Xref: utzoo comp.arch:14426 comp.lang.lisp:2870 comp.lang.misc:4344 comp.lang.smalltalk:1738
Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!hellgate.utah.edu!cdr.utah.edu!moore
From: moore%cdr.utah.edu@cs.utah.edu (Tim Moore)
Newsgroups: comp.arch,comp.lang.lisp,comp.lang.misc,comp.lang.smalltalk
Subject: Re: CORRECTION to: heaps of numbers (tagged immediate floats)
Keywords: floating point, garbage collection, tags, IEEE floats, whoops
Message-ID: <1990Mar5.221226.4841@hellgate.utah.edu>
Date: 6 Mar 90 05:12:25 GMT
References: <1990Mar5.220724.3718@Neon.Stanford.EDU> <14261@lambda.UUCP> <1990Mar6.015230.20068@Neon.Stanford.EDU>
Organization: University of Utah CS Dept
Lines: 37

In article <1990Mar6.015230.20068@Neon.Stanford.EDU> wilson@carcoar.Stanford.EDU (Paul Wilson) writes:

>So it looks like each marginal bit would be more important than I thought,
>favoring stealing _very_few_ bits from the exponent. 2**(2**4) would
>only give you a range of 1/32K to 32K.  But 2**(2**5) would give you
>a range from 1/2nano- to 2giga- which seems pretty reasonable.  And 2**(2**6)
>goes from pretty seriously small to pretty seriously big (by my standards).
>
>I interpret this to mean that I can steal 2 bits from the 8-bit exponent, 
>leaving 6, or maybe 3, leaving 5.
>
>It would be awkward to use less than two bits for primary tags, given that
>you probably want separate tags for pointers, immediate ints, and other
>immediates.  So it looks like the question comes down to this:  is the
>1/2nano- to 2giga- range enough for the large majority of floats that
>get stored into memory, or should I go with the less convenient scheme 
>of only stealing 2 bits?  In the latter case I'd have to use up one of our 
>four primary (2-bit low-) tags, but we could live with it.
>

I've been thinking about implementing short floats for Utah Common
Lisp. I think it would be better to steal bits from the mantissa than
the exponent. With our low tags scheme, a simple mask operation would
restore a short float to an IEEE 32-bit float with the least
significant bits of the mantissa zeroed, allowing the floating point
hardware to do its thing.

The original post by Paul Wilson discussed the problem of deciding
when to use short floats vs. when to use long floats to avoid losing
precision. This isn't an issue in Common Lisp (I'm reading this in
comp.lang.lisp); the rules of floating point contagion (promotion) are
defined by the language, and the user can chose the reader's default
float type. 

Tim Moore                     moore@cs.utah.edu {bellcore,hplabs}!utah-cs!moore
"Ah, youth. Ah, statute of limitations."
		-John Waters