[Python-Dev] Accessing globals without dict lookup (original) (raw)

Tim Peters tim.one@comcast.net
Mon, 11 Feb 2002 19:01:04 -0500

[Guido]

... I think you're right that we might be able to use just a PyObject **; but I haven't fully digested Tim's more aggressive idea.)

The overwhelming thrust of Tim's variant is to reduce the (by far) most frequent namespace operation to this:

   case LOAD_GLOBAL_CELL:
       cell = func_cells[oparg];
       x = cell->objptr;  /* note: not two levels, just one */
       if (x != NULL) {
           Py_INCREF(x);
           continue;
       }
       ... error recovery ...
       break;

Everything else follows from that; it's really a quite minor variant of the original proposal, consisting mostly of changes to spelling details due to having a different kind of cell. The sole big change is requiring that mutations to builtins propagate at once to their cached values in module celldicts.

I believe Jeremy's scheme could do better than this for builtins, but not the way it's currently set up (I don't see why we can't define a fixed bijection between the standard builtin names and a range of contiguous little integers, and use that fixed bijection everywhere; the compiler could exploit this global (in the cross-module sense) bijection directly for LOAD_BUILTIN offsets, eliminating all the indirection for standard builtins, and eliminating the code-object-specific vectors of referenced builtin names; note that I don't care about speeding access to builtins with non-standard names -- fine by me if they're handled via LOAD_GLOBAL instead, and fall into its "oops! it's not a module global after all" case).