Override disjoint_or in the LLVM backend

compiler/rustc_codegen_llvm/src/builder.rs

@@ -421,6 +421,14 @@ impl<'a, 'll, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'll, 'tcx> {
         unchecked_umul(x, y) => LLVMBuildNUWMul,
     }
 
+    fn or_disjoint(&mut self, a: &'ll Value, b: &'ll Value) -> &'ll Value {
+        unsafe {
+            let or = llvm::LLVMBuildOr(self.llbuilder, a, b, UNNAMED);
+            llvm::LLVMSetIsDisjoint(or, True);
+            or
+        }
+    }
+
     set_math_builder_methods! {
         fadd_fast(x, y) => (LLVMBuildFAdd, LLVMRustSetFastMath),
         fsub_fast(x, y) => (LLVMBuildFSub, LLVMRustSetFastMath),

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -1380,6 +1380,9 @@ unsafe extern "C" {
     pub fn LLVMBuildFNeg<'a>(B: &Builder<'a>, V: &'a Value, Name: *const c_char) -> &'a Value;
     pub fn LLVMBuildNot<'a>(B: &Builder<'a>, V: &'a Value, Name: *const c_char) -> &'a Value;
 
+    // Extra flags on arithmetic
+    pub fn LLVMSetIsDisjoint(Instr: &Value, IsDisjoint: Bool);
+
     // Memory
     pub fn LLVMBuildAlloca<'a>(B: &Builder<'a>, Ty: &'a Type, Name: *const c_char) -> &'a Value;
     pub fn LLVMBuildArrayAlloca<'a>(

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()) {

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;

tests/codegen/bigint-helpers.rs

@@ -0,0 +1,13 @@
+//@ compile-flags: -C opt-level=3
+
+#![crate_type = "lib"]
+#![feature(bigint_helper_methods)]
+
+// CHECK-LABEL: @u32_carrying_add
+#[no_mangle]
+pub fn u32_carrying_add(a: u32, b: u32, c: bool) -> (u32, bool) {
+    // CHECK: @llvm.uadd.with.overflow.i32
+    // CHECK: @llvm.uadd.with.overflow.i32
+    // CHECK: or disjoint i1
+    u32::carrying_add(a, b, c)
+}

tests/codegen/intrinsics/disjoint_bitor.rs

@@ -0,0 +1,20 @@
+//@ compile-flags: -C no-prepopulate-passes
+
+#![crate_type = "lib"]
+#![feature(core_intrinsics)]
+
+use std::intrinsics::disjoint_bitor;
+
+// CHECK-LABEL: @disjoint_bitor_signed
+#[no_mangle]
+pub unsafe fn disjoint_bitor_signed(x: i32, y: i32) -> i32 {
+    // CHECK: or disjoint i32 %x, %y
+    disjoint_bitor(x, y)
+}
+
+// CHECK-LABEL: @disjoint_bitor_unsigned
+#[no_mangle]
+pub unsafe fn disjoint_bitor_unsigned(x: u64, y: u64) -> u64 {
+    // CHECK: or disjoint i64 %x, %y
+    disjoint_bitor(x, y)
+}