diff --git a/crossbeam-utils/src/atomic/atomic_cell.rs b/crossbeam-utils/src/atomic/atomic_cell.rs
index 0b62b89d5..c7876a6d6 100644
--- a/crossbeam-utils/src/atomic/atomic_cell.rs
+++ b/crossbeam-utils/src/atomic/atomic_cell.rs
@@ -295,7 +295,7 @@ impl<T: Copy + Eq> AtomicCell<T> {
 }
 
 macro_rules! impl_arithmetic {
-    ($t:ty, $example:tt) => {
+    ($t:ty, fallback, $example:tt) => {
         impl AtomicCell<$t> {
             /// Increments the current value by `val` and returns the previous value.
             ///
@@ -313,10 +313,10 @@ macro_rules! impl_arithmetic {
             /// ```
             #[inline]
             pub fn fetch_add(&self, val: $t) -> $t {
-                if can_transmute::<$t, atomic::AtomicUsize>() {
-                    let a = unsafe { &*(self.value.get() as *const atomic::AtomicUsize) };
-                    a.fetch_add(val as usize, Ordering::AcqRel) as $t
-                } else {
+                #[cfg(crossbeam_loom)]
+                unimplemented!("loom does not support non-atomic atomic ops");
+                #[cfg(not(crossbeam_loom))]
+                {
                     let _guard = lock(self.value.get() as usize).write();
                     let value = unsafe { &mut *(self.value.get()) };
                     let old = *value;
@@ -341,10 +341,10 @@ macro_rules! impl_arithmetic {
             /// ```
             #[inline]
             pub fn fetch_sub(&self, val: $t) -> $t {
-                if can_transmute::<$t, atomic::AtomicUsize>() {
-                    let a = unsafe { &*(self.value.get() as *const atomic::AtomicUsize) };
-                    a.fetch_sub(val as usize, Ordering::AcqRel) as $t
-                } else {
+                #[cfg(crossbeam_loom)]
+                unimplemented!("loom does not support non-atomic atomic ops");
+                #[cfg(not(crossbeam_loom))]
+                {
                     let _guard = lock(self.value.get() as usize).write();
                     let value = unsafe { &mut *(self.value.get()) };
                     let old = *value;
@@ -367,10 +367,10 @@ macro_rules! impl_arithmetic {
             /// ```
             #[inline]
             pub fn fetch_and(&self, val: $t) -> $t {
-                if can_transmute::<$t, atomic::AtomicUsize>() {
-                    let a = unsafe { &*(self.value.get() as *const atomic::AtomicUsize) };
-                    a.fetch_and(val as usize, Ordering::AcqRel) as $t
-                } else {
+                #[cfg(crossbeam_loom)]
+                unimplemented!("loom does not support non-atomic atomic ops");
+                #[cfg(not(crossbeam_loom))]
+                {
                     let _guard = lock(self.value.get() as usize).write();
                     let value = unsafe { &mut *(self.value.get()) };
                     let old = *value;
@@ -393,10 +393,10 @@ macro_rules! impl_arithmetic {
             /// ```
             #[inline]
             pub fn fetch_or(&self, val: $t) -> $t {
-                if can_transmute::<$t, atomic::AtomicUsize>() {
-                    let a = unsafe { &*(self.value.get() as *const atomic::AtomicUsize) };
-                    a.fetch_or(val as usize, Ordering::AcqRel) as $t
-                } else {
+                #[cfg(crossbeam_loom)]
+                unimplemented!("loom does not support non-atomic atomic ops");
+                #[cfg(not(crossbeam_loom))]
+                {
                     let _guard = lock(self.value.get() as usize).write();
                     let value = unsafe { &mut *(self.value.get()) };
                     let old = *value;
@@ -419,10 +419,10 @@ macro_rules! impl_arithmetic {
             /// ```
             #[inline]
             pub fn fetch_xor(&self, val: $t) -> $t {
-                if can_transmute::<$t, atomic::AtomicUsize>() {
-                    let a = unsafe { &*(self.value.get() as *const atomic::AtomicUsize) };
-                    a.fetch_xor(val as usize, Ordering::AcqRel) as $t
-                } else {
+                #[cfg(crossbeam_loom)]
+                unimplemented!("loom does not support non-atomic atomic ops");
+                #[cfg(not(crossbeam_loom))]
+                {
                     let _guard = lock(self.value.get() as usize).write();
                     let value = unsafe { &mut *(self.value.get()) };
                     let old = *value;
@@ -541,9 +541,15 @@ impl_arithmetic!(i32, atomic::AtomicI32, "let a = AtomicCell::new(7i32);");
 impl_arithmetic!(u64, atomic::AtomicU64, "let a = AtomicCell::new(7u64);");
 #[cfg(not(crossbeam_no_atomic_64))]
 impl_arithmetic!(i64, atomic::AtomicI64, "let a = AtomicCell::new(7i64);");
+#[cfg(crossbeam_no_atomic_64)]
+impl_arithmetic!(u64, fallback, "let a = AtomicCell::new(7u64);");
+#[cfg(crossbeam_no_atomic_64)]
+impl_arithmetic!(i64, fallback, "let a = AtomicCell::new(7i64);");
 // TODO: AtomicU128 is unstable
 // impl_arithmetic!(u128, atomic::AtomicU128, "let a = AtomicCell::new(7u128);");
 // impl_arithmetic!(i128, atomic::AtomicI128, "let a = AtomicCell::new(7i128);");
+impl_arithmetic!(u128, fallback, "let a = AtomicCell::new(7u128);");
+impl_arithmetic!(i128, fallback, "let a = AtomicCell::new(7i128);");
 
 impl_arithmetic!(
     usize,