg(target_pointer_width = "16")] | ||
#[inline] | ||
fn repeat_byte(b: u8) -> usize { | ||
(b as usize) << 8 | b as usize | ||
} | ||
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#[cfg(target_pointer_width = "32")] | ||
#[inline] | ||
fn repeat_byte(b: u8) -> usize { | ||
let mut rep = (b as usize) << 8 | b as usize; | ||
rep = rep << 16 | rep; | ||
rep | ||
} | ||
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#[cfg(target_pointer_width = "64")] | ||
#[inline] | ||
fn repeat_byte(b: u8) -> usize { | ||
let mut rep = (b as usize) << 8 | b as usize; | ||
rep = rep << 16 | rep; | ||
rep = rep << 32 | rep; | ||
rep | ||
} | ||
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/// Return the first index matching the byte `a` in `text`. | ||
pub fn memchr(x: u8, text: &[u8]) -> Option<usize> { | ||
// Scan for a single byte value by reading two `usize` words at a time. | ||
// | ||
// Split `text` in three parts | ||
// - unaligned initial part, before the first word aligned address in text | ||
// - body, scan by 2 words at a time | ||
// - the last remaining part, < 2 word size | ||
let len = text.len(); | ||
let ptr = text.as_ptr(); | ||
let usize_bytes = mem::size_of::<usize>(); | ||
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// search up to an aligned boundary | ||
let mut offset = ptr.align_offset(usize_bytes); | ||
if offset > 0 { | ||
offset = cmp::min(offset, len); | ||
if let Some(index) = text[..offset].iter().position(|elt| *elt == x) { | ||
return Some(index); | ||
} | ||
} | ||
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// search the body of the text | ||
let repeated_x = repeat_byte(x); | ||
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if len >= 2 * usize_bytes { | ||
while offset <= len - 2 * usize_bytes { | ||
unsafe { | ||
let u = *(ptr.offset(offset as isize) as *const usize); | ||
let v = *(ptr.offset((offset + usize_bytes) as isize) as *const usize); | ||
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// break if there is a matching byte | ||
let zu = contains_zero_byte(u ^ repeated_x); | ||
let zv = contains_zero_byte(v ^ repeated_x); | ||
if zu || zv { | ||
break; | ||
} | ||
} | ||
offset += usize_bytes * 2; | ||
} | ||
} | ||
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// find the byte after the point the body loop stopped | ||
text[offset..].iter().position(|elt| *elt == x).map(|i| offset + i) | ||
} | ||
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/// Return the last index matching the byte `a` in `text`. | ||
pub fn memrchr(x: u8, text: &[u8]) -> Option<usize> { | ||
// Scan for a single byte value by reading two `usize` words at a time. | ||
// | ||
// Split `text` in three parts | ||
// - unaligned tail, after the last word aligned address in text | ||
// - body, scan by 2 words at a time | ||
// - the first remaining bytes, < 2 word size | ||
let len = text.len(); | ||
let ptr = text.as_ptr(); | ||
let usize_bytes = mem::size_of::<usize>(); | ||
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// search to an aligned boundary | ||
let end_align = (ptr as usize + len) & (usize_bytes - 1); | ||
let mut offset; | ||
if end_align > 0 { | ||
offset = if end_align >= len { 0 } else { len - end_align }; | ||
if let Some(index) = text[offset..].iter().rposition(|elt| *elt == x) { | ||
return Some(offset + index); | ||
} | ||
} else { | ||
offset = len; | ||
} | ||
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// search the body of the text | ||
let repeated_x = repeat_byte(x); | ||
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while offset >= 2 * usize_bytes { | ||
unsafe { | ||
let u = *(ptr.offset(offset as isize - 2 * usize_bytes as isize) as *const usize); | ||
let v = *(ptr.offset(offset as isize - usize_bytes as isize) as *const usize); | ||
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// break if there is a matching byte | ||
let zu = contains_zero_byte(u ^ repeated_x); | ||
let zv = contains_zero_byte(v ^ repeated_x); | ||
if zu || zv { | ||
break; | ||
} | ||
} | ||
offset -= 2 * usize_bytes; | ||
} | ||
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// find the byte before the point the body loop stopped | ||
text[..offset].iter().rposition(|elt| *elt == x) | ||
} | ||
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// test fallback implementations on all platforms | ||
#[test] | ||
fn matches_one() { | ||
assert_eq!(Some(0), memchr(b'a', b"a")); | ||
} | ||
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#[test] | ||
fn matches_begin() { | ||
assert_eq!(Some(0), memchr(b'a', b"aaaa")); | ||
} | ||
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#[test] | ||
fn matches_end() { | ||
assert_eq!(Some(4), memchr(b'z', b"aaaaz")); | ||
} | ||
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#[test] | ||
fn matches_nul() { | ||
assert_eq!(Some(4), memchr(b'\x00', b"aaaa\x00")); | ||
} | ||
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#[test] | ||
fn matches_past_nul() { | ||
assert_eq!(Some(5), memchr(b'z', b"aaaa\x00z")); | ||
} | ||
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#[test] | ||
fn no_match_empty() { | ||
assert_eq!(None, memchr(b'a', b"")); | ||
} | ||
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#[test] | ||
fn no_match() { | ||
assert_eq!(None, memchr(b'a', b"xyz")); | ||
} | ||
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#[test] | ||
fn matches_one_reversed() { | ||
assert_eq!(Some(0), memrchr(b'a', b"a")); | ||
} | ||
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#[test] | ||
fn matches_begin_reversed() { | ||
assert_eq!(Some(3), memrchr(b'a', b"aaaa")); | ||
} | ||
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#[test] | ||
fn matches_end_reversed() { | ||
assert_eq!(Some(0), memrchr(b'z', b"zaaaa")); | ||
} | ||
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#[test] | ||
fn matches_nul_reversed() { | ||
assert_eq!(Some(4), memrchr(b'\x00', b"aaaa\x00")); | ||
} | ||
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#[test] | ||
fn matches_past_nul_reversed() { | ||
assert_eq!(Some(0), memrchr(b'z', b"z\x00aaaa")); | ||
} | ||
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#[test] | ||
fn no_match_empty_reversed() { | ||
assert_eq!(None, memrchr(b'a', b"")); | ||
} | ||
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#[test] | ||
fn no_match_reversed() { | ||
assert_eq!(None, memrchr(b'a', b"xyz")); | ||
} | ||
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#[test] | ||
fn each_alignment_reversed() { | ||
let mut data = [1u8; 64]; | ||
let needle = 2; | ||
let pos = 40; | ||
data[pos] = needle; | ||
for start in 0..16 { | ||
assert_eq!(Some(pos - start), memrchr(needle, &data[start..])); | ||
} | ||
} |
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