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Make is_ascii_hexdigit branchless #103024

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Make is_ascii_hexdigit branchless #103024

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acb42cf
Make is_ascii_hexdigit branchless
GKFX Oct 13, 2022
5b624ac
Fix typo in range
GKFX Oct 13, 2022
f624897
Clarify comment and use character literals in char::is_ascii_hexdigit
GKFX Oct 14, 2022
8d1d17f
Fully optimize is_ascii_hexdigit by hand
GKFX Oct 22, 2022
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3 changes: 2 additions & 1 deletion library/core/src/char/methods.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1510,7 +1510,8 @@ impl char {
#[rustc_const_stable(feature = "const_ascii_ctype_on_intrinsics", since = "1.47.0")]
#[inline]
pub const fn is_ascii_hexdigit(&self) -> bool {
matches!(*self, '0'..='9' | 'A'..='F' | 'a'..='f')
// Bitwise or can avoid need for branches in compiled code.
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matches!(*self, '0'..='9') || matches!(*self as u32 | 0x20, 0x61..=0x66)
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To make this more maintainable, how about using (b'a' as u32)..=(b'f' as u32) instead?

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Unfortunately that would be a syntax error. I can't figure out a nice-looking way to use character literals here; RangeInclusive::contains isn't const, ('a' as u32 <= *self as u32 | 0x20) && ('f' as u32 >= *self as u32 | 0x20) is very long.

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You can still use that range in a pattern if you define it as a const item first.

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@eggyal eggyal Oct 14, 2022 •
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One could make it slightly more concise:

let lower = *self as u32 | 0x20;
matches!(*self, '0'..='9') || (lower >= 'a' as u32 && lower <= 'f' as u32)

As an aside, there's a (currently private) const ASCII_CASE_MASK: u8 in core::num which might be preferable to a literal 0x20.

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You can still use that range in a pattern if you define it as a const item first.

    const LOWER_ASCII: RangeInclusive<u32> = ('a' as u32)..=('f' as u32);
    matches!(*c, '0'..='9') || matches!(*c as u32 | 0x20, LOWER_ASCII)

doesn't seem to compile? I can get it to work with A and F const items though which I have done.

}

/// Checks if the value is an ASCII punctuation character:
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3 changes: 2 additions & 1 deletion library/core/src/num/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -688,7 +688,8 @@ impl u8 {
#[rustc_const_stable(feature = "const_ascii_ctype_on_intrinsics", since = "1.47.0")]
#[inline]
pub const fn is_ascii_hexdigit(&self) -> bool {
matches!(*self, b'0'..=b'9' | b'A'..=b'F' | b'a'..=b'f')
// Bitwise or can avoid need for branches in compiled code.
matches!(*self, b'0'..=b'9') || matches!(*self | 0x20, b'a'..=b'f')
}

/// Checks if the value is an ASCII punctuation character:
Expand Down