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From: sundman@ttds.UUCP (Bo Sundman)
Newsgroups: net.lang.f77
Subject: Problem with EQUIVALENCE
Message-ID: <803@ttds.UUCP>
Date: Tue, 22-Jan-85 14:50:33 EST
Article-I.D.: ttds.803
Posted: Tue Jan 22 14:50:33 1985
Date-Received: Fri, 25-Jan-85 08:03:49 EST
Reply-To: sundman@ttds.UUCP (Bo Sundman)
Organization: The Royal Inst. of Techn., Stockholm
Lines: 71

C The following piece of code is extracted from a numerical routine for which
C I am not guilty. However, I am expected to make it work on f77 under
C UNIX and I do not understand why it does not work. 
C The code works perfectly on a number of machines, including Fortran
C on VAX/VMS, but f77 does not give the correct value on the same VAX.
C
C The intention of the code is, I believe, to store integer indices for matrix
C elements in the only available workspace, a real array. I guess the reason
C for the equivalence is to avoid conversion from real to integer and back
C either to increase speed or to avoid rounding off errors.
C
	IMPLICIT DOUBLE PRECISION (A-H,O-Z)
	DIMENSION W(10),NW(2)
	EQUIVALENCE (WW,NW(1)),(NQ,NW(2))
C MEMORY MAP OF WW, NW AND NQ
C WORD:		1	2
C WW	 (..............)
C NW     (...1..)(...2..)
C NQ		 (......)
C
C Storing an integer value into NQ is thus equivalent to storing the same
C bit pattern into the least significant word of WW.
C
	WRITE(*,*)'Test 1'
	NQ=10
	WRITE(*,*)'Values should be     0  10:',NW
	W(5)=WW
	NQ=100
	WRITE(*,*)'Values should be     0 100:',NW
	WW=1.
	WRITE(*,*)'Values should be 16512   0:',NW
	WW=W(5)
	WRITE(*,*)'Values should be     0  10:',NW,' <<< wrong'
	WRITE(*,*)'Test 2, WW=1.'
	WW=1.
	NQ=10
	WRITE(*,*)'Values should be 16512  10:',NW
	W(5)=WW
	NQ=100
	WRITE(*,*)'Values should be 16512 100:',NW
	WW=1.
	WRITE(*,*)'Values should be 16512   0:',NW
	WW=W(5)
	WRITE(*,*)'Values should be 16512  10:',NW
	END
C
C Output:
C
  Test 1
  Values should be     0  10:  0  10
  Values should be     0 100:  0  100
  Values should be 16512   0:  16512  0
  Values should be     0  10:  0  0        <<< THIS IS WRONG
  Test 2, WW=1.
  Values should be 16512  10:  16512  10
  Values should be 16512 100:  16512  100
  Values should be 16512   0:  16512  0
  Values should be 16512  10:  16512  10

My conclusion is that the run time system is "smart" and when it
shall make the assignment
	WW=W(5)
it discovers that the exponent of W(5) is zero, which normally means
that W(5) IS zero, and it assigns zero to WW rather than copying the
value of W(5) to WW. It may save some nanoseconds of execution time but
is nontrivial in a case like this.
-- 
	Bo Sundman                      (..mcvax!enea!ttds!sundman)
        Thermo-Calc group, Division of Physical Metallurgy
	The Royal Institute of Technology
	Stockholm, Sweden