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| author | Mario Carneiro <di.gama@gmail.com> | 2022-02-17 20:27:53 -0800 |
|---|---|---|
| committer | GitHub <noreply@github.com> | 2022-02-17 20:27:53 -0800 |
| commit | 0f14bea448dfdafccbecbb6302a55191a763562a (patch) | |
| tree | 94d258d596ea34dc45441d850bb5321c36f40335 | |
| parent | 73a7423e77b49a99e270531fbadda5b8899df3f6 (diff) | |
| download | rust-0f14bea448dfdafccbecbb6302a55191a763562a.tar.gz rust-0f14bea448dfdafccbecbb6302a55191a763562a.zip | |
Optimize char_try_from_u32
The optimization was proposed by @falk-hueffner in https://rust-lang.zulipchat.com/#narrow/stream/219381-t-libs/topic/Micro-optimizing.20char.3A.3Afrom_u32/near/272146171, and I simplified it a bit and added an explanation of why the optimization is correct.
| -rw-r--r-- | library/core/src/char/convert.rs | 15 |
1 files changed, 14 insertions, 1 deletions
diff --git a/library/core/src/char/convert.rs b/library/core/src/char/convert.rs index 1774ddd7cbb..56dc2a594e1 100644 --- a/library/core/src/char/convert.rs +++ b/library/core/src/char/convert.rs @@ -271,7 +271,20 @@ impl FromStr for char { #[inline] const fn char_try_from_u32(i: u32) -> Result<char, CharTryFromError> { - if (i > MAX as u32) || (i >= 0xD800 && i <= 0xDFFF) { + // This is an optimized version of the check + // (i > MAX as u32) || (i >= 0xD800 && i <= 0xDFFF), + // which can also be written as + // i >= 0x110000 || (i >= 0xD800 && i < 0xE000). + // + // The XOR with 0xD800 permutes the ranges such that 0xD800..0xE000 is + // mapped to 0x0000..0x0800, while keeping all the high bits outside 0xFFFF the same. + // In particular, numbers >= 0x110000 stay in this range. + // + // Subtracting 0x800 causes 0x0000..0x0800 to wrap, meaning that a single + // unsigned comparison against 0x110000 - 0x800 will detect both the wrapped + // surrogate range as well as the numbers originally larger than 0x110000. + // + if (i ^ 0xD800).wrapping_sub(0x800) >= 0x110000 - 0x800 { Err(CharTryFromError(())) } else { // SAFETY: checked that it's a legal unicode value |