[Python-Dev] constant folding of -0

[Eugene Toder]

Hello,

I've noticed since version 3.2 python doesn't fold -0:

Python 3.1.3 (r313:86834, Nov 28 2010, 10:01:07)
>>> def foo(): return -0
>>> dis(foo)
  1           0 LOAD_CONST               1 (0)
              3 RETURN_VALUE

Python 3.2 (r32:88445, Feb 20 2011, 21:30:00)
>>> def foo(): return -0
>>> dis(foo)
  1           0 LOAD_CONST               1 (0)
              3 UNARY_NEGATIVE
              4 RETURN_VALUE

(version built from head behaves the same way).

It looks like folding -0 is disabled in peephole since this commit
http://hg.python.org/cpython/diff/660419bdb4ae/Python/compile.c
which was a long time ago. Before 3.2 -0 was likely folded in the
parser -- in a more complex case no folding happens in either version:

>>> def foo(): return -(1-1)
>>> dis(foo)
  1           0 LOAD_CONST               2 (0)
              3 UNARY_NEGATIVE
              4 RETURN_VALUE

In 3.2 parser no longer folds -0.

So I wanted to ask why folding of -0 was disabled in peephole? Commit
message makes me think this was a work-around for a problem in marshal
-- perhaps it couldn't save -0.0 properly and so not creating -0.0 in
the code objects was a simple fix. (This would mean the change
predates folding in the parser.) Was marshal fixed? If I revert the
change everything seems to work and all tests pass. Since tests are
run with .pyc-s I assume they test marshal?
Maybe this check is no longer needed and can be reverted? Or is it
there for some different reason which still holds?

Regards,
Eugene