Path: utzoo!mnetor!uunet!lll-winken!lll-lcc!ames!pasteur!ucbvax!hplabs!sdcrdcf!darrelj
From: darrelj@sdcrdcf.UUCP (Darrel VanBuer)
Newsgroups: comp.lang.lisp
Subject: Re: GC and time critical code? (use of numbers)
Message-ID: <5243@sdcrdcf.UUCP>
Date: 19 Apr 88 15:47:52 GMT
References: <452@cvaxa.sussex.ac.uk>
Reply-To: darrelj@sdcrdcf.UUCP (Darrel VanBuer)
Organization: Unisys - System Development Group, Santa Monica
Lines: 24
Keywords: GC, time-critical, number representation, Poplog

The range of numbers varies quite widely between different Lisp
implementations.  There are three catagories in many Lisps:
SMALLP  --  stored in a reserved range of (fake) pointers, these only cost a
	few cycles to add/subtract/shift to/from naked number
FIXP -- a full machine word with a pointer to it. 32 to 36 bits on most.
BIGNUM -- usually open ended, often implemented with arrays or lists

Implementation	SMALLP		FIXP		BIGNUM
==============	======		====		======
Interlisp 10	-1536..1535	36 bits		no
Interlisp VAX	-2^30..2^30-1	no		no
Interlisp D	-65535..65535	32 bits		yes
Interlisp 360	24 bits		32 bits		no
CWIC 360	-4096..12287	32 bits		no

I have also seen a few implementations with NO smallp.
Commonlisp only talks of FIXNUM and BIGNUM, but discourages knowing the
dividing line since it is unportable (though it suggests that fixnums should
provide at least 16 bits), and is even vague about how much of an efficiency
difference there is (or how you would split up the three catagories into the
two Commonlisp sets).
-- 
Darrel J. Van Buer, PhD; unisys; 2400 Colorado Ave; Santa Monica, CA 90406
(213)829-7511 x5449        KI6VY        darrel@CAM.UNISYS.COM   or
...{allegra,burdvax,cbosgd,hplabs,ihnp4}!sdcrdcf!darrelj