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
@@ -363,6 +363,7 @@
 #define OPTIMISED_CASE(n, lockfree, type) ATOMIC_RMW(n, lockfree, type, xor, ^)
 OPTIMISED_CASES
 #undef OPTIMISED_CASE
+// Allow build with clang without __c11_atomic_fetch_nand builtin (pre-14)
 #if __has_builtin(__c11_atomic_fetch_nand)
 #define OPTIMISED_CASE(n, lockfree, type) ATOMIC_RMW_NAND(n, lockfree, type)
 OPTIMISED_CASES