Path: utzoo!attcan!uunet!lll-winken!lll-lcc!ames!mailrus!cwjcc!gatech!gitpyr!loligo!mccalpin
From: mccalpin@loligo.fsu.edu (John McCalpin)
Newsgroups: comp.arch
Subject: Re: MIPS supports 80- & 128-bit floats.
Message-ID: <350@loligo.fsu.edu>
Date: 4 Jan 89 13:34:46 GMT
References: <10452@obiwan.mips.COM> <325@loligo.fsu.edu> <13142@cup.portal.com> <345@loligo.fsu.edu> <83596@sun.uucp>
Reply-To: mccalpin@loligo.UUCP (John McCalpin)
Organization: Supercomputer Computations Research Institute
Lines: 40

In article <83596@sun.uucp> khb@sun.UUCP (Keith Bierman) writes:
>In article <345@loligo.fsu.edu> (John McCalpin) writes:
>
>>I think that The IEEE standard is a very good choice in a new design.
>>It is rather expensive to do correctly, and this often translates into
>>slower speed at a fixed price.
>
>Gradual underflow and divide are sticking points for very high (cost
>is no object performance). ...they will cause the biggest "baddest"
>machines to be slower.
>Seymour's machines do very low quality divides, but they are very
>fast. If speed is the name of the game (and in supercomputing it has
>been) then a non-ieee divide is a fact of life.
>
>Keith H. Bierman
>It's Not My Fault ---- I Voted for Bill & Opus

An option that seems to be taken by many vendors is to adopt the IEEE
*format* without adopting all of the *rules*.  Admittedly, this is a
dangerous choice, but it does aid portability.  A particular example is
to simply not calculate guard, round, and sticky bits in divides.  The
user should try to replace divides with multiplies, which retain full
IEEE precision.  If the divide can't be replaced, then you just get a
less accurate answer (as on the Crays).  I don't know how to handle
gradual underflow, though I agree it is important....

The advantage of using the IEEE *formats* is that there is at least hope
that binary data files could be read on the front end.  Many, many hours
of CPU time on supercomputers are wasted on scalar data analysis/graphics
programs that should be run on a more cost-effective front-end, which is
often where the files are actually stored anyway.

I recently ran the PARANOIA floating-point validation test on a wide
variety of machines, and found precisely ONE that passed all the
tests.  I later found two more, out of about 15 machines/vendors
tested. The Sun workstations, HP workstations, and MIPS machines passed
OK.  All the supercomputers failed, of course! :-)

John D. McCalpin	Supercomputer Computations Research Institute
mccalpin@masig1.ocean.fsu.edu		mccalpin@fsu	(BITNET or MFENET)