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
@@ -233,10 +233,19 @@
   return;
   LOCK_FREE_CASES(ptr);
 #undef LOCK_FREE_ACTION
+  int i;
   Lock *l = lock_for_pointer(ptr);
   lock(l);
-  memcpy(old, ptr, size);
-  memcpy(ptr, val, size);
+  if (val != old) {
+    memcpy(old, ptr, size);
+    memcpy(ptr, val, size);
+  } else {
+    for (i = 0; i < size; ++i) {
+      *((char *)ptr + i) ^= *((char *)old + i);
+      *((char *)old + i) ^= *((char *)ptr + i);
+      *((char *)ptr + i) ^= *((char *)old + i);
+    }
+  }
   unlock(l);
 }