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From: Beebe@SCIENCE (Nelson H.F. Beebe)
Newsgroups: comp.lang.fortran
Subject: Re: Re:FORTRAN-77 XLA PLUS BITWISE OPERATORS
Message-ID: <12296045502.17.BEEBE@SCIENCE.UTAH.EDU>
Date: Mon, 20-Apr-87 11:35:16 EST
Article-I.D.: SCIENCE.12296045502.17.BEEBE
Posted: Mon Apr 20 11:35:16 1987
Date-Received: Fri, 24-Apr-87 03:23:31 EST
Lines: 36
In-Reply-To: <1731@imag.UUCP>

X-Us-Mail: "Center for Scientific Computation, South Physics, University of Utah, Salt Lake City, UT 84112"

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Regarding this subterfuge for fullword bit twiddling:
>> integer function AND(i1,i2)
>> integer k1, k2,k3
>> logical j1,j2,j3
>> equivalence (k1,j1), (k2,j2), (k3,j3)
>> 
>> k1=i1
>> k2=i2
>> j3=j1.and.j2
>> AND=k3
>> end		    
>> 

IT WON'T WORK ON MANY MACHINES.  

First, a compiler is free to implement FORTRAN LOGICAL  data
as any  object  of size  at  least  one bit.   I  have  seen
compilers that tested  the sign  bit, others  that test  the
right-most bit, others  that test either  the right-most  or
left-most   byte,   and   still   others   that   test   for
zero/non-zero.  The LOGICAL .OP. operators will rarely  work
on fullword data.

Second, there may be no guarantee  that a LOGICAL is of  the
same size as an INTEGER, and I could name one compiler where
they are not (apparently in  violation of ANSI F77,  Section
4.7, p. 4-4 "A logical datum has one numeric storage unit in
a storage sequence").

The code  above will  work on  some machines,  but  comments
should  definitely   flag  it   as  non-portable.    It   is
regrettable that  ANSI  never  included a  standard  set  of
integer bit operations  (.AND., .OR.,  .NOT., .XOR.),  since
almost all compilers (IBM notably excepted) have implemented
them in some fashion.
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