Move from fftw3 to pocketfft?

[bjeurissen]

Recent versions of eigen provide an interface to pocketfft (https://gitlab.mpcdf.mpg.de/mtr/pocketfft/tree/cpp), a header-only fft library that is widely adopted by scientific computing packages such as numpy, scipy and pytorch.

Switching is as easy as defining EIGEN_POCKETFFT_DEFAULT (see: https://gitlab.com/libeigen/eigen/-/blob/master/unsupported/Eigen/FFT#L31)

Since pocketfft is header-only, this means we could drop the fftw3 library dependency, and since it is much more optimized than kissfft which is bundled with eigen, we can probably do so without much downsides.

[bjeurissen]

Tested on master with the following config:

#!/usr/bin/python
  #
  # autogenerated by MRtrix configure script
  #
  # configure output:
  #
# MRtrix build type requested: release version with nogui
#
# WARNING: FSL detected in PATH ("/Users/ben/Software/fsl/share/fsl/bin") - removed to avoid conflicts
# OS X deployment target: 26.1
# NOTE: if you intend to build with the FSL toolchain (not recommended), pass the -fsl flag to ./configure
#
# Detecting OS: darwin
# Looking for compiler [clang++]: Apple clang version 17.0.0 (clang-1700.4.4.1)
# Checking for C++14 compliance: ok
# Checking shared library generation: ok
# Detecting pointer size: 64 bit
# Detecting byte order: little-endian
# Checking for variable-length array support: ok
# Checking for non-POD variable-length array support: ok
# Checking for ::max_align_t: 8 bytes
# Checking for std::max_align_t: 8 bytes
# Checking for Eigen3 library: 3.5.0
# Checking for Eigen3 Unsupported: Present
# Checking for zlib compression library: 1.2.12
# Checking for "JSON for Modern C++" requirements: ok
# Checking for TIFF library: LIBTIFF, Version 4.7.1
# Checking for PNG library: Header: 1.6.51; library: 1.6.51
# Checking for FFTW library: not found - FFTW support disabled


PATH = r'/opt/homebrew/bin:/opt/homebrew/sbin:/Users/ben/Software/dcm2niix/bin:/Users/ben/Software/ants/bin:/usr/local/bin:/System/Cryptexes/App/usr/bin:/usr/bin:/bin:/usr/sbin:/sbin:/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/local/bin:/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/bin:/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/appleinternal/bin:/opt/pmk/env/global/bin:/Applications/iTerm.app/Contents/Resources/utilities'
obj_suffix = '.o'
exe_suffix = ''
lib_prefix = 'lib'
lib_suffix = '.dylib'
cpp = [ 'clang++', '-c', 'CFLAGS', 'SRC', '-o', 'OBJECT' ]
cpp_flags = [ '-std=c++14', '-DMRTRIX_BUILD_TYPE="release version with nogui"', '-DMRTRIX_MACOSX', '-fPIC', '-mmacosx-version-min=26.1', '-DMRTRIX_WORD64', '-DMRTRIX_HAVE_EIGEN_UNSUPPORTED_SPECIAL_FUNCTIONS', '-DMRTRIX_TIFF_SUPPORT', '-isystem', '/opt/homebrew/opt/libtiff/include', '-isystem', '/opt/homebrew/opt/zstd/include', '-isystem', '/opt/homebrew/Cellar/xz/5.8.1/include', '-isystem', '/opt/homebrew/opt/jpeg-turbo/include', '-DMRTRIX_PNG_SUPPORT', '-isystem', '/opt/homebrew/opt/libpng/include/libpng16', '-Wall', '-O3', '-DNDEBUG' ]
ld = [ 'clang++', 'OBJECTS', 'LINKFLAGS', '-o', 'EXECUTABLE' ]
ld_flags = [ '-mmacosx-version-min=26.1', '-lz', '-L/opt/homebrew/opt/libtiff/lib', '-ltiff', '-L/opt/homebrew/opt/libpng/lib', '-lpng16' ]
runpath = '-Wl,-rpath,@loader_path/'
ld_enabled = True
ld_lib = [ 'clang++', 'OBJECTS', 'LINKLIB_FLAGS', '-o', 'LIB' ]
ld_lib_flags = [ '-dynamiclib', '-install_name', '@rpath/LIBNAME', '-mmacosx-version-min=26.1', '-lz', '-L/opt/homebrew/opt/libtiff/lib', '-ltiff', '-L/opt/homebrew/opt/libpng/lib', '-lpng16' ]
eigen_cflags = [ '-isystem', '/opt/homebrew/Cellar/eigen/5.0.1/include/eigen3', '-DEIGEN_DONT_PARALLELIZE', '-isystem', '/Users/ben/mrtrix3_master/pocketfft', '-DEIGEN_POCKETFFT_DEFAULT']
moc = ''
rcc = ''
qt_cflags = []
qt_ldflags = []
nogui = True

mrdegibbs on a (120 x 120 x 68 x 151) file, completed in 54s with pocketfft and in 57s with fftw3, so at first glance, pocketfft looks very competitive (my system was under some other load during testing).