Plan execution in Fortran

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7.4 Plan execution in Fortran

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In C, in order to use a plan, one normally calls fftw_execute, Chris@10: which executes the plan to perform the transform on the input/output Chris@10: arrays passed when the plan was created (see Using Plans). The Chris@10: corresponding subroutine call in modern Fortran is: Chris@10:

      call fftw_execute(plan)
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Chris@10: However, we have had reports that this causes problems with some Chris@10: recent optimizing Fortran compilers. The problem is, because the Chris@10: input/output arrays are not passed as explicit arguments to Chris@10: fftw_execute, the semantics of Fortran (unlike C) allow the Chris@10: compiler to assume that the input/output arrays are not changed by Chris@10: fftw_execute. As a consequence, certain compilers end up Chris@10: repositioning the call to fftw_execute, assuming incorrectly Chris@10: that it does nothing to the arrays. Chris@10: Chris@10:

There are various workarounds to this, but the safest and simplest Chris@10: thing is to not use fftw_execute in Fortran. Instead, use the Chris@10: functions described in New-array Execute Functions, which take Chris@10: the input/output arrays as explicit arguments. For example, if the Chris@10: plan is for a complex-data DFT and was created for the arrays Chris@10: in and out, you would do: Chris@10:

      call fftw_execute_dft(plan, in, out)
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Chris@10: There are a few things to be careful of, however: Chris@10: Chris@10:

You must use the correct type of execute function, matching the way Chris@10: the plan was created. Complex DFT plans should use Chris@10: fftw_execute_dft, Real-input (r2c) DFT plans should use use Chris@10: fftw_execute_dft_r2c, and real-output (c2r) DFT plans should Chris@10: use fftw_execute_dft_c2r. The various r2r plans should use Chris@10: fftw_execute_r2r. Fortunately, if you use the wrong one you Chris@10: will get a compile-time type-mismatch error (unlike legacy Fortran). Chris@10: Chris@10:
You should normally pass the same input/output arrays that were used when Chris@10: creating the plan. This is always safe. Chris@10: Chris@10:
If you pass different input/output arrays compared to Chris@10: those used when creating the plan, you must abide by all the Chris@10: restrictions of the new-array execute functions (see New-array Execute Functions). The most tricky of these is the Chris@10: requirement that the new arrays have the same alignment as the Chris@10: original arrays; the best (and possibly only) way to guarantee this Chris@10: is to use the ‘fftw_alloc’ functions to allocate your arrays (see Allocating aligned memory in Fortran). Alternatively, you can Chris@10: use the FFTW_UNALIGNED flag when creating the Chris@10: plan, in which case the plan does not depend on the alignment, but Chris@10: this may sacrifice substantial performance on architectures (like x86) Chris@10: with SIMD instructions (see SIMD alignment and fftw_malloc). Chris@10: Chris@10:

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