py: Fix compiling and decoding of *args at large arg positions.

There were two issues with the existing code:

1. "1 << i" is computed as a 32-bit number so would overflow when
   executed on 64-bit machines (when mp_uint_t is 64-bit).  This meant that
   *args beyond 32 positions would not be handled correctly.

2. star_args must fit as a positive small int so that it is encoded
   correctly in the emitted code.  MP_SMALL_INT_BITS is too big because it
   overflows a small int by 1 bit.  MP_SMALL_INT_BITS - 1 does not work
   because it produces a signed small int which is then sign extended when
   extracted (even by mp_obj_get_int_truncated), and this sign extension
   means that any position arg after *args is also treated as a star-arg.
   So the maximum bit position is MP_SMALL_INT_BITS - 2.  This means that
   MP_OBJ_SMALL_INT_VALUE() can be used instead of
   mp_obj_get_int_truncated() to get the value of star_args.

These issues are fixed by this commit, and a test added.

Signed-off-by: Damien George <damien@micropython.org>

1 files changed, 3 insertions, 3 deletions

diff --git a/py/runtime.c b/py/runtime.c
index 594e63082..ad066acb1 100644
--- a/py/runtime.c
+++ b/py/runtime.c
@@ -702,7 +702,7 @@ void mp_call_prepare_args_n_kw_var(bool have_self, size_t n_args_n_kw, const mp_
     }
     size_t n_args = n_args_n_kw & 0xff;
     size_t n_kw = (n_args_n_kw >> 8) & 0xff;
-    mp_uint_t star_args = mp_obj_get_int_truncated(args[n_args + 2 * n_kw]);
+    mp_uint_t star_args = MP_OBJ_SMALL_INT_VALUE(args[n_args + 2 * n_kw]);
 
     DEBUG_OP_printf("call method var (fun=%p, self=%p, n_args=%u, n_kw=%u, args=%p, map=%u)\n", fun, self, n_args, n_kw, args, star_args);
 
@@ -720,7 +720,7 @@ void mp_call_prepare_args_n_kw_var(bool have_self, size_t n_args_n_kw, const mp_
 
     if (star_args != 0) {
         for (size_t i = 0; i < n_args; i++) {
-            if (star_args & (1 << i)) {
+            if ((star_args >> i) & 1) {
                 mp_obj_t len = mp_obj_len_maybe(args[i]);
                 if (len != MP_OBJ_NULL) {
                     // -1 accounts for 1 of n_args occupied by this arg
@@ -773,7 +773,7 @@ void mp_call_prepare_args_n_kw_var(bool have_self, size_t n_args_n_kw, const mp_
 
         for (size_t i = 0; i < n_args; i++) {
             mp_obj_t arg = args[i];
-            if (star_args & (1 << i)) {
+            if ((star_args >> i) & 1) {
                 // star arg
                 if (mp_obj_is_type(arg, &mp_type_tuple) || mp_obj_is_type(arg, &mp_type_list)) {
                     // optimise the case of a tuple and list