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Improve codegen of String::retain method.

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Using unwrap_unchecked helps the optimizer to not generate panicking
path, that will never be taken for valid UTF-8 like string.

Using encode_utf8 saves us a call to a memcpy, as the optimizer is
unable to realize that ch_len <= 4 and so can generate much better
assembly code.

https://rust.godbolt.org/z/z73ohenfc
This commit is contained in:
Loïc BRANSTETT 2022-05-01 19:25:14 +02:00
parent 0e7915d11f
commit a98abe83eb
1 changed files with 17 additions and 8 deletions
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25
library/alloc/src/string.rs
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@ -1466,19 +1466,28 @@ fn drop(&mut self) {
let mut guard = SetLenOnDrop { s: self, idx: 0, del_bytes: 0 };
while guard.idx < len {
let ch = unsafe { guard.s.get_unchecked(guard.idx..len).chars().next().unwrap() };
let ch =
// SAFETY: `guard.idx` is positive-or-zero and less that len so the `get_unchecked`
// is in bound. `self` is valid UTF-8 like string and the returned slice starts at
// a unicode code point so the `Chars` always return one character.
unsafe { guard.s.get_unchecked(guard.idx..len).chars().next().unwrap_unchecked() };
let ch_len = ch.len_utf8();
if !f(ch) {
guard.del_bytes += ch_len;
} else if guard.del_bytes > 0 {
unsafe {
ptr::copy(
guard.s.vec.as_ptr().add(guard.idx),
guard.s.vec.as_mut_ptr().add(guard.idx - guard.del_bytes),
ch_len,
);
}
// SAFETY: `guard.idx` is in bound and `guard.del_bytes` represent the number of
// bytes that are erased from the string so the resulting `guard.idx -
// guard.del_bytes` always represent a valid unicode code point.
//
// `guard.del_bytes` >= `ch.len_utf8()`, so taking a slice with `ch.len_utf8()` len
// is safe.
ch.encode_utf8(unsafe {
crate::slice::from_raw_parts_mut(
guard.s.as_mut_ptr().add(guard.idx - guard.del_bytes),
ch.len_utf8(),
)
});
}
// Point idx to the next char