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From: lewitt@Alliant.COM (Martin E. Lewitt)
Newsgroups: comp.lang.c,comp.unix.cray
Subject: Re: Vectorizing C compilers
Message-ID: <3297@alliant.Alliant.COM>
Date: 27 Jul 89 07:43:26 GMT
References: <15388@ut-emx.UUCP>
Reply-To: lewitt@alliant.Alliant.COM (Martin E. Lewitt)
Organization: Alliant Computer Systems, Littleton, MA
Lines: 41

In article <15388@ut-emx.UUCP> eric@chaos.utexas.edu (Eric Kostelich) writes:

>What is the state of the art regarding vectorizing C compilers 
>for Crays and other "minisuper" machines like Alliant, Convex, etc.?  
>I have seen one or two C compilers (on a Unicos Cray) that vectorize
>loops like
>
>	for(j = 0; j < n; j++)
>		y[j] = x[j] + z[j];
>
>but never a C compiler that can vectorize something like
>
>	for(j = 0; j < n; j++)
>		y[j] = sqrt(x[j]);
>
I'm not a C programmer myself, but Mark Buxbaum (buxbaum@alliant.com)
of our compiler group informs me that

" the FX/C compiler V2.1 not only runs that sqrt concurrent, but
  vector-concurrent.  I believe that FX/C version 2.0 ran it
  concurrently. "

For those not familiar with Alliant, our vector-concurrent compiler
optimization, is where a loop has been vectorized and then spread either
horizontally or vertically across multiple processors, utilizing the vector
capability of each.   The compiler selects the horizontal or vertical
strip mining based upon which accessing pattern will perform best
when multiple processors are accessing the memory system simultaneously.
Special hardware support facilitates this break-up, a separate concurrency
bus and a group of instructions (known within Alliant as "do the right thing
instructions") which implicitly "know" how many other processors are working
on this job and which processor they are on in this multi-processor strip
mine scheme.  Sweet stuff.

I expect that other vendor's C compilers have also progressed since your
experience with them.  Its a competitive marketplace and none of us
is standing still (a cliche, but apropos).     :-)
-- 
Phone: (206) 931-8364			Martin E. Lewitt      My opinions are
Domain: lewitt@alliant.COM		2945 Scenic Dr. SE    my own, not my
UUCP: {linus|mit-eddie}!alliant!lewitt  Auburn, WA 98002      employer's.