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I'm porting Python 2.7.* on AIX.
I am now using a version 13.1.3.2 of XLC.
I was previously using version 12.1.0.14 .
It appears that this new compiler shows a potential issue in Python v2 code. This issue seems to appear when -O2 is used.
I was porting Python 2.7.11 with -O2 instead of -O0 when I got it.
The issue appears when running tests involving power() with Decimals where the x_sub() routine is used.
The following Python code shows the issue:
 from decimal import *
 c = Context(7, ROUND_UP)
 d = c.power(Decimal(0.7), Decimal(3.4))
 print d
Good result is: 0.2973948 .
The bug returns: 2.440099 
However, I have about 22 errors when running:
./python Lib/test/regrtest.py -v test_decimal
It is random in 64bit and quite constant in 32bit.
The issue deals with using x->ob-size.
There is a macro:
Include/object.h:#define Py_SIZE(ob) (((PyVarObject*)(ob))->ob_size)
The issue appears in last lines of Objects/longobject.c:x_sub() routine, when changing the sign: z->ob_size = -(z->ob_size);
I have looked at version 2.7.12 and 2.7.10 of Python: they contain the same code as in 2.7.11.
And ->ob_size is used about 45 times, mainly in Objects/longobject.c. However, Py_SIZE(...) is used ten times more (475).
- Looking at version 3.5.1 of Python, I see that ->ob_size is now used only once, in: Objects/object.c .
And Py_SIZE(...) is now used 616 times.
And I also see that the Python 2.7.11-12 x_sub() code:
 if (sign < 0)
 z->ob_size = -(z->ob_size);
 return long_normalize(z);
has been replaced in Python 3.5.1 by:
 if (sign < 0) {
 _PyLong_Negate(&z);
 if (z == NULL)
 return NULL;
 }
 return long_normalize(z);
 /* If a freshly-allocated int is already shared, it must
 be a small integer, so negating it must go to PyLong_FromLong */
 Py_LOCAL_INLINE(void)
 _PyLong_Negate(PyLongObject **x_p)
 {
 PyLongObject *x;
 x = (PyLongObject *)*x_p;
 if (Py_REFCNT(x) == 1) {
 Py_SIZE(x) = -Py_SIZE(x);
 return;
 }
 *x_p = (PyLongObject *)PyLong_FromLong(-MEDIUM_VALUE(x));
 Py_DECREF(x);
 }
So, it looks to me that Python v2 version should do the same that Python v3 has done: replace x->ob-size by Py_SIZE(x) everywhere, and improve the code of x_sub() by using some _PyLong_Negate() routine or macro.