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Eliminate WideWord casts from borrowing_sub #1029

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Merged
tarcieri merged 1 commit into from
Dec 10, 2025
Merged

Eliminate WideWord casts from borrowing_sub #1029

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10 changes: 5 additions & 5 deletions src/primitives.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -31,11 +31,11 @@ pub(crate) const fn overflowing_add(lhs: Word, rhs: Word) -> (Word, Word) {
/// Computes `lhs - (rhs + borrow)`, returning the result along with the new borrow.
#[inline(always)]
pub(crate) const fn borrowing_sub(lhs: Word, rhs: Word, borrow: Word) -> (Word, Word) {
let a = lhs as WideWord;
let b = rhs as WideWord;
let borrow = (borrow >> (Word::BITS - 1)) as WideWord;
let ret = a.wrapping_sub(b + borrow);
(ret as Word, (ret >> Word::BITS) as Word)
// XXX we cannot use WideWord casts here: https://github.com/rust-lang/rust/issues/149522
// rustc 1.87 through 1.91 incorrectly optimize some WideWord bit arithmetic.
let (ret, b2) = lhs.overflowing_sub(borrow >> (Word::BITS - 1));
let (ret, b1) = ret.overflowing_sub(rhs);
(ret, Word::MIN.wrapping_sub((b1 | b2) as Word))
}

/// Computes `lhs * rhs`, returning the low and the high words of the result.
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