[Python-Dev] Optimization of Python ASTs: How should we deal with constant values?

Adam Olsen rhamph at gmail.com
Fri May 9 03:38:29 CEST 2008

On Thu, May 8, 2008 at 5:54 PM, Thomas Lee <tom at vector-seven.com> wrote:
> Adam Olsen wrote:
>> On Thu, May 8, 2008 at 5:22 PM, Thomas Lee <tom at vector-seven.com> wrote:
>>> Nick Coghlan wrote:
>>>> There are a lot of micro-optimisations that are actually context
>>>> independent, so moving them before the symtable pass should be quite
>>>> feasible - e.g. replacing "return None" with "return", stripping dead
>>>> code
>>>> after a return statement, changing a "if not" statement into an "if"
>>>> statement with the two suites reversed, changing "(1, 2, 3)" into a
>>>> stored
>>>> constant, folding "1 + 2" into the constant "3".
>>>> I believe the goal is to see how many of the current bytecode
>>>> optimisations can actually be brought forward to the AST generation
>>>> stage,
>>>> rather than waiting until after the bytecode symtable calculation and
>>>> compilation passes.
>>> That's been the aim so far. It's been largely successful with the
>>> exception
>>> of a few edge cases (most notably the functions vs. generator stuff). The
>>> elimination of unreachable paths (whether they be things like "if 0: ..."
>>> or
>>> "return; ... more code ...") completely breaks generators since we might
>>> potentially be blowing away "yield" statements during the elimination
>>> process.
>> Also breaks various sanity checks relating to the global statement.
> What sanity checks are these exactly? Is this related to the lnotab?

Here we are.  In 2.4.4:

>>> def foo():
...   print test
...   if 0:
...     import test
>>> foo()
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "<stdin>", line 2, in foo
NameError: global name 'test' is not defined

2.5.1 correctly raises an UnboundLocalError instead.

Searching the bug DB for old bugs involving the optimizer showed
several variations on this, such as "if 0: yield None" at module scope
not raising a SyntaxError (Issue 1875, still open in trunk).  Clearly
there needs to be a general approach to avoid affecting these checks
with optimizations.

Adam Olsen, aka Rhamphoryncus

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