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Use disjoint_bitor inside borrowing_sub #138228

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Merged
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Mar 9, 2025
Merged

Use disjoint_bitor inside borrowing_sub #138228

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11 changes: 8 additions & 3 deletions library/core/src/num/uint_macros.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -2533,15 +2533,20 @@ macro_rules! uint_impl {
#[doc = concat!("assert_eq!((diff1, diff0), (3, ", stringify!($SelfT), "::MAX));")]
/// ```
#[unstable(feature = "bigint_helper_methods", issue = "85532")]
#[rustc_const_unstable(feature = "bigint_helper_methods", issue = "85532")]
#[must_use = "this returns the result of the operation, \
without modifying the original"]
#[inline]
pub const fn borrowing_sub(self, rhs: Self, borrow: bool) -> (Self, bool) {
// note: longer-term this should be done via an intrinsic, but this has been shown
// to generate optimal code for now, and LLVM doesn't have an equivalent intrinsic
let (a, b) = self.overflowing_sub(rhs);
let (c, d) = a.overflowing_sub(borrow as $SelfT);
(c, b | d)
let (a, c1) = self.overflowing_sub(rhs);
let (b, c2) = a.overflowing_sub(borrow as $SelfT);
// SAFETY: Only one of `c1` and `c2` can be set.
// For c1 to be set we need to have underflowed, but if we did then
// `a` is nonzero, which means that `c2` cannot possibly
// underflow because it's subtracting at most `1` (since it came from `bool`)
(b, unsafe { intrinsics::disjoint_bitor(c1, c2) })
}

/// Calculates `self` - `rhs` with a signed `rhs`
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