Use bitwise XOR in to_ascii_uppercase #95831
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This saves an instruction compared to the previous approach, which was to unset the fifth bit with bitwise OR.
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Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @m-ou-se (or someone else) soon. Please see the contribution instructions for more information. |
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The Godbolt checks out, and MCA says it will be less cycles. Nice catch! |
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📌 Commit 1e6365d has been approved by |
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Use bitwise XOR in to_ascii_uppercase This saves an instruction compared to the previous approach, which was to unset the fifth bit with bitwise OR. Comparison of generated assembly on x86: https://godbolt.org/z/GdfvdGs39 This can also affect autovectorization, saving SIMD instructions as well: https://godbolt.org/z/cnPcz75T9 Not sure if `u8::to_ascii_lowercase` should also be changed, since using bitwise OR for that function does not require an extra bitwise negate since the code is setting a bit rather than unsetting a bit. `char::to_ascii_uppercase` already uses XOR, so no change seems to be required there.
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Rollup of 7 pull requests Successful merges: - rust-lang#94794 (Clarify indexing into Strings) - rust-lang#95361 (Make non-power-of-two alignments a validity error in `Layout`) - rust-lang#95369 (Fix `x test src/librustdoc` with `download-rustc` enabled ) - rust-lang#95805 (Left overs of rust-lang#95761) - rust-lang#95808 (expand: Remove `ParseSess::missing_fragment_specifiers`) - rust-lang#95817 (hide another #[allow] directive from a docs example) - rust-lang#95831 (Use bitwise XOR in to_ascii_uppercase) Failed merges: r? `@ghost` `@rustbot` modify labels: rollup
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Nice. If you're in the area @redzic have a look at rust/library/core/src/char/methods.rs Line 1129 in edba282 |
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This saves an instruction compared to the previous approach, which
was to unset the fifth bit with bitwise OR.
Comparison of generated assembly on x86: https://godbolt.org/z/GdfvdGs39
This can also affect autovectorization, saving SIMD instructions as well: https://godbolt.org/z/cnPcz75T9
Not sure if
u8::to_ascii_lowercaseshould also be changed, since using bitwise OR for that function does not require an extra bitwise negate since the code is setting a bit rather than unsetting a bit.char::to_ascii_uppercasealready uses XOR, so no change seems to be required there.