class.jam performance issues

From: Vladimir Prus (ghost_at_[hidden])
Date: 2002-07-22 07:05:44

I'm afraid we have a serious performance problem. I've wrote a simple 
example, where a rule calls itself recursively until a recursion limit is 
reached. In each invocation, a simple class is created. There's a table,
giving the number of time jam's function 'hashitem' is called, which is 
responsible for a large fraction of jam's running time.

Limit	Calls
1	1082194
200 1276412
300 1433818
400	1631218
500	1868618
600	2146018
700	2463418
800	2820818

(about 2179 per level)

Now let's try without creating the class.
1 1081336
800 1104008

(about 28 per level w/o)

This is very slow! It essentially means that if I switch from using jam list 
to using contaner.vector in one place, I'll have 100x slowdown. Actually... 
it seems worse than that: each construction of class causes a call to 
BACKTRACE and that rule returns the list of all stack frames -- I suspect a 
quadratic running time is possible.

As an experiment, I've replaced the call to BACKTRACE in class.__init__ to 
call of a new builtin which directly returns the caller. This
resulted in 1485639 calls for limit or 800 ( 504 calls per level). 
That's clearly not enough, and we need to do something more. Personally, I'd 
prefer introducing 10 new builtins to having such overhead.

At this stage it is still possible to work on build system, but once we start 
trying real examples, this will become problem #1.

- Volodya