[Python-bugs-list] [ python-Bugs-458941 ] Looks like a unary minus bug

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Bugs item #458941, was opened at 2001-09-05 16:29
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Category: Parser/Compiler
Group: Python 2.2
Status: Open
Resolution: None
Priority: 6
Submitted By: Tim Peters (tim_one)
>Assigned to: Tim Peters (tim_one)
Summary: Looks like a unary minus bug

Initial Comment:
The attached program should print the same stuff in 
both calls to drive().  It actually prints

Using literals:
0.0
0.0
0.0
0.0

Using computation:
0.0
3.14159265359
0.0
-3.14159265359

on Windows.

The disassembly of driver() suggests the compiler is 
optimizing

-0.0

into

0.0, but the sign of a *floating* zero is significant.

This works as expected in 2.1.1; the behavior changed 
after that.

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>Comment By: Guido van Rossum (gvanrossum)
Date: 2001-09-06 02:24

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The com_addconst code was never intended to deal with
anything except positive literals.

I wonder if we are doing ourselves a favor with the
unary-minus optimization?  This is the second time we have a
problem in it.

If we want to keep it, making the exception for 0.0 and 0j
makes sense. Back to Tim (or to Fred?)


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Comment By: Tim Peters (tim_one)
Date: 2001-09-05 21:45

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Reassigned to Guido for pondering.  The unary minus 
optimization doesn't appear to be the true cause here, but 
rather unmasked a different problem:  com_addconst uses a 
dict to map (value, type) pairs to consts, and +0.0 
compares equal to -0.0.  Thus in the test case, compilation 
of the tuple

    (+0.0, -0.0)

finds +0.0 first, and despite that "-0.0" is correctly 
converted to a negative 0 on my box, add_const thinks it's 
the same thing:  *all* literal float zeroes are treated 
like +0.0 just because +0.0 was the first one added to the 
consts.

If I change the tuple to

    (-0.0, +0.0)

instead, then all literal 0.0 thingies get treated like 
minus 0 instead, and the test case prints 4 instances of -
pi (instead of 4 instances of 0).

A sufficient hack to hide this again may be to disable the 
unary minus optimization when the NUMBER is a float or 
imaginary zero.

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Comment By: Skip Montanaro (montanaro)
Date: 2001-09-05 19:50

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>From the checkin comments, it looks like v 2.216 of
Python/compile.c is the culprit.  Getting rid of
com_invert_constant and reverting com_factor to the version
from 2.215 fixes the problem.  Looks like it was strictly an
optimization, so it can probably be safely removed.



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