200 lines
10 KiB
Plaintext
200 lines
10 KiB
Plaintext
@node Upgrading from FFTW version 2, Installation and Customization, Calling FFTW from Legacy Fortran, Top
|
|
@chapter Upgrading from FFTW version 2
|
|
|
|
In this chapter, we outline the process for updating codes designed for
|
|
the older FFTW 2 interface to work with FFTW 3. The interface for FFTW
|
|
3 is not backwards-compatible with the interface for FFTW 2 and earlier
|
|
versions; codes written to use those versions will fail to link with
|
|
FFTW 3. Nor is it possible to write ``compatibility wrappers'' to
|
|
bridge the gap (at least not efficiently), because FFTW 3 has different
|
|
semantics from previous versions. However, upgrading should be a
|
|
straightforward process because the data formats are identical and the
|
|
overall style of planning/execution is essentially the same.
|
|
|
|
Unlike FFTW 2, there are no separate header files for real and complex
|
|
transforms (or even for different precisions) in FFTW 3; all interfaces
|
|
are defined in the @code{<fftw3.h>} header file.
|
|
|
|
@heading Numeric Types
|
|
|
|
The main difference in data types is that @code{fftw_complex} in FFTW 2
|
|
was defined as a @code{struct} with macros @code{c_re} and @code{c_im}
|
|
for accessing the real/imaginary parts. (This is binary-compatible with
|
|
FFTW 3 on any machine except perhaps for some older Crays in single
|
|
precision.) The equivalent macros for FFTW 3 are:
|
|
|
|
@example
|
|
#define c_re(c) ((c)[0])
|
|
#define c_im(c) ((c)[1])
|
|
@end example
|
|
|
|
This does not work if you are using the C99 complex type, however,
|
|
unless you insert a @code{double*} typecast into the above macros
|
|
(@pxref{Complex numbers}).
|
|
|
|
Also, FFTW 2 had an @code{fftw_real} typedef that was an alias for
|
|
@code{double} (in double precision). In FFTW 3 you should just use
|
|
@code{double} (or whatever precision you are employing).
|
|
|
|
@heading Plans
|
|
|
|
The major difference between FFTW 2 and FFTW 3 is in the
|
|
planning/execution division of labor. In FFTW 2, plans were found for a
|
|
given transform size and type, and then could be applied to @emph{any}
|
|
arrays and for @emph{any} multiplicity/stride parameters. In FFTW 3,
|
|
you specify the particular arrays, stride parameters, etcetera when
|
|
creating the plan, and the plan is then executed for @emph{those} arrays
|
|
(unless the guru interface is used) and @emph{those} parameters
|
|
@emph{only}. (FFTW 2 had ``specific planner'' routines that planned for
|
|
a particular array and stride, but the plan could still be used for
|
|
other arrays and strides.) That is, much of the information that was
|
|
formerly specified at execution time is now specified at planning time.
|
|
|
|
Like FFTW 2's specific planner routines, the FFTW 3 planner overwrites
|
|
the input/output arrays unless you use @code{FFTW_ESTIMATE}.
|
|
|
|
FFTW 2 had separate data types @code{fftw_plan}, @code{fftwnd_plan},
|
|
@code{rfftw_plan}, and @code{rfftwnd_plan} for complex and real one- and
|
|
multi-dimensional transforms, and each type had its own @samp{destroy}
|
|
function. In FFTW 3, all plans are of type @code{fftw_plan} and all are
|
|
destroyed by @code{fftw_destroy_plan(plan)}.
|
|
|
|
Where you formerly used @code{fftw_create_plan} and @code{fftw_one} to
|
|
plan and compute a single 1d transform, you would now use
|
|
@code{fftw_plan_dft_1d} to plan the transform. If you used the generic
|
|
@code{fftw} function to execute the transform with multiplicity
|
|
(@code{howmany}) and stride parameters, you would now use the advanced
|
|
interface @code{fftw_plan_many_dft} to specify those parameters. The
|
|
plans are now executed with @code{fftw_execute(plan)}, which takes all
|
|
of its parameters (including the input/output arrays) from the plan.
|
|
|
|
In-place transforms no longer interpret their output argument as scratch
|
|
space, nor is there an @code{FFTW_IN_PLACE} flag. You simply pass the
|
|
same pointer for both the input and output arguments. (Previously, the
|
|
output @code{ostride} and @code{odist} parameters were ignored for
|
|
in-place transforms; now, if they are specified via the advanced
|
|
interface, they are significant even in the in-place case, although they
|
|
should normally equal the corresponding input parameters.)
|
|
|
|
The @code{FFTW_ESTIMATE} and @code{FFTW_MEASURE} flags have the same
|
|
meaning as before, although the planning time will differ. You may also
|
|
consider using @code{FFTW_PATIENT}, which is like @code{FFTW_MEASURE}
|
|
except that it takes more time in order to consider a wider variety of
|
|
algorithms.
|
|
|
|
For multi-dimensional complex DFTs, instead of @code{fftwnd_create_plan}
|
|
(or @code{fftw2d_create_plan} or @code{fftw3d_create_plan}), followed by
|
|
@code{fftwnd_one}, you would use @code{fftw_plan_dft} (or
|
|
@code{fftw_plan_dft_2d} or @code{fftw_plan_dft_3d}). followed by
|
|
@code{fftw_execute}. If you used @code{fftwnd} to to specify strides
|
|
etcetera, you would instead specify these via @code{fftw_plan_many_dft}.
|
|
|
|
The analogues to @code{rfftw_create_plan} and @code{rfftw_one} with
|
|
@code{FFTW_REAL_TO_COMPLEX} or @code{FFTW_COMPLEX_TO_REAL} directions
|
|
are @code{fftw_plan_r2r_1d} with kind @code{FFTW_R2HC} or
|
|
@code{FFTW_HC2R}, followed by @code{fftw_execute}. The stride etcetera
|
|
arguments of @code{rfftw} are now in @code{fftw_plan_many_r2r}.
|
|
|
|
Instead of @code{rfftwnd_create_plan} (or @code{rfftw2d_create_plan} or
|
|
@code{rfftw3d_create_plan}) followed by
|
|
@code{rfftwnd_one_real_to_complex} or
|
|
@code{rfftwnd_one_complex_to_real}, you now use @code{fftw_plan_dft_r2c}
|
|
(or @code{fftw_plan_dft_r2c_2d} or @code{fftw_plan_dft_r2c_3d}) or
|
|
@code{fftw_plan_dft_c2r} (or @code{fftw_plan_dft_c2r_2d} or
|
|
@code{fftw_plan_dft_c2r_3d}), respectively, followed by
|
|
@code{fftw_execute}. As usual, the strides etcetera of
|
|
@code{rfftwnd_real_to_complex} or @code{rfftwnd_complex_to_real} are no
|
|
specified in the advanced planner routines,
|
|
@code{fftw_plan_many_dft_r2c} or @code{fftw_plan_many_dft_c2r}.
|
|
|
|
@heading Wisdom
|
|
|
|
In FFTW 2, you had to supply the @code{FFTW_USE_WISDOM} flag in order to
|
|
use wisdom; in FFTW 3, wisdom is always used. (You could simulate the
|
|
FFTW 2 wisdom-less behavior by calling @code{fftw_forget_wisdom} after
|
|
every planner call.)
|
|
|
|
The FFTW 3 wisdom import/export routines are almost the same as before
|
|
(although the storage format is entirely different). There is one
|
|
significant difference, however. In FFTW 2, the import routines would
|
|
never read past the end of the wisdom, so you could store extra data
|
|
beyond the wisdom in the same file, for example. In FFTW 3, the
|
|
file-import routine may read up to a few hundred bytes past the end of
|
|
the wisdom, so you cannot store other data just beyond it.@footnote{We
|
|
do our own buffering because GNU libc I/O routines are horribly slow for
|
|
single-character I/O, apparently for thread-safety reasons (whether you
|
|
are using threads or not).}
|
|
|
|
Wisdom has been enhanced by additional humility in FFTW 3: whereas FFTW
|
|
2 would re-use wisdom for a given transform size regardless of the
|
|
stride etc., in FFTW 3 wisdom is only used with the strides etc. for
|
|
which it was created. Unfortunately, this means FFTW 3 has to create
|
|
new plans from scratch more often than FFTW 2 (in FFTW 2, planning
|
|
e.g. one transform of size 1024 also created wisdom for all smaller
|
|
powers of 2, but this no longer occurs).
|
|
|
|
FFTW 3 also has the new routine @code{fftw_import_system_wisdom} to
|
|
import wisdom from a standard system-wide location.
|
|
|
|
@heading Memory allocation
|
|
|
|
In FFTW 3, we recommend allocating your arrays with @code{fftw_malloc}
|
|
and deallocating them with @code{fftw_free}; this is not required, but
|
|
allows optimal performance when SIMD acceleration is used. (Those two
|
|
functions actually existed in FFTW 2, and worked the same way, but were
|
|
not documented.)
|
|
|
|
In FFTW 2, there were @code{fftw_malloc_hook} and @code{fftw_free_hook}
|
|
functions that allowed the user to replace FFTW's memory-allocation
|
|
routines (e.g. to implement different error-handling, since by default
|
|
FFTW prints an error message and calls @code{exit} to abort the program
|
|
if @code{malloc} returns @code{NULL}). These hooks are not supported in
|
|
FFTW 3; those few users who require this functionality can just
|
|
directly modify the memory-allocation routines in FFTW (they are defined
|
|
in @code{kernel/alloc.c}).
|
|
|
|
@heading Fortran interface
|
|
|
|
In FFTW 2, the subroutine names were obtained by replacing @samp{fftw_}
|
|
with @samp{fftw_f77}; in FFTW 3, you replace @samp{fftw_} with
|
|
@samp{dfftw_} (or @samp{sfftw_} or @samp{lfftw_}, depending upon the
|
|
precision).
|
|
|
|
In FFTW 3, we have begun recommending that you always declare the type
|
|
used to store plans as @code{integer*8}. (Too many people didn't notice
|
|
our instruction to switch from @code{integer} to @code{integer*8} for
|
|
64-bit machines.)
|
|
|
|
In FFTW 3, we provide a @code{fftw3.f} ``header file'' to include in
|
|
your code (and which is officially installed on Unix systems). (In FFTW
|
|
2, we supplied a @code{fftw_f77.i} file, but it was not installed.)
|
|
|
|
Otherwise, the C-Fortran interface relationship is much the same as it
|
|
was before (e.g. return values become initial parameters, and
|
|
multi-dimensional arrays are in column-major order). Unlike FFTW 2, we
|
|
do provide some support for wisdom import/export in Fortran
|
|
(@pxref{Wisdom of Fortran?}).
|
|
|
|
@heading Threads
|
|
|
|
Like FFTW 2, only the execution routines are thread-safe. All planner
|
|
routines, etcetera, should be called by only a single thread at a time
|
|
(@pxref{Thread safety}). @emph{Unlike} FFTW 2, there is no special
|
|
@code{FFTW_THREADSAFE} flag for the planner to allow a given plan to be
|
|
usable by multiple threads in parallel; this is now the case by default.
|
|
|
|
The multi-threaded version of FFTW 2 required you to pass the number of
|
|
threads each time you execute the transform. The number of threads is
|
|
now stored in the plan, and is specified before the planner is called by
|
|
@code{fftw_plan_with_nthreads}. The threads initialization routine used
|
|
to be called @code{fftw_threads_init} and would return zero on success;
|
|
the new routine is called @code{fftw_init_threads} and returns zero on
|
|
failure. The current number of threads used by the planner can be
|
|
checked with @code{fftw_planner_nthreads}. @xref{Multi-threaded FFTW}.
|
|
|
|
There is no separate threads header file in FFTW 3; all the function
|
|
prototypes are in @code{<fftw3.h>}. However, you still have to link to
|
|
a separate library (@code{-lfftw3_threads -lfftw3 -lm} on Unix), as well as
|
|
to the threading library (e.g. POSIX threads on Unix).
|
|
|