Add unchecked_disjoint_bitor with fallback intrinsic implementation

core/src/intrinsics/fallback.rs

@@ -110,3 +110,43 @@ impl const CarryingMulAdd for i128 {
         (low, high)
     }
 }
+
+#[const_trait]
+#[rustc_const_unstable(feature = "core_intrinsics_fallbacks", issue = "none")]
+pub trait DisjointBitOr: Copy + 'static {
+    /// This is always just `assume((self & other) == 0); self | other`.
+    ///
+    /// It's essential that the assume is there so that this is sufficient to
+    /// specify the UB for MIRI, rather than it needing to re-implement it.
+    ///
+    /// # Safety
+    /// See [`super::disjoint_bitor`].
+    unsafe fn disjoint_bitor(self, other: Self) -> Self;
+}
+macro_rules! zero {
+    (bool) => {
+        false
+    };
+    ($t:ident) => {
+        0
+    };
+}
+macro_rules! impl_disjoint_bitor {
+    ($($t:ident,)+) => {$(
+        #[rustc_const_unstable(feature = "core_intrinsics_fallbacks", issue = "none")]
+        impl const DisjointBitOr for $t {
+            #[inline]
+            unsafe fn disjoint_bitor(self, other: Self) -> Self {
+                // SAFETY: our precondition is that there are no bits in common,
+                // so this is just telling that to the backend.
+                unsafe { super::assume((self & other) == zero!($t)) };
+                self | other
+            }
+        }
+    )+};
+}
+impl_disjoint_bitor! {
+    bool,
+    u8, u16, u32, u64, u128, usize,
+    i8, i16, i32, i64, i128, isize,
+}

core/src/intrinsics/mod.rs

@@ -3248,6 +3248,25 @@ pub const fn three_way_compare<T: Copy>(_lhs: T, _rhss: T) -> crate::cmp::Orderi
     unimplemented!()
 }
 
+/// Combine two values which have no bits in common.
+///
+/// This allows the backend to implement it as `a + b` *or* `a | b`,
+/// depending which us easier to implement on a specific target.
+///
+/// # Safety
+///
+/// Requires that `(a & b) == 0`, or equivalently that `(a | b) == (a + b)`.
+///
+/// Otherwise it's immediate UB.
+#[rustc_const_unstable(feature = "disjoint_bitor", issue = "135758")]
+#[rustc_nounwind]
+#[cfg_attr(not(bootstrap), rustc_intrinsic)]
+#[miri::intrinsic_fallback_is_spec] // the fallbacks all `assume` to tell MIRI
+pub const unsafe fn disjoint_bitor<T: ~const fallback::DisjointBitOr>(a: T, b: T) -> T {
+    // SAFETY: same preconditions as this function.
+    unsafe { fallback::DisjointBitOr::disjoint_bitor(a, b) }
+}
+
 /// Performs checked integer addition.
 ///
 /// Note that, unlike most intrinsics, this is safe to call;

core/src/lib.rs

@@ -117,6 +117,7 @@
 #![feature(const_eval_select)]
 #![feature(core_intrinsics)]
 #![feature(coverage_attribute)]
+#![feature(disjoint_bitor)]
 #![feature(internal_impls_macro)]
 #![feature(ip)]
 #![feature(is_ascii_octdigit)]

core/src/num/uint_macros.rs

@@ -1187,6 +1187,52 @@ macro_rules! uint_impl {
             self % rhs
         }
 
+        /// Same value as
+        #[doc = concat!("`<", stringify!($SelfT), " as BitOr>::bitor(self, other)`")]
+        /// but UB if any bit position is set in both inputs.
+        ///
+        /// This is a situational μoptimization for places where you'd rather use
+        /// addition on some platforms and bitwise or on other platforms, based on
+        /// exactly which instructions combine better with whatever else you're
+        /// doing.  Note that there's no reason to bother using this for places
+        /// where it's clear from the operations involved that they can't overlap.
+        /// For example, if you're combining `u16`s into a `u32` with
+        /// `((a as u32) << 16) | (b as u32)`, that's fine, as the backend will
+        /// know those sides of the `|` are disjoint without needing help.
+        ///
+        /// # Examples
+        ///
+        /// ```
+        /// #![feature(disjoint_bitor)]
+        ///
+        /// // SAFETY: `1` and `4` have no bits in common.
+        /// unsafe {
+        #[doc = concat!("    assert_eq!(1_", stringify!($SelfT), ".unchecked_disjoint_bitor(4), 5);")]
+        /// }
+        /// ```
+        ///
+        /// # Safety
+        ///
+        /// Requires that `(self | other) == 0`, otherwise it's immediate UB.
+        ///
+        /// Equivalently, requires that `(self | other) == (self + other)`.
+        #[unstable(feature = "disjoint_bitor", issue = "135758")]
+        #[rustc_const_unstable(feature = "disjoint_bitor", issue = "135758")]
+        #[inline]
+        pub const unsafe fn unchecked_disjoint_bitor(self, other: Self) -> Self {
+            assert_unsafe_precondition!(
+                check_language_ub,
+                concat!(stringify!($SelfT), "::unchecked_disjoint_bitor cannot have overlapping bits"),
+                (
+                    lhs: $SelfT = self,
+                    rhs: $SelfT = other,
+                ) => (lhs & rhs) == 0,
+            );
+
+            // SAFETY: Same precondition
+            unsafe { intrinsics::disjoint_bitor(self, other) }
+        }
+
         /// Returns the logarithm of the number with respect to an arbitrary base,
         /// rounded down.
         ///
@@ -2346,15 +2392,22 @@ macro_rules! uint_impl {
         /// assert_eq!((sum1, sum0), (9, 6));
         /// ```
         #[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 carrying_add(self, rhs: Self, carry: 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_add(rhs);
-            let (c, d) = a.overflowing_add(carry as $SelfT);
-            (c, b | d)
+            let (a, c1) = self.overflowing_add(rhs);
+            let (b, c2) = a.overflowing_add(carry as $SelfT);
+            // Ideally LLVM would know this is disjoint without us telling them,
+            // but it doesn't <https://github.com/llvm/llvm-project/issues/118162>
+            // SAFETY: Only one of `c1` and `c2` can be set.
+            // For c1 to be set we need to have overflowed, but if we did then
+            // `a` is at most `MAX-1`, which means that `c2` cannot possibly
+            // overflow because it's adding at most `1` (since it came from `bool`)
+            (b, unsafe { intrinsics::disjoint_bitor(c1, c2) })
         }
 
         /// Calculates `self` + `rhs` with a signed `rhs`.