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ABS(x) == 0 can be simplified to x == 0 #52905
Description
| BPO | 8659 |
|---|---|
| Nosy | @mdickinson |
| Files |
Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.
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GitHub fields:
assignee = None
closed_at = <Date 2010-05-08.08:05:05.781>
created_at = <Date 2010-05-08.06:15:20.845>
labels = ['interpreter-core', 'performance']
title = 'ABS(x) == 0 can be simplified to x == 0'
updated_at = <Date 2010-05-08.10:07:43.271>
user = 'https://bugs.python.org/stutzbach'bugs.python.org fields:
activity = <Date 2010-05-08.10:07:43.271>
actor = 'mark.dickinson'
assignee = 'stutzbach'
closed = True
closed_date = <Date 2010-05-08.08:05:05.781>
closer = 'mark.dickinson'
components = ['Interpreter Core']
creation = <Date 2010-05-08.06:15:20.845>
creator = 'stutzbach'
dependencies = []
files = ['17259']
hgrepos = []
issue_num = 8659
keywords = ['patch', 'needs review']
message_count = 4.0
messages = ['105265', '105267', '105268', '105273']
nosy_count = 2.0
nosy_names = ['mark.dickinson', 'stutzbach']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'performance'
url = 'https://bugs.python.org/issue8659'
versions = ['Python 3.2']
Activity
stutzbach commentedon May 8, 2010
I noticed that in longobject.c, there are a few spots that take the absolute value of an int before comparing the int to 0. There's no -0 for C integers, so the absolute value isn't necessary.
I traced back through the commit history, and it looks like they're an artifact of the original one's-complement longobject.c (which COULD have -0). longobject was switched to two's-complement in 1992. ;-)
mdickinson commentedon May 8, 2010
Thanks! Fixed in r80961 (trunk) and r80962 (py3k).
BTW, are there any particular commit messages or news entries that indicate a switch to two's complement? I've looked for these in the past, but not found them.
mdickinson commentedon May 8, 2010
I found:
to give more useful results for negative operands
in Misc/HISTORY, for the Python 0.9.6 release. That's different, though: it's about Python's semantics, not C's. It's orthogonal to the question of whether the underlying C implementation uses two's complement, ones' complement or sign-magnitude.
I can't find any explicit indication of a decision to assume that the hardware integers are two's complement anywhere; as far as I know, no such decision was ever taken.
But of course your simplifications are valid regardless of the integer representation of the machine.
mdickinson commentedon May 8, 2010
Ah, now I understand :)
r2604 introduced a scheme where for a negative PyLongObject x with n digits, the value stored in x->ob_size was -1-n. A little like ones' complement, but unrelated to anything to do with the platform integer representation. So at that stage there were two possible internal representations of 0L, which is why all the ZABS() stuff was necessary to make sure that those two representations of 0L compared equal.
r2751 (in 1992, indeed!) reversed this, using -n instead of -1-n for this case, but didn't remove the extra tests for the two different representations of 0L. r2751 also changed the semantics of long bitwise operations for negative numbers, but that's not relevant to this issue.
Sorry for being slow.
It's impressive that this piece of redundant code has survived this long.