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From: fisher@Alliant.COM (Larry Fisher)
Newsgroups: comp.lang.fortran
Subject: Re: WHERE statement
Keywords: F8X
Message-ID: <3504@alliant.Alliant.COM>
Date: 27 Oct 89 14:57:39 GMT
References: <10400001@jadoube>
Reply-To: fisher@alliant.Alliant.COM (Larry Fisher)
Distribution: na
Organization: Alliant Computer Systems, Littleton, MA
Lines: 39

I have one question in regard to the "conversion" of a DO loop with nested
IFs to a Fortran 8X WHERE block:  Why do the conversion?

Working with the Alliant Fortran compiler, I have the option of using
WHERE blocks when porting and tuning code.  However, I find that it is
rarely worth the time to do the conversion for a number of reasons.

First, I assume that the reason for using WHERE is to get (force) the
DO/nested IF structure to be mapped to vector hardware instructions.  As
a practical matter on today's vector processors, vectorized IF
statements are worthwhile only if the conditionally-executed (cond-ex)
statements are "frequently" executed.  I quote the word frequently
because the definition varies by machine and by computational complexity
of the cond-ex code.  Keeping the explanation brief, the implementation
of vector IFs causes the cond-ex code to be executed *regardless* of
whether or not the result will be used.  The only part of the cond-ex
code that is conditional is the storage into destination
(left-hand-side) variables based on a mask; a mask that is the result of
the IF statement's condition expression.

Second, I (IMHO) view WHERE blocks to be a notational shorthand that is
to be used where appropriate.  The DO/nested IF example recently shown
in this group is not one I would consider appropriate for a WHERE block.
As was pointed out by the original poster, forcing the readable F78 code
into a much-less-comprehensible F8x WHERE block doesn't make sense,
never mind the fact that no equivalent to ELSEIF exists.  Finally, It
doesn't appear to me that the existing algorithm implemented in F78 is
one that, without changes, will run effectively on vector hardware.

Certainly we could discuss automatic mapping of DO/IF constructs to
WHERE blocks by a Fortran compiler, as is done, where appropriate, by
Alliant's FX/Fortran compiler, but that's just one of a number of other
considerations tied to the above comments.

"Opinions..."
-- 
Larry Fisher			Domain:	fisher@alliant.com
Alliant Computer Systems	UUCP:	{mit-eddie|linus}!alliant!fisher
Littleton, MA 01460		Phone:	(508) 486-1449