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, cannam@95: which executes the plan to perform the transform on the input/output cannam@95: arrays passed when the plan was created (see Using Plans). The cannam@95: corresponding subroutine call in modern Fortran is: cannam@95:

      call fftw_execute(plan)
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cannam@95: However, we have had reports that this causes problems with some cannam@95: recent optimizing Fortran compilers. The problem is, because the cannam@95: input/output arrays are not passed as explicit arguments to cannam@95: fftw_execute, the semantics of Fortran (unlike C) allow the cannam@95: compiler to assume that the input/output arrays are not changed by cannam@95: fftw_execute. As a consequence, certain compilers end up cannam@95: repositioning the call to fftw_execute, assuming incorrectly cannam@95: that it does nothing to the arrays. cannam@95: cannam@95:

There are various workarounds to this, but the safest and simplest cannam@95: thing is to not use fftw_execute in Fortran. Instead, use the cannam@95: functions described in New-array Execute Functions, which take cannam@95: the input/output arrays as explicit arguments. For example, if the cannam@95: plan is for a complex-data DFT and was created for the arrays cannam@95: in and out, you would do: cannam@95:

      call fftw_execute_dft(plan, in, out)
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cannam@95: There are a few things to be careful of, however: cannam@95: cannam@95:

You must use the correct type of execute function, matching the way cannam@95: the plan was created. Complex DFT plans should use cannam@95: fftw_execute_dft, Real-input (r2c) DFT plans should use use cannam@95: fftw_execute_dft_r2c, and real-output (c2r) DFT plans should cannam@95: use fftw_execute_dft_c2r. The various r2r plans should use cannam@95: fftw_execute_r2r. Fortunately, if you use the wrong one you cannam@95: will get a compile-time type-mismatch error (unlike legacy Fortran). cannam@95: cannam@95:
You should normally pass the same input/output arrays that were used when cannam@95: creating the plan. This is always safe. cannam@95: cannam@95:
If you pass different input/output arrays compared to cannam@95: those used when creating the plan, you must abide by all the cannam@95: restrictions of the new-array execute functions (see New-array Execute Functions). The most tricky of these is the cannam@95: requirement that the new arrays have the same alignment as the cannam@95: original arrays; the best (and possibly only) way to guarantee this cannam@95: is to use the ‘fftw_alloc’ functions to allocate your arrays (see Allocating aligned memory in Fortran). Alternatively, you can cannam@95: use the FFTW_UNALIGNED flag when creating the cannam@95: plan, in which case the plan does not depend on the alignment, but cannam@95: this may sacrifice substantial performance on architectures (like x86) cannam@95: with SIMD instructions (see SIMD alignment and fftw_malloc). cannam@95: cannam@95:

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