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COMP-FORTRAN-90  January 2011

COMP-FORTRAN-90 January 2011

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Subject:

Re: Finding vector size

From:

"Greenberg, Naomi" <[log in to unmask]>

Reply-To:

Fortran 90 List <[log in to unmask]>

Date:

Tue, 4 Jan 2011 14:28:54 -0500

Content-Type:

multipart/signed

Parts/Attachments:

Parts/Attachments

text/plain (66 lines) , smime.p7s (66 lines)

Bill,
  I inherited some code that was optimized for a Cray vector machine.  There
originally were very large computational loops that worked over large
arrays.  The person who vectorized it (very successfully) broke up the large
loop into a series of small simple clear vectorizable loops that computed
partials and then combined them later.  He sized the partial arrays based on
a vector size (R(nvec,2,3), with loops going in nvec chunks).  My questions
now are 1) what should nvec be set for an Itanium-based linux system or
other 32 bit system (Intel compiler)? Is there a way to compute this
automatically? 2) Is this the best way to do this still? Can I be hurt by it
on other machines? Again, it's not just the loop counters that are sized,
it's the actual data structure sizes also.

Thanks,
Naomi

-----Original Message-----
From: Fortran 90 List [mailto:[log in to unmask]] On Behalf Of
Bill Long
Sent: Tuesday, January 04, 2011 11:48 AM
To: [log in to unmask]
Subject: Re: Finding vector size

The optimal vector size for each machine should be known internally by 
the compiler.  Write the loop as the algorithm dictates.  The compiler 
will divide it up into vector "chunks" automatically if the body of the 
loop can be executed by vector hardware.  User attempts to manually 
reform loops for presumed vector lengths,  pipelining,  or cache 
blocking are generally counter-productive.  The result is code that is 
unclear to read,  difficult to maintain,  and confusing to the compiler. 
  Compiler optimizers work best on simple, clean loops.

Cheers,
Bill


On 1/4/11 9:37 AM, Greenberg, Naomi wrote:
> I am trying to find a way to configure code before compile time to set
> the optimal loop vectorization size for the user's machine and then
> (using the Fortran preprocessor) get that value and set the loop size to
> this value. For example, on Machine1, nvec might be 64, on machine2, it
> might be 1024, and the code would "do i=1,nvec" (obviously not quite
> that way). The question is whether there's a way to automatically get
> the optimal vector size from each machine (using Linux) or whether
> there's a better way to get the same result? Any suggestions are welcome!
>
> Naomi Greenberg
>
> /Member of the Research Staff/
>
> Riverside Research Institute
>
> (212) 502-1718 (ph)
>
> (212) 502-1729 (fax)
>
> [log in to unmask]
>

-- 
Bill Long                                           [log in to unmask]
Fortran Technical Support    &                 voice: 651-605-9024
Bioinformatics Software Development            fax:   651-605-9142
Cray Inc./Cray Plaza, Suite 210/380 Jackson St./St. Paul, MN 55101

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