[Python-Dev] gcc 4.2 exposes signed integer overflows

[Nick Maclaren]

"Tim Peters" <tim.peters at gmail.com> wrote:
>
> This is a wrong time in the release process to take on chance on
> discovering a flaky LONG_MIN on some box, so I want to keep the code
> as much as possible like what's already there (which worked fine for >
> 10 years on all known boxes) for now.

No, it didn't.  I reported a bug a couple of years back.

A blanket rule not to use symbols is clearly wrong, but there are
good reasons not to want to rely on LONG_MIN (or INT_MIN for that
matter).  Because of some incredibly complex issues (which I only
know some of), it hasn't been consistently -(1+LONG_MAX) on twos'
complement machines.  There are good reasons for making it -LONG_MAX,
but they aren't the ones that actually cause it to be so.

There are, however, very good reasons for using BOTH tests.  I.e.
if I have a C system which defines LONG_MIN to be -LONG_MAX because
it uses -(1+LONG_MAX) for an integer NaN indicator in some contexts,
you really DON'T want to create such a value.  I don't know if there
are any such C systems, but there have been other languages that did.

I hope that Guido wasn't saying that Python should EVER rely on
signed integer overflow wrapping in twos' complement.  Despite the
common usage, Java and all that, it is perhaps the worst systematic
architectural change to have happened in 30 years, and accounts for
a good 30% of all hard bugs in many classes of program.  Simple
buffer overflow is fairly easy to avoid by good programming style;
integer overflow causing trashing of unpredictable data isn't.

Any decent programming language (like Python!) regards integer
overflow as an error, and the moves to make C copy Java semantics
are yet another step away from software engineering in the direction
of who-gives-a-damn hacking.


Regards,
Nick Maclaren,
University of Cambridge Computing Service,
New Museums Site, Pembroke Street, Cambridge CB2 3QH, England.
Email:  nmm1 at cam.ac.uk
Tel.:  +44 1223 334761    Fax:  +44 1223 334679