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To digit simplification #82094
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Co-authored-by: Mara <[email protected]>
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Remove unused const
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To digit simplification I found out the other day that all the ascii digits have the first four bits as one would hope them to. (Eg. char `2` ends `0b0010`). There are two bits to indicate it's in the digit range ( `0b0011_0000`). If it is a true digit then all the higher bits aside from these two will be 0 (as ascii is the lowest part of the unicode u32 spectrum). So XORing with `0b11_0000` should mean we either get the number 0-9 or alternativly we get a larger number in the u32 space. If we get something that's not 0-9 then it will be discarded as it will be greater than the radix. The code seems so fast though that there's quite a lot of noise in the benchmarks so it's not that easy to prove conclusively that it's faster as well as less instructions. The non-fast path I was toying with as well wondering if we could do this as then we'd only have one return and less instructions still: ``` match self { 'a'..='z' => self as u32 - 'a' as u32 + 10, 'A'..='Z' => self as u32 - 'A' as u32 + 10, _ => { radix = 10; self as u32 ^ ASCII_DIGIT_MASK}, } ``` Here's the [godbolt](https://godbolt.org/z/883c9n). ( H/T to `@byteshadow` for pointing out xor was what I needed)
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…laumeGomez Rollup of 11 pull requests Successful merges: - rust-lang#79981 (Add 'consider using' message to overflowing_literals) - rust-lang#82094 (To digit simplification) - rust-lang#82105 (Don't fail to remove files if they are missing) - rust-lang#82136 (Fix ICE: Use delay_span_bug for mismatched subst/hir arg) - rust-lang#82169 (Document that `assert!` format arguments are evaluated lazily) - rust-lang#82174 (Replace File::create and write_all with fs::write) - rust-lang#82196 (Add caveat to Path::display() about lossiness) - rust-lang#82198 (Use internal iteration in Iterator::is_sorted_by) - rust-lang#82204 (Update books) - rust-lang#82207 (rustdoc: treat edition 2021 as unstable) - rust-lang#82231 (Add long explanation for E0543) Failed merges: r? `@ghost` `@rustbot` modify labels: rollup
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I found out the other day that all the ascii digits have the first four bits as one would hope them to. (Eg. char
2
ends0b0010
). There are two bits to indicate it's in the digit range (0b0011_0000
). If it is a true digit then all the higher bits aside from these two will be 0 (as ascii is the lowest part of the unicode u32 spectrum). So XORing with0b11_0000
should mean we either get the number 0-9 or alternativly we get a larger number in the u32 space. If we get something that's not 0-9 then it will be discarded as it will be greater than the radix.The code seems so fast though that there's quite a lot of noise in the benchmarks so it's not that easy to prove conclusively that it's faster as well as less instructions.
The non-fast path I was toying with as well wondering if we could do this as then we'd only have one return and less instructions still:
Here's the godbolt.
( H/T to @byteshadow for pointing out xor was what I needed)