why does dead code costs time?

Ian Kelly ian.g.kelly at gmail.com
Wed Dec 5 18:59:26 CET 2012


On Wed, Dec 5, 2012 at 10:34 AM, Steven D'Aprano
<steve+comp.lang.python at pearwood.info> wrote:
> The difference is almost certain between the LOAD_CONST and the
> LOAD_GLOBAL.
>
> As to *why* there is such a difference, I believe that's a leftover from
> early Python days when None was not a keyword and could be reassigned.

I think this should even be considered a bug, not just a missing
optimization.  Consider:

>>> globals()['None'] = 42
>>> def f(x):
...     return None
...     print(x)
...
>>> f('test')
42

The use of the LOAD_GLOBAL allows None to effectively be reassigned.

It's also worth noting that:

>>> def f(x):
...     return
...     print(x)
...
>>> dis.dis(f)
  2           0 LOAD_CONST               0 (None)
              3 RETURN_VALUE

  3           4 LOAD_GLOBAL              0 (print)
              7 LOAD_FAST                0 (x)
             10 CALL_FUNCTION            1 (1 positional, 0 keyword pair)
             13 POP_TOP

So if you just write 'return' rather than 'return None', you get the
correct bytecode.  Additionally, the use of LOAD_GLOBAL instead of
LOAD_CONST seems to be linked to having unreachable code at the end of
the function.  This is fine:

>>> def f(x):
...     if x:
...         return None
...     print(x)
...
>>> dis.dis(f)
  2           0 LOAD_FAST                0 (x)
              3 POP_JUMP_IF_FALSE       10

  3           6 LOAD_CONST               0 (None)
              9 RETURN_VALUE

  4     >>   10 LOAD_GLOBAL              1 (print)
             13 LOAD_FAST                0 (x)
             16 CALL_FUNCTION            1 (1 positional, 0 keyword pair)
             19 POP_TOP
             20 LOAD_CONST               0 (None)
             23 RETURN_VALUE

But this is not.  Note here that *both* loads of None become
LOAD_GLOBAL in this case:

>>> def f(x):
...     if x:
...         return None
...     return None
...     print(x)
...
>>> dis.dis(f)
  2           0 LOAD_FAST                0 (x)
              3 POP_JUMP_IF_FALSE       13

  3           6 LOAD_GLOBAL              0 (None)
              9 RETURN_VALUE
             10 JUMP_FORWARD             0 (to 13)

  4     >>   13 LOAD_GLOBAL              0 (None)
             16 RETURN_VALUE

  5          17 LOAD_GLOBAL              1 (print)
             20 LOAD_FAST                0 (x)
             23 CALL_FUNCTION            1 (1 positional, 0 keyword pair)
             26 POP_TOP



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