diff --git a/mult/imult64.c b/mult/imult64.c
index 961d226..1c3c0dd 100644
--- a/mult/imult64.c
+++ b/mult/imult64.c
@@ -1,5 +1,6 @@
 #ifdef __x86_64__
 
+/* WARNING: this only works if compiled with -fomit-frame-pointer */
 void imult64() {
   asm volatile(
     "xchgq %rdx,%rsi\n"
diff --git a/mult/umult64.c b/mult/umult64.c
index 5b644dd..82bf2ab 100644
--- a/mult/umult64.c
+++ b/mult/umult64.c
@@ -1,5 +1,6 @@
 #ifdef __x86_64__
 
+/* WARNING: this only works if compiled with -fomit-frame-pointer */
 void umult64() {
   asm volatile(
     "xchgq %rdx,%rsi\n"