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
@@ -225,8 +225,10 @@
 }
 
 /// Performs an atomic exchange operation between two pointers.  This is atomic
-/// with respect to the target address.
-void __atomic_exchange_c(int size, void *ptr, void *val, void *old, int model) {
+/// with respect to the target address. ptr, val and old should not alias each
+/// other.
+void __atomic_exchange_c(int size, void *restrict ptr, void *restrict val,
+                         void *restrict old, int model) {
 #define LOCK_FREE_ACTION(type)                                                 \
   *(type *)old =                                                               \
       __c11_atomic_exchange((_Atomic(type) *)ptr, *(type *)val, model);        \