Subject: Buglet in PyLong_AsLong

Subject: Buglet in PyLong_AsLong
From: "Tim Peters" <tim_one@email.msn.com>
To: "Guido van Rossum" <guido@CNRI.Reston.VA.US>
Date: Sat, 23 May 1998 21:45:53 -0400

Guido, the overflow checking in PyLong_AsLong is off a little:

1) If the C in use sign-extends right shifts on signed longs, there's a
spurious overflow error when converting the most-negative int:

Python 1.5.1 (#0, Apr 13 1998, 20:22:04) [MSC 32 bit (Intel)] on win32
Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam
>>> x = -1L << 31
>>> x
-2147483648L
>>> int(x)
Traceback (innermost last):
  File "<stdin>", line 1, in ?
OverflowError: long int too long to convert
>>>

2) If C does not sign-extend, some genuine overflows won't be caught.

The attached should repair both, and, because I installed a new disk and a C
compiler today, it's even been compiled this time <wink>.

Python 1.5.1 (#0, May 23 1998, 20:24:58) [MSC 32 bit (Intel)] on win32
Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam
>>> x = -1L << 31
>>> x
-2147483648L
>>> int(x)
-2147483648
>>> int(-x)
Traceback (innermost last):
  File "<stdin>", line 1, in ?
OverflowError: long int too long to convert
>>> int(-x-1)
2147483647
>>> int(x-1)
Traceback (innermost last):
  File "<stdin>", line 1, in ?
OverflowError: long int too long to convert
>>>

end-casing-ly y'rs  - tim