[Python-Dev] ints not overflowing into longs?

[Derek Shockey]

I believe you're right. The 2.7.2 MacPorts portfile definitely passes
the -fwrapv flag to clang, but the bad behavior still occurs with
exponents. I verified the current head of the 2.7 branch does not have
this problem when built with clang, so I'm assuming that issue12973
resolved this with a patch to int_pow() and that it will be out in the
next release.

-Derek

On Thu, Nov 3, 2011 at 4:30 AM, Antoine Pitrou <solipsis at pitrou.net> wrote:
> 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)
>
>
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