Path: utzoo!attcan!uunet!elroy.jpl.nasa.gov!usc!zaphod.mps.ohio-state.edu!think.com!barmar
From: barmar@think.com (Barry Margolin)
Newsgroups: comp.lang.lisp
Subject: Re: Self-modifying function
Message-ID: <1991Feb28.053846.1858@Think.COM>
Date: 28 Feb 91 05:38:46 GMT
References: <11609@jpl-devvax.JPL.NASA.GOV>
Sender: news@Think.COM
Organization: Thinking Machines Corporation, Cambridge MA, USA
Lines: 32

In article <11609@jpl-devvax.JPL.NASA.GOV> charest@ai-cyclops.jpl.nasa.gov writes:
>More specifically, is it safe to modify the contents
>of a symbols' function cell even as the contents are being executed?

I believe it is, but there are some rules.  CLtL doesn't address these
problems, but the X3J13 Compiler Committee did some work to tighten up this
area.  Here are relevant excerpts from the proposal that was approved:

    (b) The compiler may assume that, within a named function, a
	recursive call to a function of the same name refers to the
	same function, unless that function has been declared NOTINLINE.

    (c) COMPILE-FILE may assume that, in the absence of NOTINLINE
	declarations, a call within the file being compiled to a named
	function which is defined in that file refers to that function.
	(This permits "block compilation" of files.)  The behavior of
	the program is unspecified if functions are redefined individually 
	at runtime.

Point (b) is not directly applicable to your example, but it would be to a
modified version that ends with a recursive call to the function.

However, point (c) is applicable if there are calls to the function in
other parts of the file.

In both cases, preceding the definition with a (proclaim '(notinline
mutate)) should make things safe.
--
Barry Margolin, Thinking Machines Corp.

barmar@think.com
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