Hardcode accepting 0 as a valid str char boundary #32456
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Thanks! Could you add a comment as well indicating that it's redundant, but nice for perf? Other than that r=me |
bluss
changed the title
Index 0 must be a valid char boundary (invariant of str that it contains
valid UTF-8 data).
If we check explicitly for index == 0, that removes the need to read the
byte at index 0, so it avoids a trip to the string's memory, and it
optimizes out the slicing index' bounds check whenever it is zero.
With this change, the following examples all change from having a read of
the byte at 0 and a branch to possibly panicing, to having the bounds
checking optimized away.
```rust
pub fn split(s: &str) -> (&str, &str) {
s.split_at(0)
}
pub fn both(s: &str) -> &str {
&s[0..s.len()]
}
pub fn first(s: &str) -> &str {
&s[..0]
}
pub fn last(s: &str) -> &str {
&s[0..]
}
```
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Sure, that's a good idea. Updated with the comment. @bors r=alexcrichton |
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📌 Commit f621193 has been approved by |
Manishearth
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Hardcode accepting 0 as a valid str char boundary If we check explicitly for index == 0, that removes the need to read the byte at index 0, so it avoids a trip to the string's memory, and it optimizes out the slicing index' bounds check whenever it is (a constant) zero.
bors
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Mar 26, 2016
Manishearth
added a commit
to Manishearth/rust
that referenced
this pull request
Mar 26, 2016
Hardcode accepting 0 as a valid str char boundary If we check explicitly for index == 0, that removes the need to read the byte at index 0, so it avoids a trip to the string's memory, and it optimizes out the slicing index' bounds check whenever it is (a constant) zero.
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If we check explicitly for index == 0, that removes the need to read the
byte at index 0, so it avoids a trip to the string's memory, and it
optimizes out the slicing index' bounds check whenever it is (a constant) zero.