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From: levy@ttrdc.UUCP (Daniel R. Levy)
Newsgroups: net.arch
Subject: Re: Floating point
Message-ID: <767@ttrdc.UUCP>
Date: Sun, 2-Mar-86 16:24:59 EST
Article-I.D.: ttrdc.767
Posted: Sun Mar  2 16:24:59 1986
Date-Received: Fri, 7-Mar-86 06:30:20 EST
References: <5100014@ccvaxa> <5059@alice.uUCp>
Organization: AT&T, Computer Systems Division, Skokie, IL
Lines: 57

<Oh oh here it comes.  Watch out boy, it'll chew you up! \
Oh oh here it comes.  The LINE EATER!  [Line eater]>

In article <5059@alice.uUCp>, ark@alice.UucP (Andrew Koenig) writes:
>> Now, is this an architectural question: should or could randomized rounding be
>> provided by a floating point unit? Assuming you can cheaply obtain a random
>> number in hardware (amplify transistor noise?) doing randomized rounding in
>> the floating point unit would be a lot faster than doing so in software,
>> where you'd have to extract and play around with the guard bits.
>If randomized rounding is provided by a floating-point unit,
>it had better be easy to turn off because using it is likely
>to make debugging impossible.  Consider: if you run the same
>program twice and get slightly different results, how will you
>ever know if it is a bug in the program?

Random floating point thoughts... :-)

I can just see it, a new mauctl() system call (for the 3B2/5/15'ers :-).
What about OTHER processes which were also using the floating point unit?
They'd get screwed, wouldn't they, unless the floating point behavior was
modifiable on a per-process basis?

Would it work as well to set up the floating point unit so that the way it
rounds would alternate from one way to the other each time that a given
PROCESS used it (not just every time it was used, since several processes
might all be calling for it) and that it would always start the same way?
If so might this alleviate some of the debugging difficulties?

(Only trouble is then that debugging statements had better use the unit an even
number of times or you're right back where you started...)

Maybe the default random-rounding state should be OFF for the floating point
unit.

[Gentle flame on...]

I think that more important than the random rounding issue would be
to CLEAN UP THE ACT of the various math libraries in common use!  (Is there
an ANSI standard either existing or in the works for math libraries?)  In
a recent news article by (I think) Gene Spafford, it is pointed out that
the situation is horrible for many math libraries.  I have found that on the
same computer (a VAX 8600) the results of a trivial SPICE simulation run were
quite different when the VMS math libraries were used versus Unix (Eunice--
a 4.1 BSD emulation under VMS) math libraries.  (On a SysV 3B20, the results
matched the VMS ones fairly closely, which seems to point blame away from the
[Fortrash] compiler and the floating point implementation and toward the lib-
raries.)  It is my belief that results would diverge a lot less between
different machines if the math libraries were simply cleaned up and standard-
ized.
-- 
 -------------------------------    Disclaimer:  The views contained herein are
|       dan levy | yvel nad      |  my own and are not at all those of my em-
|         an engihacker @        |  ployer or the administrator of any computer
| at&t computer systems division |  upon which I may hack.
|        skokie, illinois        |
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