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DOC: add a 1.26.1 release notes section for BLAS/LAPACK build changes #24902

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Oct 11, 2023
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DOC: add a 1.26.1 release notes section for BLAS/LAPACK build changes #24902

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56 changes: 54 additions & 2 deletions doc/source/release/1.26.1-notes.rst
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Expand Up @@ -3,6 +3,58 @@
==========================
NumPy 1.26.1 Release Notes
==========================

NumPy 1.26.1 is a maintenance release that fixes bugs and regressions
discovered after the 1.26.0 release. This is the last planned release in the
The Python versions supported by this release are 3.9-3.12.
discovered after the 1.26.0 release. In addition, it adds new functionality for
detecting BLAS and LAPACK when building from source. The 1.26.release series is
the last planned minor release series before NumPy 2.0. The Python versions
supported by this release are 3.9-3.12.


Build system changes
====================

Improved BLAS/LAPACK detection and control
------------------------------------------

Auto-detection for a number of BLAS and LAPACK is now implemented for Meson.
By default, the build system will try to detect MKL, Accelerate (on macOS
>=13.3), OpenBLAS, FlexiBLAS, BLIS and reference BLAS/LAPACK. Support for MKL
was significantly improved, and support for FlexiBLAS was added.

New command-line flags are available to further control the selection of the
BLAS and LAPACK libraries to build against.

To select a specific library, use the config-settings interface via ``pip`` or
``pypa/build``. E.g., to select ``libblas``/``liblapack``, use::

$ pip install numpy -Csetup-args=-Dblas=blas -Csetup-args=-Dlapack=lapack
$ # OR
$ python -m build . -Csetup-args=-Dblas=blas -Csetup-args=-Dlapack=lapack

This works not only for the libraries named above, but for any library that
Meson is able to detect with the given name through ``pkg-config`` or CMake.

Besides ``-Dblas`` and ``-Dlapack``, a number of other new flags are available
to control BLAS/LAPACK selection and behavior:

- ``-Dblas-order`` and ``-Dlapack-order``: a list of library names to search
for in order, overriding the default search order.
- ``-Duse-ilp64``: if set to ``true``, use ILP64 (64-bit integer) BLAS and
LAPACK. Note that with this release, ILP64 support has been extended to
include MKL and FlexiBLAS. OpenBLAS and Accelerate were supported in previous
releases.
- ``-Dallow-noblas``: if set to ``true``, allow NumPy to build with its
internal (very slow) fallback routines instead of linking against an external
BLAS/LAPACK library. *The default for this flag may be changed to ``false``
in a future 1.26.x release, however for 1.26.1 we'd prefer to keep it as
``true`` because if failures to detect an installed library are happening,
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Oops, spotted a mistake here. The first word here should be false, and two lines up it should be true. @charris would you prefer me to open a new PR to fix it, or can you take it along in your release notes update that is in the pipeline I think?

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I'll do it now before I forget :)

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Done.

we'd like a bug report for that, so we can quickly assess whether the new
auto-detection machinery needs further improvements.*
- ``-Dmkl-threading``: to select the threading layer for MKL. There are four
options: ``seq``, ``iomp``, ``gomp`` and ``tbb``. The default is ``auto``,
which selects from those four as appropriate given the version of MKL
selected.
- ``-Dblas-symbol-suffix``: manually select the symbol suffix to use for the
library - should only be needed for linking against libraries built in a
non-standard way.