diff --git a/library/aesce.c b/library/aesce.c
index 752e008..e36c6d4 100644
--- a/library/aesce.c
+++ b/library/aesce.c
@@ -568,7 +568,7 @@
     /* use 'asm' as an optimisation barrier to prevent loading MODULO from
      * memory. It is for GNUC compatible compilers.
      */
-    asm ("" : "+w" (r));
+    asm volatile ("" : "+w" (r));
 #endif
     uint8x16_t const MODULO = vreinterpretq_u8_u64(vshrq_n_u64(r, 64 - 8));
     uint8x16_t h, m, l; /* input high/middle/low 128b */