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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