[Python-Dev] ints not overflowing into longs?

[Antoine Pitrou]

On Wed, 2 Nov 2011 19:41:30 -0700
Guido van Rossum <guido at python.org> wrote:
> Apparently Macports is still using a buggy compiler.

If I understand things correctly, this is technically not a buggy
compiler but Python making optimistic assumptions about the C standard.
(from issue11149: "clang (as with gcc 4.x) assumes signed integer
overflow is undefined. But Python depends on the fact that signed
integer overflow wraps")

I'd happily call that a buggy C standard, though :-)

Regards

Antoine.


> I reported a
> similar issue before and got this reply from Ned Delly:
> 
> """
> Thanks for the pointer.  That looks like a duplicate of Issue11149 (and
> Issue12701).  Another manifestation of this was reported in Issue13061
> which also originated from MacPorts.  I'll remind them that the
> configure change is likely needed for all Pythons.  It's still safest to
> stick with good old gcc-4.2 on OS X at the moment.
> """
> 
> (Those issues are on bugs.python.org.)
> 
> --Guido
> 
> On Wed, Nov 2, 2011 at 7:32 PM, Derek Shockey <derek.shockey at gmail.com> wrote:
> > I just found an unexpected behavior and I'm wondering if it is a bug.
> > In my 2.7.2 interpreter on OS X, built and installed via MacPorts, it
> > appears that integers are not correctly overflowing into longs and
> > instead are yielding bizarre results. I can only reproduce this when
> > using the exponent operator with two ints (declaring either operand
> > explicitly as long prevents the behavior).
> >
> >>>> 2**100
> > 0
> >>>> 2**100L
> > 1267650600228229401496703205376L
> >
> >>>> 20**20
> > -2101438300051996672
> >>>> 20L**20
> > 104857600000000000000000000L
> >
> >>>> 10**20
> > 7766279631452241920
> >>>> 10L**20L
> > 100000000000000000000L
> >
> > To confirm I'm not crazy, I tried in the 2.7.1 and 2.6.7 installations
> > included in OS X 10.7, and also a 2.7.2+ (not sure what the + is) on
> > an Ubuntu machine and didn't see this behavior. This looks like some
> > kind of truncation error, but I don't know much about the internals of
> > Python and have no idea what's going on. I assume since it's only in
> > my MacPorts installation, it must be build configuration issue that is
> > specific to OS X, perhaps only 10.7, or MacPorts.
> >
> > Am I doing something wrong, and is there a way to fix it before I
> > compile? I could find any references to this problem as a known issue.
> >
> > Thanks,
> > Derek
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> 
> 
> 
> -- 
> --Guido van Rossum (python.org/~guido)