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From: shebs@utah-cs.UUCP (Stanley Shebs)
Newsgroups: net.arch,net.ai,net.lang,net.lang.lisp
Subject: Re: Lisp Machines
Message-ID: <3345@utah-cs.UUCP>
Date: Mon, 20-May-85 10:08:08 EDT
Article-I.D.: utah-cs.3345
Posted: Mon May 20 10:08:08 1985
Date-Received: Tue, 21-May-85 04:43:15 EDT
References: <922@noscvax.UUCP> <5604@utzoo.UUCP>
Reply-To: shebs@utah-cs.UUCP (Stanley shebs)
Organization: Univ of Utah CS Dept
Lines: 28
Xref: linus net.arch:1034 net.ai:2548 net.lang:1362 net.lang.lisp:404

In article <5604@utzoo.UUCP> henry@utzoo.UUCP (Henry Spencer) writes:
>In the recent AI issue of Byte, there was an article by some folks at a
>company (Fairchild?) that is looking at building a super-fast AI machine.
>Their analysis of supporting AI languages (notably Lisp) really well on
>conventional machines ultimately boiled down to "efficient simulation of
>tagged memory is the major stumbling block".  Their conclusion was that
>conventional machines will be good for Lisp etc. in direct proportion to
>how quickly they can (a) pick out some bits from a word and branch to one
>of several places depending on the value of those bits, and (b) do an
>indirect fetch which ignores some of the bits in the address register.

This is true only if one has a relatively stupid compiler that does
no type inference of any sort (sadly, this is the case for most compilers
today).  The nice thing about a really high-level language is that
a compiler can look at something like (car quux) and turn that into
a single instruction that does offset addressing without any tag
checking at all!  Of course, the compiler has to be smart, so that
it doesn't try that trick with (car 12)...  Compiled PSL code wins
big because it sacrifices some reliability and debuggability of code
for speed.  Also, to get the best performance, one must use several
kinds of declarations and flags.  The net effect is to compile away
a large percentage of function calls and type checks.

The moral?  Fixing the compiler is a better approach than sinking
millions of dollars into hardware, then trying to sell it as an
"advanced AI engine" or "powerful Lisp machine"...

						stan shebs