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Auto merge of #96605 - Urgau:string-retain-codegen, r=thomcc

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Improve codegen of String::retain method

This pull-request improve the codegen of the `String::retain` method.

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

Using `encode_utf8` saves us from an expensive call to `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:
bors 2022-05-21 01:56:51 +00:00
commit 4a86c7907b
1 changed files with 17 additions and 8 deletions
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25
library/alloc/src/string.rs
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@ -1469,19 +1469,28 @@ impl String {
let mut guard = SetLenOnDrop { s: self, idx: 0, del_bytes: 0 }; let mut guard = SetLenOnDrop { s: self, idx: 0, del_bytes: 0 };
while guard.idx < len { 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(); let ch_len = ch.len_utf8();
if !f(ch) { if !f(ch) {
guard.del_bytes += ch_len; guard.del_bytes += ch_len;
} else if guard.del_bytes > 0 { } else if guard.del_bytes > 0 {
unsafe { // SAFETY: `guard.idx` is in bound and `guard.del_bytes` represent the number of
ptr::copy( // bytes that are erased from the string so the resulting `guard.idx -
guard.s.vec.as_ptr().add(guard.idx), // guard.del_bytes` always represent a valid unicode code point.
guard.s.vec.as_mut_ptr().add(guard.idx - guard.del_bytes), //
ch_len, // `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 // Point idx to the next char