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Asked: 2026-05-31T20:19:05+00:00

as it is so common with Fortran, I’m writing a massively parallel scientific code.

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as it is so common with Fortran, I’m writing a massively parallel scientific code. In the beginning of my code I read my configuration file which tells me which type of solver I want to use. Now that means that in a subroutine (during the main run) I have

if(solver.eq.1)then
  call solver1()
elseif(solver.eq.2)then
  call solver2()
else
  call solver3()
endif

Edit to avoid some confusion: This if is inside my time integration loop and I have one that is inside 3 nested loops.

Now my question is, wouldn’t it be more efficient to use function pointers instead as the solver variable will not change during execution, except at the initialisation procedure.

Obviously function pointers are F2003. That shouldn’t be a problem as long as I use gfortran 4.6. But I’m mainly using a BlueGene P, there is a f2003 compiler, so I suppose it’s going to work there as well although I couldn’t find any conclusive evidence on the web.

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1 Answer

  1. Editorial Team

    Knowing nothing about Fortran, this is my answer: The main problem with branching is that a CPU potentially cannot speculatively execute code across them. To mitigate this problem, branch prediction was introduced (which is very sophisticated in modern CPUs).

    Indirect calls through a function pointer can be a problem for the prediction unit of the CPU. If it can’t predict where the call will actually go, this will stall the pipeline.

    I am quite sure that the CPU will correctly predict that your branch will always be taken or not taken because it is a trivial case of prediction.

    Maybe the CPU can speculate across the indirect call, maybe it can’t. This is why you need to test which is better.

    If it cannot, you will certainly notice in your benchmark.

    In addition, maybe you can hoist the if test out of your inner loop so it won’t be called often. This will make the actual performance of the branch irrelevant.

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