8.3 FFTW Execution in Fortran

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

        call dfftw_execute(plan)

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

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

        call dfftw_execute_dft(plan, in, out)

There are a few things to be careful of, however:

• You must use the correct type of execute function, matching the way the plan was created. Complex DFT plans should use dfftw_execute_dft, Real-input (r2c) DFT plans should use use dfftw_execute_dft_r2c, and real-output (c2r) DFT plans should use dfftw_execute_dft_c2r. The various r2r plans should use dfftw_execute_r2r.
• You should normally pass the same input/output arrays that were used when creating the plan. This is always safe.
• If you pass different input/output arrays compared to those used when creating the plan, you must abide by all the restrictions of the new-array execute functions (see New-array Execute Functions). The most difficult of these, in Fortran, is the requirement that the new arrays have the same alignment as the original arrays, because there seems to be no way in legacy Fortran to obtain guaranteed-aligned arrays (analogous to fftw_malloc in C). You can, of course, use the FFTW_UNALIGNED flag when creating the plan, in which case the plan does not depend on the alignment, but this may sacrifice substantial performance on architectures (like x86) with SIMD instructions (see SIMD alignment and fftw_malloc).