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>

2 files changed, 38 insertions, 0 deletions

diff --git a/tests/stress/fun_call_limit.py b/tests/stress/fun_call_limit.py
new file mode 100644
index 000000000..b802aadd5
--- /dev/null
+++ b/tests/stress/fun_call_limit.py
@@ -0,0 +1,36 @@
+# Test the limit of the number of arguments to a function call.
+# This currently tests the case of *args after many positional args.
+
+
+def f(*args):
+    return len(args)
+
+
+def test(n):
+    pos_args = ",".join(str(i) for i in range(n))
+    s = "f({}, *(100, 101), 102, 103)".format(pos_args)
+    try:
+        return eval(s)
+    except SyntaxError:
+        return "SyntaxError"
+
+
+# If the port has at least 32-bits then this test should pass.
+print(test(29))
+
+# This test should fail on all ports (overflows a small int).
+print(test(70))
+
+# Check that there is a correct transition to the limit of too many args before *args.
+reached_limit = False
+for i in range(30, 70):
+    result = test(i)
+    if reached_limit:
+        if result != "SyntaxError":
+            print("FAIL")
+    else:
+        if result == "SyntaxError":
+            reached_limit = True
+        else:
+            if result != i + 4:
+                print("FAIL")
diff --git a/tests/stress/fun_call_limit.py.exp b/tests/stress/fun_call_limit.py.exp
new file mode 100644
index 000000000..53d2b2804
--- /dev/null
+++ b/tests/stress/fun_call_limit.py.exp
@@ -0,0 +1,2 @@
+33
+SyntaxError