Path: utzoo!mnetor!tmsoft!torsqnt!news-server.csri.toronto.edu!cs.utexas.edu!uunet!mcsun!cernvax!chx400!chx400!bernina!bloepfe
From: bloepfe@bernina.ethz.ch (Bruno Loepfe)
Newsgroups: comp.lang.fortran
Subject: Re: Cray conditional vector merge
Message-ID: <1991Feb25.122147.430@bernina.ethz.ch>
Date: 25 Feb 91 12:21:47 GMT
References: <1991Feb24.050433.21694@ariel.unm.edu>
Reply-To: bloepfe@bernina.UUCP (Bruno Loepfe)
Organization: Swiss Federal Institute of Technology (ETH), Zuerich
Lines: 31

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))
>
>where f,g, and h are some arithmetic functions.  Does it compute both
>f(x) and g(x) and then return only one depending on what the value
>of h(x) is or does it only compute f(x) or g(x) as required depending
>on the value of h(x)?  
>

Yes, it does indeed. It evaluates f(x) to some vector register, evaluates
g(x) to another vector register, creates a bit mask of (h(x) .GT. 0.0) and
then stores the aproppriate value in a, according to the mask. This means,
if for example h(x) > 0.0 for 90% or more of all x, f(x) = const and
g(x) = VERY time consuming function of x, such a loop might execute slower
in vector mode than in scalar mode...

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Bruno Loepfe                     	u36@czheth5a.bitnet
Computing Center                 	loepfe@rz.id.ethz.ch
Federal Institute of Technology  	bloepfe@ethz.uucp
Zuerich, Switzerland		 	..!uunet!mcsun!ethz!bloepfe (UUCP)

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Bruno Loepfe                     	u36@czheth5a.bitnet
Computing Center                 	loepfe@rz.id.ethz.ch
Federal Institute of Technology  	bloepfe@ethz.uucp