fftw3: FFTW Execution in Fortran
8.3 FFTW Execution in Fortran
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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 (
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 (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 (SIMD alignment and
fftw_malloc).
Info Catalog
fftw3: FFTW Constants in Fortran
fftw3: Calling FFTW from Legacy Fortran
fftw3: Fortran Examples