diff --git a/crypto/modes/internal.h b/crypto/modes/internal.h
index 4fa0ec6..4659eab 100644
--- a/crypto/modes/internal.h
+++ b/crypto/modes/internal.h
@@ -194,6 +194,13 @@
 #endif
 
 
+#if defined(OPENSSL_X86) || defined(OPENSSL_X86_64)
+/* crypto_gcm_clmul_enabled returns one if the CLMUL implementation of GCM is
+ * used. */
+int crypto_gcm_clmul_enabled(void);
+#endif
+
+
 #if defined(__cplusplus)
 } /* extern C */
 #endif