Subject: Re: What is expensive about consing
From: (Rob Warnock)
Date: Wed, 10 Sep 2008 10:12:38 -0500
Newsgroups: comp.lang.lisp
Message-ID: <>
Bakul Shah  <> wrote:
| Rob Warnock wrote:
| > What CL *really* needs here is just a small tweak to REDUCE, to add
| > one additional keyword argument, :MAX-ARITY, that permits REDUCE to
| > use a larger arity than 2 on the intermediate applications of the
| > function being REDUCEd. It might be defined this way:
| No need to complexify its interface as reduce can be implemented
| using a fixed amount of temp space even without TCO...

CL:REDUCE's interface is *already* quite complex, with five ANSI-mandated
keyword args already [:KEY :FROM-END :START :END :INITIAL-VALUE]. One more
keyword would hardly be "complexifying" it.

As others have noted, in the case of #'+ if all the compiler would use
is an underlying 2-arg machine instruction (or 2-in/1-out, for RISC),
then :MAX-ARGS doesn't buy you much. But that's just the simplest case.
A :MAX-ARGS keyword might be very useful if either:

1. The compiler knew about & could use some underlying vector arithmetic,
   such as x86 SSE{2,3}; or,

2. The function being REDUCE'd is both *expensive* to call and
   inherently variadic [but with little incremental overhead per
   additional argument]. In that case, operating on "bunches" of
   args at a time could be a real performance boost. A possibly
   worst-case example of this would be if each function call required
   a setup/teardown of a network connection [think SQL database access],
   or allocating/releasing a bunch of slave processors [Google-style
   MAP/REDUCE], etc.


Rob Warnock			<>
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