New submission from Oren Milman:
------------ the proposed changes ------------
I believe the following checks are redundant:
1. in Objects/longobject.c in long_add:
In case both a and b are negative, their absolute values are added
using x_add, with the result stored in z.
If (z != NULL), it must be that x_add succeeded, and also it must be
that (Py_SIZE(z) > 0), as it is guaranteed that the absolute values of a and b
are both bigger than zero.
Thus, the check (Py_SIZE(z) != 0) here is redundant.
2. in Objects/longobject.c in long_sub:
In case a is negative, the absolute values of a and b are subtracted or
added using x_sub or x_add, with the result stored in z.
Later on, if (z != NULL && Py_SIZE(z) != 0), then Py_SIZE(z) is
negated. However, even though it might be that Py_SIZE(z) == 0, it doesn't
really matter.
doing 'Py_SIZE(z) = -(Py_SIZE(z));' in that case would do nothing.
Thus, the check (Py_SIZE(z) != 0) here is redundant.
The original versions of both of these checks were added in revision 443
(November 1991!). Back then, ob_size's was implemented using one's complement,
and negating it was actually doing 'z->ob_size = ~z->ob_size;'.
Of course, in that case the check (z->ob_size != 0) was necessary, but
then, in revision 590, ob_size was changed to use two's complement, and the
check (z->ob_size != 0) was left untouched, and remained there to this day.
------------ diff ------------
The patches diff is attached.
------------ tests ------------
I built the patched CPython for x86, and played with it a little. Everything
seemed to work as usual.
In addition, I ran 'python -m test' (on my 64-bit Windows 10) before and after
applying the patch, and got quite the same output.
the outputs of both runs are attached.
----------
components: Interpreter Core
files: issue.diff
keywords: patch
messages: 265989
nosy: Oren Milman
priority: normal
severity: normal
status: open
title: redundant checks in long_add and long_sub
Added file: http://bugs.python.org/file42920/issue.diff
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<http://bugs.python.org/issue27073>
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