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Installation and Customization

This chapter describes the installation and customization of FFTW, the latest version of which may be downloaded from the FFTW home page.

As distributed, FFTW makes very few assumptions about your system. All you need is an ANSI C compiler (gcc is fine, although vendor-provided compilers often produce faster code). However, installation of FFTW is somewhat simpler if you have a Unix or a GNU system, such as Linux. In this chapter, we first describe the installation of FFTW on Unix and non-Unix systems. We then describe how you can customize FFTW to achieve better performance. Specifically, you can I) enable gcc/x86-specific hacks that improve performance on Pentia and PentiumPro's; II) adapt FFTW to use the high-resolution clock of your machine, if any; III) produce code (codelets) to support fast transforms of sizes that are not supported efficiently by the standard FFTW distribution.

Installation on Unix

FFTW comes with a configure program in the GNU style. Installation can be as simple as:

make install

This will build the uniprocessor complex and real transform libraries along with the test programs. We strongly recommend that you use GNU make if it is available; on some systems it is called gmake. The "make install" command installs the fftw and rfftw libraries in standard places, and typically requires root privileges (unless you specify a different install directory with the --prefix flag to configure). You can also type "make check" to put the FFTW test programs through their paces. If you have problems during configuration or compilation, you may want to run "make distclean" before trying again; this ensures that you don't have any stale files left over from previous compilation attempts.

The configure script knows good CFLAGS (C compiler flags) for a few systems. If your system is not known, the configure script will print out a warning. (9) In this case, you can compile FFTW with the command

make CFLAGS="<write your CFLAGS here>"

If you do find an optimal set of CFLAGS for your system, please let us know what they are (along with the output of config.guess) so that we can include them in future releases.

The configure program supports all the standard flags defined by the GNU Coding Standards; see the INSTALL file in FFTW or the GNU web page. Note especially --help to list all flags and --enable-shared to create shared, rather than static, libraries. configure also accepts a few FFTW-specific flags, particularly:

To force configure to use a particular C compiler (instead of the default, usually cc), set the environment variable CC to the name of the desired compiler before running configure; you may also need to set the flags via the variable CFLAGS.

Installation on non-Unix Systems

It is quite straightforward to install FFTW even on non-Unix systems lacking the niceties of the configure script. The FFTW Home Page may include some FFTW packages preconfigured for particular systems/compilers, and also contains installation notes sent in by users. All you really need to do, though, is to compile all of the .c files in the appropriate directories of the FFTW package. (You needn't worry about the many extraneous files lying around.)

For the complex transforms, compile all of the .c files in the fftw directory and link them into a library. Similarly, for the real transforms, compile all of the .c files in the rfftw directory into a library. Note that these sources #include various files in the fftw and rfftw directories, so you may need to set up the #include paths for your compiler appropriately. Be sure to enable the highest-possible level of optimization in your compiler.

By default, FFTW is compiled for double-precision transforms. To work in single precision rather than double precision, #define the symbol FFTW_ENABLE_FLOAT in fftw.h (in the fftw directory) and (re)compile FFTW.

These libraries should be linked with any program that uses the corresponding transforms. The required header files, fftw.h and rfftw.h, are located in the fftw and rfftw directories respectively; you may want to put them with the libraries, or wherever header files normally go on your system.

FFTW includes test programs, fftw_test and rfftw_test, in the tests directory. These are compiled and linked like any program using FFTW, except that they use additional header files located in the fftw and rfftw directories, so you will need to set your compiler #include paths appropriately. fftw_test is compiled from fftw_test.c and test_main.c, while rfftw_test is compiled from rfftw_test.c and test_main.c. When you run these programs, you will be prompted interactively for various possible tests to perform; see also tests/README for more information.

Installing FFTW in both single and double precision

It is often useful to install both single- and double-precision versions of the FFTW libraries on the same machine, and we provide a convenient mechanism for achieving this on Unix systems.

When the --enable-type-prefix option of configure is used, the FFTW libraries and header files are installed with a prefix of `d' or `s', depending upon whether you compiled in double or single precision. Then, instead of linking your program with -lrfftw -lfftw, for example, you would link with -ldrfftw -ldfftw to use the double-precision version or with -lsrfftw -lsfftw to use the single-precision version. Also, you would #include <drfftw.h> or <srfftw.h> instead of <rfftw.h>, and so on.

The names of FFTW functions, data types, and constants remain unchanged! You still call, for instance, fftw_one and not dfftw_one. Only the names of header files and libraries are modified. One consequence of this is that you cannot use both the single- and double-precision FFTW libraries in the same program, simultaneously, as the function names would conflict.

So, to install both the single- and double-precision libraries on the same machine, you would do:

./configure --enable-type-prefix [ other options ]
make install
make clean
./configure --enable-float --enable-type-prefix [ other options ]
make install

gcc and Pentium hacks

The configure option --enable-i386-hacks enables specific optimizations for the Pentium and later x86 CPUs under gcc, which can significantly improve performance of double-precision transforms. Specifically, we have tested these hacks on Linux with gcc 2.[789] and versions of egcs since 1.0.3. These optimizations affect only the performance and not the correctness of FFTW (i.e. it is always safe to try them out).

These hacks provide a workaround to the incorrect alignment of local double variables in gcc. The compiler aligns these variables to multiples of 4 bytes, but execution is much faster (on Pentium and PentiumPro) if doubles are aligned to a multiple of 8 bytes. By carefully counting the number of variables allocated by the compiler in performance-critical regions of the code, we have been able to introduce dummy allocations (using alloca) that align the stack properly. The hack depends crucially on the compiler flags that are used. For example, it won't work without -fomit-frame-pointer.

In principle, these hacks are no longer required under gcc versions 2.95 and later, which automatically align the stack correctly (see -mpreferred-stack-boundary in the gcc manual). However, we have encountered a bug in the stack alignment of versions 2.95.[012] that causes FFTW's stack to be misaligned under some circumstances. The configure script automatically detects this bug and disables gcc's stack alignment in favor of our own hacks when --enable-i386-hacks is used.

The fftw_test program outputs speed measurements that you can use to see if these hacks are beneficial.

The configure option --enable-pentium-timer enables the use of the Pentium and PentiumPro cycle counter for timing purposes. In order to get correct results, you must define FFTW_CYCLES_PER_SEC in fftw/config.h to be the clock speed of your processor; the resulting FFTW library will be nonportable. The use of this option is deprecated. On serious operating systems (such as Linux), FFTW uses gettimeofday(), which has enough resolution and is portable. (Note that Win32 has its own high-resolution timing routines as well. FFTW contains unsupported code to use these routines.)

Customizing the timer

FFTW needs a reasonably-precise clock in order to find the optimal way to compute a transform. On Unix systems, configure looks for gettimeofday and other system-specific timers. If it does not find any high resolution clock, it defaults to using the clock() function, which is very portable, but forces FFTW to run for a long time in order to get reliable measurements.

If your machine supports a high-resolution clock not recognized by FFTW, it is therefore advisable to use it. You must edit fftw/fftw-int.h. There are a few macros you must redefine. The code is documented and should be self-explanatory. (By the way, fftw-int stands for fftw-internal, but for some inexplicable reason people are still using primitive systems with 8.3 filenames.)

Even if you don't install high-resolution timing code, we still recommend that you look at the FFTW_TIME_MIN constant in fftw/fftw-int.h. This constant holds the minimum time interval (in seconds) required to get accurate timing measurements, and should be (at least) several hundred times the resolution of your clock. The default constants are on the conservative side, and may cause FFTW to take longer than necessary when you create a plan. Set FFTW_TIME_MIN to whatever is appropriate on your system (be sure to set the right FFTW_TIME_MIN...there are several definitions in fftw-int.h, corresponding to different platforms and timers).

As an aid in checking the resolution of your clock, you can use the tests/fftw_test program with the -t option (c.f. tests/README). Remember, the mere fact that your clock reports times in, say, picoseconds, does not mean that it is actually accurate to that resolution.

Generating your own code

If you know that you will only use transforms of a certain size (say, powers of 2) and want to reduce the size of the library, you can reconfigure FFTW to support only those sizes you are interested in. You may even generate code to enable efficient transforms of a size not supported by the default distribution. The default distribution supports transforms of any size, but not all sizes are equally fast. The default installation of FFTW is best at handling sizes of the form 2a 3b 5c 7d 11e 13f, where e+f is either 0 or 1, and the other exponents are arbitrary. Other sizes are computed by means of a slow, general-purpose routine. However, if you have an application that requires fast transforms of size, say, 17, there is a way to generate specialized code to handle that.

The directory gensrc contains all the programs and scripts that were used to generate FFTW. In particular, the program gensrc/genfft.ml was used to generate the code that FFTW uses to compute the transforms. We do not expect casual users to use it. genfft is a rather sophisticated program that generates directed acyclic graphs of FFT algorithms and performs algebraic simplifications on them. genfft is written in Objective Caml, a dialect of ML. Objective Caml is described at http://pauillac.inria.fr/ocaml/ and can be downloaded from from ftp://ftp.inria.fr/lang/caml-light.

If you have Objective Caml installed, you can type sh bootstrap.sh in the top-level directory to re-generate the files. If you change the gensrc/config file, you can optimize FFTW for sizes that are not currently supported efficiently (say, 17 or 19).

We do not provide more details about the code-generation process, since we do not expect that users will need to generate their own code. However, feel free to contact us at fftw@fftw.org if you are interested in the subject.

You might find it interesting to learn Caml and/or some modern programming techniques that we used in the generator (including monadic programming), especially if you heard the rumor that Java and object-oriented programming are the latest advancement in the field. The internal operation of the codelet generator is described in the paper, "A Fast Fourier Transform Compiler," by M. Frigo, which is available from the FFTW home page and will appear in the Proceedings of the 1999 ACM SIGPLAN Conference on Programming Language Design and Implementation (PLDI).

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