Re: Yes, they are special, but...

[Barry Margolin <barmar@THINK.COM>]

    Date: Wed, 21 Jun 89 09:12 PDT
    From: Richard Lamson <rsl@max-fleischer.ila-sf.dialnet.symbolics.com>

	Date: Mon, 19 Jun 89 19:34 EDT
	From: barmar@Think.COM (Barry Margolin)

	If the CASE statement you're talking about looks something like

		(case <expression>
		  (0 ...)
		  (1 ...)
		  ...
		  (t ...))

	then the 3600 compiler will compile this into a computed go (I believe
	this was introduced in Genera 7.1 or 7.2).  It does this when all the
	keys are non-negative integers and the key space isn't too sparse (more
	than half of the keys from 0 to the largest key are used).  

    Have you tried this on a machine with an IFU (including, I think, the G
    machines)?

The 3600 compiler doesn't distinguish between different models of 3600,
so this transformation is guaranteed to take place on a machine with an
IFU, too.  If you're asking whether this actually improves performance
on IFU machines, I have no idea -- I'd expect that metering to have been
done by Symbolics before installing it.

I don't know much about the design of the IFU, but I can certainly
imagine how it might be able to speed up the COND version of the
expansion.  If it prefetches conditional branch targets in parallel with
the condition computation and test, then a series of COND tests can be
optimized rather well.  The %DISPATCH-ELT presumably can't prefetch all
the potential targets.  Clearing the instruction cache, however, seems
pretty drastic.

It probably depends heavily on the number of cases, the amount of code
in each case (i.e. host many cases fit into the I-cache), and which are
the more common cases (if the later cases are more common, dispatching
is probably faster than COND).

Finally, since there's only one 36xx compiler, it's necessary to make
compromises when open coding.  It might be useful to have a switch that
specifies the model of most of the target machines of a compilation, so
that it can perform optimizations best suited to that implementation.  I
don't know how many such differences there are, though, so I don't know
how much payoff there would be for such a compiler feature.

                                                barmar