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>

0 files changed, 0 insertions, 0 deletions