diff --git a/tccasm.c b/tccasm.c
index e8bc357..50fb794 100644
--- a/tccasm.c
+++ b/tccasm.c
@@ -946,7 +946,7 @@ static void subst_asm_operands(ASMOperand *operands, int nb_operands,
             modifier = 0;
             if (*str == 'c' || *str == 'n' ||
                 *str == 'b' || *str == 'w' ||
-                *str == 'h' || *str == 'k')
+                *str == 'h' || *str == 'k' || *str == 'q')
                 modifier = *str++;
             index = find_constraint(operands, nb_operands, str, &str);
             if (index < 0)
diff --git a/tests/tcctest.c b/tests/tcctest.c
index 5851fb4..7fd8171 100644
--- a/tests/tcctest.c
+++ b/tests/tcctest.c
@@ -2584,6 +2584,17 @@ unsigned long mconstraint_test(struct struct1231 *r)
     return ret + a[0];
 }
 
+#ifdef __x86_64__
+int fls64(unsigned long long x)
+{
+  int bitpos = -1;
+  asm("bsrq %1,%q0"
+      : "+r" (bitpos)
+      : "rm" (x));
+  return bitpos + 1;
+}
+#endif
+
 unsigned int set;
 
 void asm_test(void)