diff --git a/compiler-rt/lib/builtins/atomic.c b/compiler-rt/lib/builtins/atomic.c
--- a/compiler-rt/lib/builtins/atomic.c
+++ b/compiler-rt/lib/builtins/atomic.c
@@ -92,6 +92,8 @@
 static Lock locks[SPINLOCK_COUNT]; // initialized to OS_SPINLOCK_INIT which is 0
 
 #else
+_Static_assert(__atomic_always_lock_free(sizeof(uintptr_t), 0),
+               "Implementation assumes lock-free pointer-size cmpxchg");
 typedef _Atomic(uintptr_t) Lock;
 /// Unlock a lock.  This is a release operation.
 __inline static void unlock(Lock *l) {