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mecej4 · Joined: 31 Oct 2006 Posts: 1897

The software package HST3D developed by the USGS is used to model groundwater flows with coupled heat and salt transport. It has been widely used for decades, and is standard Fortran 95 with no dependence on external libraries. See https://wwwbrr.cr.usgs.gov/projects/GW_Solute/hst/index.shtml .

The HST3D source code contains several instances where a pointer to an array section is passed as an actual argument to a subprogram where the corresponding dummy argument is an assumed shape array. Such subprograms have explicit interfaces available to their callers, as required.

Here is an example of such usage.

PaulLaidler · Posted: Fri Feb 09, 2024 8:29 am Post subject:

mecej4

Many thanks for the feedback. I have logged this for investigation.

mecej4 · Joined: 31 Oct 2006 Posts: 1897

Paul,

Encouraged by your agreeing to look into this issue, I continued my attempts to produce a shorter reproducer, and this time I succeeded.

I diverted the arrays in HST3D to an unformatted file at a point where I felt that the performance bug might surface, and created a reproducer that started by reading that file rather than creating the arrays in question by going through a long calculation as HST3D does.

The reproducer is less than 200 lines long, in a single source file. The unformatted file is "efact.bin" and I have provided a batch file to build two versions of the program, one with array pointers and the other with array sections as arguments. The three files are contained in the following zip file

https://www.dropbox.com/scl/fi/ivxm461g4j8vlbzfgdbou/ArraySec.zip?rlkey=chaxi7d7432f17egfecl6la4k&dl=0

On my PC, the array section version runs in 0.3 second, and the pointer version takes 22.4 seconds.

It may be more convenient for you to use this new reproducer until a fix is found, and then try the fixed compiler/DLLs on the larger HST3D test program.

PaulLaidler · Posted: Fri Feb 09, 2024 5:14 pm Post subject:

mecej4

Thank you. That is very helpful.

PaulLaidler · Posted: Thu Feb 15, 2024 4:38 pm Post subject:

mecej4

An initial investigation indicates that the calls

mecej4 · Joined: 31 Oct 2006 Posts: 1897

Thanks, Paul.

Indeed, I tried adding the CONTIGUOUS attribute to the dummy argument declarations to which these array sections are being passed, and the program ran fine when compiled with other compilers that support CONTIGUOUS (Intel, NAG, Gfortran do, but not Lahey). If FTN95 can be helped to avoid copy-in/copy-out by being told that the argument in question is contiguous, that would suffice until the compiler becomes more sophisticated in deciding on its own whether copy-in/copy-out is needed or not.

Whether a solution is possible may depend on the compiler's array descriptor (dope vector) details. A fallback remedy is to pass the first array element as the actual argument, but that remedy has the drawback that /check will not permit such usage (unless there is a magical /inhibit_check number that will allow it). Is there such a check number corresponding to scalar actual/array dummy mismatch being tolerated, with other checks in place as usual?

PaulLaidler · Posted: Fri Feb 16, 2024 10:35 am Post subject:

mecej4

Sorry there is no magical check number that I know of. Adding some support for the CONTIGUOUS attribute looks like a useful idea.

mecej4 · Joined: 31 Oct 2006 Posts: 1897

Thanks, Paul; there are no surprises in what you found.

Implementing CONTIGUOUS could be done in two stages. In the first stage, the compiler would recognise CONTIGUOUS as a standard-approved attribute, but take no further action.

For users, this would be helpful by allowing them to use the same source code with FTN95 as with other compilers that recognise CONTIGUOUS (and, perhaps, those compilers may output faster machine code as a consequence).

As a later stage, FTN95 may be enhanced to generate faster machine code for passing contiguous actual arguments without copy-in/copy-out or temporary arrays. Users may see speed improvement without any source code changes being needed.

PaulLaidler · Posted: Mon Feb 19, 2024 8:28 am Post subject:

Support for the CONTIGUOUS attribute has now been added for the next release of FTN95. It will have the effect of supressing some "copy in" and "copy out" processes for array sections when the compiler is not provided with enough information to be able to do this automatically.

On its own this will not fix the primary issue on this thread which remains outstanding.

PaulLaidler · Posted: Wed Feb 21, 2024 3:45 pm Post subject:

A provisional fix has now been implemented for this issue. Provisional in the sense that it works for the cut-down code but I have not tested it against the orginal project. This fix requires /opt and works for both Win32 and x64. It will be in the next release of FTN95.

mecej4 · Joined: 31 Oct 2006 Posts: 1897

Great news! I look forward to the release of the compiler with the fixes.