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Re: gcc trunk vs python

Sorry for the formatting weirdness. I should be more careful with
Thunderbird's auto-conversion to plain text. Here is the correct
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:


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)

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.


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