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From: jerry@violet.berkeley.edu (Jerry Berkman)
Newsgroups: comp.lang.fortran
Subject: Re: Cray conditional vector merge
Message-ID: <1991Mar3.041831.11088@agate.berkeley.edu>
Date: 3 Mar 91 04:18:31 GMT
References: <1991Feb24.050433.21694@ariel.unm.edu>
Sender: usenet@agate.berkeley.edu (USENET Administrator)
Organization: University of California, Berkeley
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In article <1991Feb24.050433.21694@ariel.unm.edu> john@ghostwheel.unm.edu (John Prentice) writes:
>I have a question for the Cray guru's out there.  When you execute
>a conditional vector merge in Cray Fortran, what is really happening?
>For example, consider:
>
>    a=cvmgp(f(x),g(x),h(x))
>
> ...
>John K. Prentice    john@unmfys.unm.edu (Internet)
>Dept. of Physics and Astronomy, University of New Mexico, Albuquerque, NM, USA
>Computational Physics Group, Amparo Corporation, Albuquerque, NM, USA

As others have noted, first all three expressions are computed,
then a mask is formed, and then the result is returned (to be stored)
depending on the mask.  The cvmg* family of functions are all typeless,
consider:

	do i = 1, n
		x(i) = cvmgt( x(i), i-5, y(i).gt.z )
	end do

this is almost the same as:

	do i = 1, n
		if( y(i).gt.z ) x(i) = i-5
	end do

except that since cvmgt() is typeless, no type conversion is made when
storing i-5 in x(i).

In a lot of cases, Cray's CFT77 compiler will compile if()s the same
as cvmg*() calls.  It's generally better to stay standard and avoid these
except when the compiler gets confused and won't vectorize code
unless you use them.

	- Jerry Berkman, U.C. Berkeley