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
@@ -328,6 +328,18 @@
     return tmp;                                                                \
   }
 
+// No lockfree atomic_fetch_*_explicit variation.
+#define ATOMIC_RMW_NEG(n, lockfree, type, opname, op)                          \
+  type __atomic_fetch_##opname##_##n(_Atomic(type) * ptr, type val,            \
+                                     memory_order model) {                     \
+    Lock *l = lock_for_pointer(ptr);                                           \
+    lock(l);                                                                   \
+    type tmp = *ptr;                                                           \
+    *ptr = ~(tmp op val);                                                      \
+    unlock(l);                                                                 \
+    return tmp;                                                                \
+  }
+
 #define OPTIMISED_CASE(n, lockfree, type) ATOMIC_RMW(n, lockfree, type, add, +)
 OPTIMISED_CASES
 #undef OPTIMISED_CASE
@@ -343,3 +355,7 @@
 #define OPTIMISED_CASE(n, lockfree, type) ATOMIC_RMW(n, lockfree, type, xor, ^)
 OPTIMISED_CASES
 #undef OPTIMISED_CASE
+#define OPTIMISED_CASE(n, lockfree, type)                                      \
+  ATOMIC_RMW_NEG(n, lockfree, type, nand, &)
+OPTIMISED_CASES
+#undef OPTIMISED_CASE