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Auto merge of #91244 - dtolnay:lossy, r=Mark-Simulacrum

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Eliminate bunch of copies of error codepath from Utf8LossyChunksIter

Using a macro to stamp out 7 identical copies of the nontrivial slicing logic to exit this loop didn't seem like a necessary use of a macro. The early return case can be handled by `break` without practically any changes to the logic inside the loop.

All this code is from early 2014 (#12062—nearly 8 years ago; pre-1.0) so it's possible there were compiler limitations that forced the macro way at the time.

Confirmed that `x.py bench library/alloc --stage 0 --test-args from_utf8_lossy` is unaffected on my machine.
This commit is contained in:
bors 2021-11-30 01:08:56 +00:00
commit 94bec90702
1 changed files with 37 additions and 38 deletions
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75
library/core/src/str/lossy.rs
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@ -61,36 +61,26 @@ impl<'a> Iterator for Utf8LossyChunksIter<'a> {
}
let mut i = 0;
let mut valid_up_to = 0;
while i < self.source.len() {
let i_ = i;
// SAFETY: `i` starts at `0`, is less than `self.source.len()`, and
// only increases, so `0 <= i < self.source.len()`.
// SAFETY: `i < self.source.len()` per previous line.
// For some reason the following are both significantly slower:
// while let Some(&byte) = self.source.get(i) {
// while let Some(byte) = self.source.get(i).copied() {
let byte = unsafe { *self.source.get_unchecked(i) };
i += 1;
if byte < 128 {
// This could be a `1 => ...` case in the match below, but for
// the common case of all-ASCII inputs, we bypass loading the
// sizeable UTF8_CHAR_WIDTH table into cache.
} else {
let w = utf8_char_width(byte);
macro_rules! error {
() => {{
// SAFETY: We have checked up to `i` that source is valid UTF-8.
unsafe {
let r = Utf8LossyChunk {
valid: from_utf8_unchecked(&self.source[0..i_]),
broken: &self.source[i_..i],
};
self.source = &self.source[i..];
return Some(r);
}
}};
}
match w {
2 => {
if safe_get(self.source, i) & 192 != TAG_CONT_U8 {
error!();
break;
}
i += 1;
}
@ -100,13 +90,11 @@ impl<'a> Iterator for Utf8LossyChunksIter<'a> {
(0xE1..=0xEC, 0x80..=0xBF) => (),
(0xED, 0x80..=0x9F) => (),
(0xEE..=0xEF, 0x80..=0xBF) => (),
_ => {
error!();
}
_ => break,
}
i += 1;
if safe_get(self.source, i) & 192 != TAG_CONT_U8 {
error!();
break;
}
i += 1;
}
@ -115,34 +103,45 @@ impl<'a> Iterator for Utf8LossyChunksIter<'a> {
(0xF0, 0x90..=0xBF) => (),
(0xF1..=0xF3, 0x80..=0xBF) => (),
(0xF4, 0x80..=0x8F) => (),
_ => {
error!();
}
_ => break,
}
i += 1;
if safe_get(self.source, i) & 192 != TAG_CONT_U8 {
error!();
break;
}
i += 1;
if safe_get(self.source, i) & 192 != TAG_CONT_U8 {
error!();
break;
}
i += 1;
}
_ => {
error!();
}
_ => break,
}
}
valid_up_to = i;
}
let r = Utf8LossyChunk {
// SAFETY: We have checked that the entire source is valid UTF-8.
valid: unsafe { from_utf8_unchecked(self.source) },
broken: &[],
};
self.source = &[];
Some(r)
// SAFETY: `i <= self.source.len()` because it is only ever incremented
// via `i += 1` and in between every single one of those increments, `i`
// is compared against `self.source.len()`. That happens either
// literally by `i < self.source.len()` in the while-loop's condition,
// or indirectly by `safe_get(self.source, i) & 192 != TAG_CONT_U8`. The
// loop is terminated as soon as the latest `i += 1` has made `i` no
// longer less than `self.source.len()`, which means it'll be at most
// equal to `self.source.len()`.
let (inspected, remaining) = unsafe { self.source.split_at_unchecked(i) };
self.source = remaining;
// SAFETY: `valid_up_to <= i` because it is only ever assigned via
// `valid_up_to = i` and `i` only increases.
let (valid, broken) = unsafe { inspected.split_at_unchecked(valid_up_to) };
Some(Utf8LossyChunk {
// SAFETY: All bytes up to `valid_up_to` are valid UTF-8.
valid: unsafe { from_utf8_unchecked(valid) },
broken,
})
}
}