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Rollup merge of #81837 - gilescope:to_ascii_speedups, r=dtolnay

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Slight perf improvement on char::to_ascii_lowercase

`char::to_ascii_lowercase()` was checking if it was ascii and then if it was in the right range. Instead propose to check once (I think removing a compare and a shift in the process: [godbolt](https://godbolt.org/z/e5Tora) ).

before:
```
        test char::methods::bench_to_ascii_lowercase                    ... bench:      11,196 ns/iter (+/- 632)
        test char::methods::bench_to_ascii_uppercase                    ... bench:      11,656 ns/iter (+/- 671)
```
after:
```
         test char::methods::bench_to_ascii_lowercase                    ... bench:       9,612 ns/iter (+/- 979)
         test char::methods::bench_to_ascii_uppercase                    ... bench:       8,241 ns/iter (+/- 701)
```

(calling u8::to_ascii_lowercase and letting that flip the 5th bit is also an option, but it's more instructions. I'm thinking for things around ascii and char we want to be as efficient as possible.)
This commit is contained in:
Dylan DPC 2021-02-23 02:51:47 +01:00 • committed by GitHub
commit 4af965e732
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4 changed files with 35 additions and 6 deletions
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12
library/core/src/char/methods.rs
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@ -1088,7 +1088,11 @@ impl char {
#[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
#[inline]
pub fn to_ascii_uppercase(&self) -> char {
if self.is_ascii() { (*self as u8).to_ascii_uppercase() as char } else { *self }
if self.is_ascii_lowercase() {
(*self as u8).ascii_change_case_unchecked() as char
} else {
*self
}
}
/// Makes a copy of the value in its ASCII lower case equivalent.
@ -1116,7 +1120,11 @@ impl char {
#[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
#[inline]
pub fn to_ascii_lowercase(&self) -> char {
if self.is_ascii() { (*self as u8).to_ascii_lowercase() as char } else { *self }
if self.is_ascii_uppercase() {
(*self as u8).ascii_change_case_unchecked() as char
} else {
*self
}
}
/// Checks that two values are an ASCII case-insensitive match.