Auto merge of #135760 - scottmcm:disjoint-bitor, r=WaffleLapkin

Add `unchecked_disjoint_bitor` per ACP373

Following the names from libs-api in https://github.com/rust-lang/libs-team/issues/373#issuecomment-2085686057

Includes a fallback implementation so this doesn't have to update cg_clif or cg_gcc, and overrides it in cg_llvm to use `or disjoint`, which [is available in LLVM 18](https://releases.llvm.org/18.1.0/docs/LangRef.html#or-instruction) so hopefully we don't need any version checks.

2 files changed, 10 insertions, 0 deletions

diff --git a/compiler/rustc_codegen_ssa/src/mir/intrinsic.rs b/compiler/rustc_codegen_ssa/src/mir/intrinsic.rs
index 304ac4544ee..6e7fbe62c8d 100644
--- a/compiler/rustc_codegen_ssa/src/mir/intrinsic.rs
+++ b/compiler/rustc_codegen_ssa/src/mir/intrinsic.rs
@@ -225,6 +225,11 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                 args[1].val.unaligned_volatile_store(bx, dst);
                 return Ok(());
             }
+            sym::disjoint_bitor => {
+                let a = args[0].immediate();
+                let b = args[1].immediate();
+                bx.or_disjoint(a, b)
+            }
             sym::exact_div => {
                 let ty = arg_tys[0];
                 match int_type_width_signed(ty, bx.tcx()) {
diff --git a/compiler/rustc_codegen_ssa/src/traits/builder.rs b/compiler/rustc_codegen_ssa/src/traits/builder.rs
index bbf87a59942..b7dcf16fa2b 100644
--- a/compiler/rustc_codegen_ssa/src/traits/builder.rs
+++ b/compiler/rustc_codegen_ssa/src/traits/builder.rs
@@ -167,6 +167,11 @@ pub trait BuilderMethods<'a, 'tcx>:
     fn unchecked_umul(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
     fn and(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
     fn or(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
+    /// Defaults to [`Self::or`], but guarantees `(lhs & rhs) == 0` so some backends
+    /// can emit something more helpful for optimizations.
+    fn or_disjoint(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value {
+        self.or(lhs, rhs)
+    }
     fn xor(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
     fn neg(&mut self, v: Self::Value) -> Self::Value;
     fn fneg(&mut self, v: Self::Value) -> Self::Value;