Re: Adding Identities to Peval

[David Kastrup <dak@gnu.org>]

David Kastrup <dak@gnu.org> writes:

> Noah Lavine <noah.b.lavine@gmail.com> writes:
>
>> Hello,
>>
>> I've been working on a patch to add a new sort of optimization to
>> peval, and I think it's almost ready. It's based on some of the ideas
>> in "Environment Analysis of Higher-Order Languages".
>>
>> The goal is to recognize when two quantities are equal even when we
>> don't know what they are. My working example has been this expression:
>>
>> (let* ((x (random))
>>        (y x))
>>   (eq? x y))
>>
>> The patch attached to this message lets peval optimize that to
>>
>> (begin (random) #t)
>
> I have a hard time imagining this optimization to be useful for any code
> occuring in practice.  Can you suggest an example that would make more
> sense than demonstrating that the optimization works?  Is this supposed
> to help with automatically generated code like macros?

Actually, I've been just racking my brain over how to write this better:

            {
	      // We end only one slur unless several ones have been
	      // caused by the same event, like with double slurs.
              if (!ended || scm_is_eq (starter,
				       slurs_[j]->get_property ("cause")))
		{
		  ended = true;
		  starter = slurs_[j]->get_property ("cause");
		  end_slurs_.push_back (slurs_[j]);
		  slurs_.erase (slurs_.begin () + j);
		}
            }

The problem is that if you enter the "if" through the second alternative
condition, you can just jump past the first two assignments (because
they assign values that are already known to be in the respective
variables).  I've not found a good way to handoptimize this short of
using goto or abusing do ... while;

-- 
David Kastrup