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Use unrolled loop for searching NULL in [u16] on Windows #67705

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merged 2 commits into from
Apr 9, 2020
Merged

Use unrolled loop for searching NULL in [u16] on Windows #67705

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041e170
use of wmemchr for faster searching in [u16]
tesuji Dec 29, 2019
89bc236
Use unrolled loop
tesuji Apr 2, 2020
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48 changes: 46 additions & 2 deletions src/libstd/sys/windows/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -81,10 +81,54 @@ pub fn decode_error_kind(errno: i32) -> ErrorKind {
}
}

pub fn unrolled_find_u16s(needle: u16, haystack: &[u16]) -> Option<usize> {
let ptr = haystack.as_ptr();
let mut len = haystack.len();
let mut start = &haystack[..];

// For performance reasons unfold the loop eight times.
while len >= 8 {
if start[0] == needle {
return Some((start.as_ptr() as usize - ptr as usize) / 2);
}
if start[1] == needle {
return Some((start[1..].as_ptr() as usize - ptr as usize) / 2);
}
if start[2] == needle {
return Some((start[2..].as_ptr() as usize - ptr as usize) / 2);
}
if start[3] == needle {
return Some((start[3..].as_ptr() as usize - ptr as usize) / 2);
}
if start[4] == needle {
return Some((start[4..].as_ptr() as usize - ptr as usize) / 2);
}
if start[5] == needle {
return Some((start[5..].as_ptr() as usize - ptr as usize) / 2);
}
if start[6] == needle {
return Some((start[6..].as_ptr() as usize - ptr as usize) / 2);
}
if start[7] == needle {
return Some((start[7..].as_ptr() as usize - ptr as usize) / 2);
}

start = &start[8..];
len -= 8;
}

for (i, c) in start.iter().enumerate() {
if *c == needle {
return Some((start.as_ptr() as usize - ptr as usize) / 2 + i);
}
}
None
}

pub fn to_u16s<S: AsRef<OsStr>>(s: S) -> crate::io::Result<Vec<u16>> {
fn inner(s: &OsStr) -> crate::io::Result<Vec<u16>> {
let mut maybe_result: Vec<u16> = s.encode_wide().collect();
if maybe_result.iter().any(|&u| u == 0) {
if unrolled_find_u16s(0, &maybe_result).is_some() {
return Err(crate::io::Error::new(
ErrorKind::InvalidInput,
"strings passed to WinAPI cannot contain NULs",
Expand Down Expand Up @@ -214,7 +258,7 @@ fn wide_char_to_multi_byte(
}

pub fn truncate_utf16_at_nul(v: &[u16]) -> &[u16] {
match v.iter().position(|c| *c == 0) {
match unrolled_find_u16s(0, v) {
// don't include the 0
Some(i) => &v[..i],
None => v,
Expand Down