Chris@19: Next: Fortran Examples, Chris@19: Previous: FFTW Constants in Fortran, Chris@19: Up: Calling FFTW from Legacy Fortran Chris@19:
In C, in order to use a plan, one normally calls fftw_execute,
Chris@19: which executes the plan to perform the transform on the input/output
Chris@19: arrays passed when the plan was created (see Using Plans). The
Chris@19: corresponding subroutine call in legacy Fortran is:
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call dfftw_execute(plan) Chris@19:Chris@19:
Chris@19: However, we have had reports that this causes problems with some
Chris@19: recent optimizing Fortran compilers. The problem is, because the
Chris@19: input/output arrays are not passed as explicit arguments to
Chris@19: dfftw_execute, the semantics of Fortran (unlike C) allow the
Chris@19: compiler to assume that the input/output arrays are not changed by
Chris@19: dfftw_execute. As a consequence, certain compilers end up
Chris@19: optimizing out or repositioning the call to dfftw_execute,
Chris@19: assuming incorrectly that it does nothing.
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There are various workarounds to this, but the safest and simplest
Chris@19: thing is to not use dfftw_execute in Fortran. Instead, use the
Chris@19: functions described in New-array Execute Functions, which take
Chris@19: the input/output arrays as explicit arguments. For example, if the
Chris@19: plan is for a complex-data DFT and was created for the arrays
Chris@19: in and out, you would do:
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call dfftw_execute_dft(plan, in, out) Chris@19:Chris@19:
Chris@19: There are a few things to be careful of, however: Chris@19: Chris@19:
dfftw_execute_dft, Real-input (r2c) DFT plans should use use
Chris@19: dfftw_execute_dft_r2c, and real-output (c2r) DFT plans should
Chris@19: use dfftw_execute_dft_c2r. The various r2r plans should use
Chris@19: dfftw_execute_r2r.
Chris@19:
Chris@19: fftw_malloc in C). You
Chris@19: can, of course, use the FFTW_UNALIGNED flag when creating the
Chris@19: plan, in which case the plan does not depend on the alignment, but
Chris@19: this may sacrifice substantial performance on architectures (like x86)
Chris@19: with SIMD instructions (see SIMD alignment and fftw_malloc).
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