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Nested Implied DO LOOP - order of indices

 
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viroxa



Joined: 28 Jul 2017
Posts: 69

PostPosted: Mon Aug 07, 2017 4:34 pm    Post subject: Nested Implied DO LOOP - order of indices Reply with quote

Good evening, everyone,

is the order of indices within an implied DO LOOP important, regarding performance?

Example: Order of icoun, jcoun, kcoun
Code:
      ALLOCATE(bmatt(ndib1,ndib2,nlays))

!      Assemble array bmatt - contains layerwise results
!      Reshape is used, because implied DO LOOP returns 1D-array 
      bmatt=reshape([((assign_belem(...),jcoun=1,ndib1),kcoun=1,ndib2),icoun=1,nlays)],shape(bmatt))


Thanks!
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PaulLaidler
Site Admin


Joined: 21 Feb 2005
Posts: 4920
Location: Salford, UK

PostPosted: Mon Aug 07, 2017 5:34 pm    Post subject: Reply with quote

It might well be, depending on how often the assignment is performed and how many dimensions there are in the array.

Fortran uses "column major" ordering and the compiler may optimise the process of calculating the address of each element. For FTN95 further optimisation may be applied when using /opt on the command line.

I suggest that you run a simple timing test, wrapping the assignment in a do loop with (say) 10000 passes. This will give you a sensible speed comparison between different orderings and different compiler command lines options.
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viroxa



Joined: 28 Jul 2017
Posts: 69

PostPosted: Tue Aug 08, 2017 9:00 am    Post subject: Reply with quote

Thanks, I'll try that.
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