`algorithms/mod`: This only black-boxes `acc` so LLVM keeps the subtraction results

[github-actions]

On 2026-04-30 @prestwich wrote in 836a44f “Merge pull request #579 from DaniPopes/cmp-with-sub-revived”:

This only black-boxes acc so LLVM keeps the subtraction results
alive long enough for the backend to recognize the borrowing_sub chain
and generate better code: rust-lang/rust#143517
SAFETY: Writing to a local variable through a reference is safe.

    for i in 0..a.len() {
        let x;
        (x, borrow) = borrowing_sub(a[i], b[i], borrow);
        acc |= x;
    }
    // HACK: This only black-boxes `acc` so LLVM keeps the subtraction results
    // alive long enough for the backend to recognize the `borrowing_sub` chain
    // and generate better code: https://github.com/rust-lang/rust/issues/143517
    // SAFETY: Writing to a local variable through a reference is safe.
    unsafe { core::ptr::write_volatile(&mut acc, acc) };
    (acc, borrow)
}

#[inline]

From src/algorithms/mod.rs:188