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Re: gcc trunk vs python
- From: Michael Veksler <mveksler at techunix dot technion dot ac dot il>
- To: Jack Howarth <howarth at bromo dot msbb dot uc dot edu>
- Cc: mveksler at techunix dot technion dot ac dot il, gcc at gcc dot gnu dot org, geoffk at apple dot com, mrs at apple dot com
- Date: Thu, 24 Aug 2006 04:47:33 +0300
- Subject: Re: gcc trunk vs python
- References: <20060823222747.23EEE11000E@bromo.msbb.uc.edu>
Sorry for the formatting weirdness. I should be more careful with
Thunderbird's auto-conversion to plain text. Here is the correct
version:
Jack Howarth wrote:
import sys
divmod(-sys.maxint-1, -1)
(0, -2147483648)
If I add the -fwrapv flag to BASECFLAGS in the Makefile, I get...
(2147483648L, 0L)
So your analysis appears to be correct. Even more interesting is
that if I build python 2.4.3 with Apple's gcc from Xcode 2.3, the
correct results are obtained without the need to resort to the
-fwrapv flag. So either we have a regression in gcc trunk or Apple has
a patch in their branch which was never moved over
It *is* a bug in python, here is the proof:
https://codespeak.net/viewvc/vendor/cpython/Python-r243/dist/src/Objects/intobject.c?revision=25647&view=markup
Function
i_divmod(register long x, register long y,
the following lines:
/* (-sys.maxint-1)/-1 is the only overflow case. */
if (y == -1 && x < 0 && x == -x)
return DIVMOD_OVERFLOW;
If overflow is barred then x==-x may happen only when x==0.
This conflicts with x<0, which means that the compiler may assume
that
x<0 && x==-x
always yields false. This may allow the compiler to eliminate the whole if
statement. Hence, clearly python is at fault.
Michael