IMPORT_MODULE proposal

From: Vladimir Prus (ghost_at_[hidden])
Date: 2003-07-28 07:12:39

I'm currently considering some optimization, and posting my ideas to the list 
to benefit from peer review.

Looking at the bjam profile on my project, I see that IMPORT and RULENAMES 
builtins take a lot of time. The possible reason is this: each class instance 
is actually a module. So, "import" statements inside classes are executed 
once for each instance. And wholesale imports like

import target ;

cause V2 to
1. finding all rules in "targets" via RULENAMES
2. importing them all via "IMPORT"

which is probably the cause. My guess is backed up by the fact that there are 
only 7000 instances created, while RULENAMES is called 36000 times. It can't 
be due to loading of ordinary modules.

I propose to introduce new builtin IMPORT_MODULE. After "IMPORT_MODULE b " is 
called in module a, "b" will be added to the list of imported modules. Name 
lookup for rule will work like this:
- lookup the rule in this module
- if not found, and name is in the form something1.something2 and "something1" 
is in the list of imported modules, look for "something2" in module 
"something1".

I think this should reduce memory usage and time needed for RULENAMES/IMPORT. 
Are there any pitfalls I've missed?

- Volodya