[Python-Dev] Possible optimization for LOAD_FAST ?

[Guido van Rossum]

On Mon, Jan 3, 2011 at 9:52 AM, David Malcolm <dmalcolm at redhat.com> wrote:
> On Sun, 2011-01-02 at 19:18 -0800, Guido van Rossum wrote:
>> On Sun, Jan 2, 2011 at 5:50 PM, Alex Gaynor <alex.gaynor at gmail.com> wrote:
>> > No, it's singularly impossible to prove that any global load will be any given
>> > value at compile time.  Any optimization based on this premise is wrong.
>>
>> True.
>>
>> My proposed way out of this conundrum has been to change the language
>> semantics slightly so that global names which (a) coincide with a
>> builtin, and (b) have no explicit assignment to them in the current
>> module, would be fair game for such optimizations, with the
>> understanding that the presence of e.g. "len = len" anywhere in the
>> module (even in dead code!) would be sufficient to disable the
>> optimization.
>>
>> But barring someone interested in implementing something based on this
>> rule, the proposal has languished for many years.
>
> Is there a PEP for this?

Not that I know of, otherwise I'd have mentioned it. :-)

It would be useful if someone wrote it up, since the idea comes back
in one form or another regularly.

>> FWIW, this is reminiscent of Fortran's rules for "intrinsics" (its
>> name for builtins), which have a similar optimization behavior (except
>> there the potential overrides that the compiler doesn't need to take
>> into account are load-time definitions).
>
> I've been attempting another way in:
>  http://bugs.python.org/issue10399
> using a new "JUMP_IF_SPECIALIZABLE" opcode.  This compares what a value
> is against a compile-time prediction, branching to an optimized
> implementation if the guess was correct.  I use this to implement
> function-call inlining within the generated bytecode.

Yeah, that's what everybody proposes to keep the language semantics
unchanged. But I claim that an easier solution is to say to hell with
those semantics, let's change them to make the implementation simpler.
That's from the Zen of Python: "If the implementation is easy to
explain, it may be a good idea." I guess few people can seriously
propose to change Python's semantics, that's why *I* am proposing it.
:-) Note that the semantics of locals (e.g. UnboundLocalError) were
also changed specifically to allow a significant optimization -- again
by me.

> Caveat-of-doom: That code's very much a work-in-progress at this stage,
> though: sometimes it doesn't segfault :) and the way that I track the
> predicted values is taking some other liberties with semantics (see that
> URL and the dmalcolm-ast-optimization-branch in SVN).
>
> (There's probably at least 2 PEPs in the above idea, though have yet to
> write my first PEP)

If you want to write up a PEP for the semantics change I am proposing,
everything you need is in this thread.

-- 
--Guido van Rossum (python.org/~guido)