Index: contrib/llvm-project/compiler-rt/lib/builtins/atomic.c
===================================================================
--- contrib/llvm-project/compiler-rt/lib/builtins/atomic.c
+++ contrib/llvm-project/compiler-rt/lib/builtins/atomic.c
@@ -336,6 +336,7 @@
     return tmp;                                                                \
   }
 
+#if __has_builtin(__c11_atomic_fetch_nand)
 #define ATOMIC_RMW_NAND(n, lockfree, type)                                     \
   type __atomic_fetch_nand_##n(type *ptr, type val, int model) {               \
     if (lockfree(ptr))                                                         \
@@ -347,6 +348,17 @@
     unlock(l);                                                                 \
     return tmp;                                                                \
   }
+#else
+#define ATOMIC_RMW_NAND(n, lockfree, type)                                     \
+  type __atomic_fetch_nand_##n(type *ptr, type val, int model) {               \
+    Lock *l = lock_for_pointer(ptr);                                           \
+    lock(l);                                                                   \
+    type tmp = *ptr;                                                           \
+    *ptr = ~(tmp & val);                                                       \
+    unlock(l);                                                                 \
+    return tmp;                                                                \
+  }
+#endif
 
 #define OPTIMISED_CASE(n, lockfree, type) ATOMIC_RMW(n, lockfree, type, add, +)
 OPTIMISED_CASES