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Re: Issue: TAIL-RECURSION-OPTIMIZATION (Version 1)
- To: Kent M Pitman <KMP@SCRC-STONY-BROOK.ARPA>
- Subject: Re: Issue: TAIL-RECURSION-OPTIMIZATION (Version 1)
- From: David N Gray <Gray@DSG.csc.ti.com>
- Date: Thu, 1 Sep 88 16:29:42 CDT
- Cc: CL-Cleanup@SAIL.STANFORD.EDU
- In-reply-to: Msg of Wed, 31 Aug 88 18:11 EDT from Kent M Pitman <KMP@SCRC-STONY-BROOK.ARPA>
- Sender: GRAY@Kelvin.csc.ti.com
> Issue: TAIL-RECURSION-OPTIMIZATION
> References: 5.1 Forms (pp54-59), SYMBOL-FUNCTION (p90)
> Category: CHANGE
> Edit history: 31-Aug-88, Version 1 by Pitman
> Status: For Internal Discussion
> Problem Description:
> Useful tail-recursion optimizations are not permitted by CLtL because
> the compiler must assume that any opaque function call might change
> the definition of a function in between calls to that function, so a
> direct jump to the code is not appropriate.
"Tail-recursion optimizations" could mean one of two things: either
a function that calls itself being optimized into a loop, or jumping to
another function while keeping the same stack frame. Your examples only
mention the first case; I don't see that the second is any problem. I'm
not even convinced that the first case is really a problem since I haven't
seen an example of meaningful code where it would not be safe to assume
that a function didn't redefine itself (other than the DEFUN-AUTOLOADING
example, which is not a likely candidate for optimization anyway). In any
case, all one would have to do to insure a run-time indirection is to do
(FUNCALL (SYMBOL-FUNCTION 'FOO) ...) instead of (FOO ...).
Anyway, I think it would be a mistake to tie this issue with the
SPEED and NOTINLINE declarations since they are intended to affect
efficiency, not the semantics of the code.
> Current Practice:
> Symbolics Genera and Symbolics Cloe not currently do tail recursion
> optimization. As such, they are compatible with the proposal.
The TI Explorer does do tail recursion optimization (both kinds) at higher
optimization levels. It does assume that a function won't redefine
itself, but no one has reported that as being a problem.
-- David Gray