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Optimize char_try_from_u32

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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.
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Mario Carneiro 2022-02-17 20:27:53 -08:00 committed by GitHub
parent 73a7423e77
commit 0f14bea448
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1 changed files with 14 additions and 1 deletions
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15
library/core/src/char/convert.rs
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@ -271,7 +271,20 @@ fn from_str(s: &str) -> Result<Self, Self::Err> {
#[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