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Github str::is_char_boundary - slight optimization by Soveu · Pull Request #8475...

 2 years ago
source link: https://github.com/rust-lang/rust/pull/84751
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Soveu commented on Apr 30

Current str::is_char_boundary implementation emits slightly more instructions, because it includes an additional branch for index == s.len()

pub fn is_char_boundary(s: &str, index: usize) -> bool {
    if index == 0 || index == s.len() {
        return true;
    }
    match s.as_bytes().get(index) {
        None => false,
        Some(&b) => (b as i8) >= -0x40,
    }
}

Just changing the place of index == s.len() merges it with index < s.len() from s.as_bytes().get(index)

pub fn is_char_boundary2(s: &str, index: usize) -> bool {
    if index == 0 {
        return true;
    }

    match s.as_bytes().get(index) {
        // For some reason, LLVM likes this comparison here more
        None => index == s.len(),
        // This is bit magic equivalent to: b < 128 || b >= 192
        Some(&b) => (b as i8) >= -0x40,
    }
}

This one has better codegen on every platform, except powerpc

x86 codegenaarch64 codegenriscv64gc codegen

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@rustbot label: A-codegen


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