WebAssembly · tlively · Mar 18, 2026 · Mar 18, 2026
@@ -2003,6 +2003,10 @@ struct OptimizeInstructions
    if (curr->ref->type.getHeapType().isShared()) {
      return;
    }
+    if (curr->type == Type::unreachable) {
+      // Leave this to DCE.
+      return;
+    }

    // Lower the RMW to its more basic operations. Breaking the atomic
    // operation into several non-atomic operations is safe because no other
@@ -2100,6 +2104,11 @@ struct OptimizeInstructions
      return;
    }

+    if (curr->type == Type::unreachable) {
+      // Leave this to DCE.
+      return;
+    }
+
    // Just like other RMW operations, lower to basic operations when operating
    // on unshared memory.
    auto ref = builder.addVar(getFunction(), curr->ref->type);

@@ -1308,4 +1308,66 @@
      (i32.const 2)
    )
  )
+
+  ;; CHECK:      (func $struct-rmw-unreachable-lowering (type $16) (param $0 (ref null $unshared-i32)) (result i32)
+  ;; CHECK-NEXT:  (struct.atomic.rmw.add $unshared-i32 0
+  ;; CHECK-NEXT:   (local.get $0)
+  ;; CHECK-NEXT:   (unreachable)
+  ;; CHECK-NEXT:  )
+  ;; CHECK-NEXT: )
+  (func $struct-rmw-unreachable-lowering (param (ref null $unshared-i32)) (result i32)
+    ;; Check that we skip lowering if an operand is unreachable.
+    (struct.atomic.rmw.add $unshared-i32 0
+      (local.get 0)
+      (unreachable)
+    )
+  )
+
+  ;; CHECK:      (func $struct-cmpxchg-unreachable-lowering (type $16) (param $0 (ref null $unshared-i32)) (result i32)
+  ;; CHECK-NEXT:  (struct.atomic.rmw.cmpxchg $unshared-i32 0
+  ;; CHECK-NEXT:   (local.get $0)
+  ;; CHECK-NEXT:   (i32.const 1)
+  ;; CHECK-NEXT:   (unreachable)
+  ;; CHECK-NEXT:  )
+  ;; CHECK-NEXT: )
+  (func $struct-cmpxchg-unreachable-lowering (param (ref null $unshared-i32)) (result i32)
+    (struct.atomic.rmw.cmpxchg $unshared-i32 0
+      (local.get 0)
+      (i32.const 1)
+      (unreachable)
+    )
+  )
+
+  ;; CHECK:      (func $array-rmw-unreachable-lowering (type $19) (param $0 (ref null $array)) (result i32)
+  ;; CHECK-NEXT:  (array.atomic.rmw.add $array
+  ;; CHECK-NEXT:   (local.get $0)
+  ;; CHECK-NEXT:   (i32.const 0)
+  ;; CHECK-NEXT:   (unreachable)
+  ;; CHECK-NEXT:  )
+  ;; CHECK-NEXT: )
+  (func $array-rmw-unreachable-lowering (param (ref null $array)) (result i32)
+    ;; Check that we skip lowering if an operand is unreachable.
+    (array.atomic.rmw.add $array
+      (local.get 0)
+      (i32.const 0)
+      (unreachable)
+    )
+  )
+
+  ;; CHECK:      (func $cmpxchg-unreachable-lowering (type $19) (param $0 (ref null $array)) (result i32)
+  ;; CHECK-NEXT:  (array.atomic.rmw.cmpxchg $array
+  ;; CHECK-NEXT:   (local.get $0)
+  ;; CHECK-NEXT:   (i32.const 0)
+  ;; CHECK-NEXT:   (i32.const 1)
+  ;; CHECK-NEXT:   (unreachable)
+  ;; CHECK-NEXT:  )
+  ;; CHECK-NEXT: )
+  (func $cmpxchg-unreachable-lowering (param (ref null $array)) (result i32)
+    (array.atomic.rmw.cmpxchg $array
+      (local.get 0)
+      (i32.const 0)
+      (i32.const 1)
+      (unreachable)
+    )
+  )
 )