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From: naparst@cartan.UUCP
Newsgroups: comp.lang.fortran
Subject: Cray amod bug
Message-ID: <1373@cartan.Berkeley.EDU>
Date: Sun, 1-Nov-87 01:35:22 EST
Article-I.D.: cartan.1373
Posted: Sun Nov  1 01:35:22 1987
Date-Received: Tue, 3-Nov-87 01:43:18 EST
Reply-To: naparst@cartan.Berkeley.EDU (Harold Naparst)
Organization: Math Dept. UCB
Lines: 32

> 	Cray II CFT77 Fortran amod function appears to give seriously wrong
> results on certain occasions. It is possible the other Cray Fortran compiler
> uses the same amod assembly level code; perhaps even the C and Pascal
> compilers.
> 	It sometimes returns -epsilon instead of the Y-epsilon as shown in
> the following:
> 
             [ program deleted to save space] 
> 
> I am alternating between amusement, bewilderment,
> and rage about such a cavalier attitude. IF X < Y BY ANY REPRESENTABLE
> AMOUNT, THE R=AMOD(X,Y) MUST BE NEAR X NOT NEAR 0. 

The author is absolutely correct.  I have verified this bug on our Cray X-MP,
with both the CFT and CFT77 compilers.  This is definitely a hardware bug,
not just in the author's machine, but in all CRAY systems.  I have reported 
this to our systems staff here.  In some sense, this message should not
have been posted to comp.lang.fortran, since it is a hardware problem,
not a compiler problem.  But I don't care about these things.

Cray systems are renowned for their inaccuracies.  You might try dividing
240.0 by 3.0.  I believe you don't get exactly 80.  The author should obtain
a copy of Prof. William Kahan's program "Paranoia" from netlib@anl-mcs.arpa
and run it on the CRAY.  It fails miserably.  On the other hand, if you 
run it on a SUN, you will find complete conformance to the IEEE standard.

Harold Naparst  (naparst@cartan.berkeley.edu)
-- 
Harold Naparst (415)-848-4560
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