[Python-Dev] Can LOAD_GLOBAL be optimized to a simple array lookup? (original) (raw)

Noam Raphael noamraph at gmail.com
Thu Aug 24 00:50:34 CEST 2006

2006/8/24, Brett Cannon <brett at python.org>:

On 8/23/06, K.S.Sreeram <sreeram at tachyontech.net> wrote: > Hi all, > > I noticed in Python/ceval.c that LOADGLOBAL uses a dictionary lookup, > and was wondering if that can be optimized to a simple array lookup. No, not as the language stands now. > If i'm right there are 3 kinds of name lookups: locals, outer > scopes(closures), and globals. (not counting attribute lookup). Locals > are identified by, either the presence of assignments, or their presence > in the arg list. So all name lookups can be classified into the 3 types > at compile/load time. > > Since we know, at load time, which names are global.. Can't we simply > build a global name table and replace LOADGLOBALs with a lookup at the > corresponding index into the global name table? But we don't know statically what the globals will be. You can import a module and put something in its global namespace externally. That is done after load time or compile time. I think that it can be implemented for the language as it stands now. I don't know whether it will be a good thing or not.

In principle, you can add a feature to dict implementation that will allow it to notify when the value of a specific key changes. If you have that, you can change LOAD_GLOBAL implementation to:

  1. look for the global.
  2. ask for notification when the global dict changes in a way that will change the meaning of the global.
  3. change the LOAD_GLOBAL opcode to something like LOAD_CONST, and set the notification from the dict to update the LOAD_CONST opcode to the new object.

In that way, LOAD_GLOBAL will cause a dict lookup only once. Changing the value of globals will require more work, though.

Again, I'm not saying that it's desired, just that it's possible.

Have a good day, Noam

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