Path: utzoo!attcan!uunet!lll-winken!lll-lcc!ames!mailrus!uflorida!gatech!gitpyr!loligo!mccalpin
From: mccalpin@loligo.fsu.edu (John McCalpin)
Newsgroups: comp.arch
Subject: Re: MIPS supports 80- & 128-bit floats.
Message-ID: <352@loligo.fsu.edu>
Date: 4 Jan 89 22:10:31 GMT
References: <350@loligo.fsu.edu> <83722@sun.uucp>
Reply-To: mccalpin@loligo.cc.fsu.edu (John McCalpin)
Organization: Supercomputer Computations Research Institute
Lines: 39

In article <83722@sun.uucp> khb@sun.UUCP (Keith Bierman - Sun Tactical Engineering) writes:
>In article <350@loligo.fsu.edu> mccalpin@loligo.UUCP (John McCalpin) writes:
>>
>>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.
>
>If there is a 32-bit mode (and its being used) I would be very worried
>about the quality of a non-ieee "algorithm" using ieee formatted
>numbers. For 64-bit machines the problem is not as severe...
>
>Keith H. Bierman

I certainly agree with that fear, and insist on testing my 32-bit codes
pretty thoroughly before trusting them.  An interesting example (which 
I referred to in my paper in SUPERCOMPUTING a few months back) is the
1000x1000 LINPACK benchmark test. The matrix is fairly poorly
conditioned, so requires some care in 32-bit mode. I include a sample 
of my results below.  The numbers scale so that the absolute value
of the exponent of the RMS error is about the number of significant
digits of accuracy in the solution.

machine		       precision	    RMS error in solution (*)
---------------------------------------------------------------------
Cyber 205 / ETA-10	32-bit			2.22521256e-01
IBM 3081		32-bit			2.37465184e-03
IEEE standard		32-bit			2.82104476e-04
---------------------------------------------------------------------
Cyber 205 / ETA-10	64-bit			1.32111221e-08
Cray X/MP		64-bit			2.47078473e-11
IEEE standard		64-bit			2.27274978e-13
---------------------------------------------------------------------
Cyber 205/ETA-10	128-bit			1.60755733e-22
Cray X/MP		128-bit			4.15861230e-26

NOTES:
    (*) The solution vector consists of 1000 identical elements = 1.0
    (1) The IEEE standard was run on a Sun 3/280, which had passed
	the PARANOIA benchmark test.