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From: jbs@WATSON.IBM.COM
Newsgroups: comp.arch
Subject: IEEE floating point
Message-ID: <9105290600.AA28145@ucbvax.Berkeley.EDU>
Date: 29 May 91 05:15:30 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Lines: 63


        I said:
>As to wrong answers, wrong answers are generally caused by
>users not knowing what they are doing.
        D.C. Lindsay commented:
I disagree. Dr. Kahan tells excellent anecdotes on this point, with
examples of well-respected products (e.g. Mathematica, hand
calculators) giving wrong answers to simple problems.

        I don't see your point here.  In the first place I said
generally not always.  In the second place these examples support
my point since the users here are the writers of Mathematica and
the power function on calculators and they are making errors
which have nothing to do with the quality of the floating point
arithmetic they are using.
        D.C. Linsay also said
Seconded. People who think that the common alternatives are as well
designed, are probably unfamiliar with the sore points.  For example,
Cray format allows a value which will overflow if multiplied by 1 ...

        IEEE format allows values which when multiplied by 0 do not
equal 0 which is in my opinion worse.
        I said:
>...don't believe it addresses the main sources of error.  I also don't
>believe that IEEE FP is well-designed.  Exactly why do you believe nu-
>merical problems using IEEE FP are more likely to be obvious and pre-
>dictable than numerical problems using IBM hex (for example)? ...
        Henry Spencer replies
A proper response to this would basically be a detailed defence of IEEE FP's
more controversial design decisions.  I have neither the time nor, really,
the expertise to do this.  However, salvation arriveth from an unexpected
direction. :-)  Go read "What Every Computer Scientist Should Know About
Floating-Point Arithmetic", by David Goldberg, in the latest (March) issue
of ACM Computing Surveys; doing so will enlighten you in detail on the
subject.
I will confine myself to observing that IBM hex FP is the only FP format
I know of that made half the FP instructions -- the single-precision ones --
just about useless to most programmers.

        I checked the Goldberg reference.  He states in effect the pur-
pose of his paper is to explain IEEE floating point not defend it.
        Your statements about IBM hex also are not responsive to the
question I asked which to repeat is given that you have exceeded the
precision provided why does this produce "obvious and predictable"
effects with IEEE as opposed to "subtle and mysterious" effects with
IBM hex (or any other alternative).
        Bill Davidson states:
may not have more than a few significant bits for starters, and I would
rather not trust my bits to a computer designed with the marketing
department winning compromises between "right" and "fast" answers.

        I believe you have things backwards here.  Marketing depart-
ments generally love IEEE.  After all they don't have to implement it.
        Keith H. Bierman states:
Having 1% bad fp arithmetic is like playing russian roulette every
morning. It will bite you someday. Unlike RR, the bad fp will just
assist you in doing bad science. Perhaps that isn't a problem, just
think of all the extra papers/research we all get to do to disprove
the resulting drivel...

        This would imply people using Crays are more likely to pro-
duce drivel than people using IEEE systems.  I doubt this is true.
                       James B. Shearer