BLAS and LAPACK

Selecting BLAS and LAPACK libraries

BLAS and LAPACK library selection, other than the OpenBLAS default, is implemented via Meson build options. For example, to select plain libblas and liblapack (this is typically Netlib BLAS/LAPACK on Linux distros, and can be dynamically switched between implementations on conda-forge), use:

$ # for a development build
$ python dev.py build -C-Dblas=blas -C-Dlapack=lapack

$ # to build and install a wheel
$ python -m build -Csetup-args=-Dblas=blas -Csetup-args=-Dlapack=lapack
$ pip install dist/scipy*.whl

$ # Or, with pip>=23.1, this works too:
$ python -m pip -Csetup-args=-Dblas=blas -Csetup-args=-Dlapack=lapack

Other options that should work (as long as they’re installed with pkg-config or CMake support) include mkl, atlas, blis and accelerate.

Note that both Accelerate and scipy-openblas have flags in dev.py that are easier to remember, since they’re commonly used for development:

$ python dev.py build --with-accelerate
$ python dev.py build --with-scipy-openblas

The -Dlapack flag isn’t needed for Accelerate, MKL or scipy-openblas, since we can be sure that BLAS and LAPACK are the same for those options. E.g., to create a wheel with Accelerate (on macOS >=13.3 only), use:

$ python -m build -Csetup-args=-Dblas=accelerate

Using pkg-config to detect libraries in a nonstandard location

The way BLAS and LAPACK detection works under the hood is that Meson tries to discover the specified libraries first with pkg-config, and then with CMake. If all you have is a standalone shared library file (e.g., armpl_lp64.so in /a/random/path/lib/ and a corresponding header file in /a/random/path/include/), then what you have to do is craft your own pkg-config file. It should have a matching name (so in this example, armpl_lp64.pc) and may be located anywhere. The PKG_CONFIG_PATH environment variable should be set to point to the location of the .pc file. The contents of that file should be:

libdir=/path/to/library-dir      # e.g., /a/random/path/lib
includedir=/path/to/include-dir  # e.g., /a/random/path/include
version=1.2.3                    # set to actual version
extralib=-lm -lpthread -lgfortran   # if needed, the flags to link in dependencies
Name: armpl_lp64
Description: ArmPL - Arm Performance Libraries
Version: ${version}
Libs: -L${libdir} -larmpl_lp64      # linker flags
Libs.private: ${extralib}
Cflags: -I${includedir}

To check that this works as expected, you should be able to run:

$ pkg-config --libs armpl_lp64
-L/path/to/library-dir -larmpl_lp64
$ pkg-config --cflags armpl_lp64
-I/path/to/include-dir

Specifying the Fortran ABI to use

Some linear algebra libraries are built with the g77 ABI (also known as “the f2c calling convention”) and others with GFortran ABI, and these two ABIs are incompatible. Therefore, if you build SciPy with gfortran and link to a linear algebra library like MKL, which is built with a g77 ABI, there’ll be an exception or a segfault. SciPy fixes this by using ABI wrappers which rely on the CBLAS API for the few functions in the BLAS API that suffer from this issue.

Note that SciPy needs to know at build time, what needs to be done and the build system will automatically check whether linear algebra library is MKL or Accelerate (which both always use the g77 ABI) and if so, use the CBLAS API instead of the BLAS API. If autodetection fails or if the user wants to override this autodetection mechanism for building against plain libblas/liblapack (this is what conda-forge does for example), use the -Duse-g77-abi=true build option. E.g.,:

$ python -m build -C-Duse-g77-abi=true -Csetup-args=-Dblas=blas -Csetup-args=-Dlapack=lapack

Work-in-progress

These options are planned to be fully supported, but currently not usable out of the box:

• ILP64 (64-bit integer size) builds: large parts of SciPy support using ILP64 BLAS/LAPACK. Note that support is still incomplete, so SciPy also requires LP64 (32-bit integer size) BLAS/LAPACK.

• Automatically selecting from multiple possible BLAS and LAPACK options, with a user-provided order of precedence